JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D22, PAGES 28,629-28,641, NOVEMBER 27, 2001

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D22, PAGES 28,629-28,641, NOVEMBER 27, 2001 Vertical and horizontal distributions of the aerosol number concentration and size distribution over the northern Indian Ocean Marian de Reus,,2 Radovan Krejci, 2 Jonathan Williams, Horst Fischer, Rinus Scheele, 3 and Johan Str6m 2 Abstract. Airborne measurements of the aerosol number concentration and size distribution were conducted over the northern Indian Ocean during the Indian Ocean Experiment (INDOEX) in February-March Vertical profiles of the aerosol number concentration demonstrate elevated concentrations of nucleation mode particles in a layer between 8 and 12.5 km altitude. By using a novel combination of back trajectory information and cloud top temperatures retrieved from satellite images, it is shown that these particles most likely originated from the outflow of large convective clouds. Accumulation mode particles observed at these altitudes show indications for cloud processing. The aerosol size distributions observed in the layer between 4 and 8 km altitude show characteristics of an aged aerosol. In this layer the accumulation mode particle number concentration presents a minimum, a low variability, and very few particles larger than 0.7/xm diameter. The aerosol in the marine boundary layer can be characterized by high number concentrations of submicron and accumulation mode particles, which gradually decrease with distance from the Indian subcontinent. The particle loss rate is equivalent to cm -3 per hour. This decrease takes place over the whole size distribution and can therefore not be explained by coagulation, but is likely due to precipitation and entrainment of free tropospheric air. 1. Introduction been conducted in this region over the past 4 years, with an intensive field phase in February-March First results of Atmospheric aerosols are known to play an important role in the INDOEX measurementshow that an anthropogenic haze both urban haze and acid rain [Graedel and Crutzen, 1993; of gas phase and aerosol pollution spreads over much of the Kulshrestha et al., 1999], and when high concentrations of an- Arabian Sea and northern Indian Ocean from December to thropogenic aerosols are widespread, they can have a signifi- April [Ramanathan et al., this issue; Lelieveld et al., 2001]. cant effect on global climate [Charlson et al., 1992; Ra- These anthropogenic aerosols induce a strong perturbation in manathan, 1998]. Aerosols influence climate directly by the regional radiative forcing, which can exceed the radiative scattering and absorbing incoming solar radiation, and indieffect of greenhouse gases by a factor of 5-10 [Ramanathan et rectly by acting as cloud condensationuclei for cloud and fog al., this issue]. droplets, thereby affecting the droplet concentrations, optical Aerosol gradients in the Indian Ocean marine boundary properties, precipitation rate, and lifetime of clouds [Brenguier layer as a function of latitude during the winter monsoon have et al., 2000]. been characterized by shipborne measurements prior to the The northern Indian Ocean region is currently the subject of intensive research because of its large potential for growing INDOEX intensive field phase [e.g., Satheesh et al., 1998; pollution emissions, due to the rapid growth of human popu- Krishnamufti et al., 1998; Jayaraman et al., 1998]. These authors lation and industrial development in South and Southeast reported a decrease in high aerosol concentrations and optical Asia. This region also provides a good laboratory to study depth with increasing distance from the coast along the track temporal changes in aerosol and trace gas properties. Tempo- of the northeasterly trade winds. Near the Indian west coast an ral changes can be studied as polluted air is transported from aerosol optical depth as high as 0.5 was observed, while it the continent over the ocean toward the Intertropical Convergence Zone (ITCZ) with the persistent northeast trade wind, decreased to less than 0.1 over the interior Indian Ocean. prevailing during the winter monsoon period between December and April. The Indian Ocean Experiment (INDOEX) has 1Department of Air Chemistry, Max Planck Institute for Chemistry, Mainz, Germany. taining an accumulation mode with a mean mode diameter of 2Air Pollution Laboratory, Institute of Applied Environmental Re- about / m and a coarse mode at about 1-2/ m diamsearch, Stockholm University, Stockholm, Sweden. eter. This size distribution did not change significantly with 3Royal Netherlands Meteorological Institute, De Bilt, Netherlands. distance from the continent [Moorthy et al., 1999; Satheesh et Copyright 2001 by the American Geophysical Union. Paper number 2001JD /01/2001JD ,629 Concurrently, the aerosol mass concentration decreased from 80/ g m -3 to a few/ g m -3. Columnar aerosol number size distributions retrieved from radiometer measurements during the same time period over the northern Indian Ocean show a bimodal distribution, con- al., 1998]. Although Moorthy et al. [1993] and Murugavel and Kamra [1999] have noted a change in the size distribution, attributed to the sea breeze effect, little attention has been

2 28,630 DE REUS ET AL.' AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN 10 --I Longitude (øe) Figure 1. Map of the measurement area with the flight tracks of all flights performed by the Citation during the INDOEX intensive field phase in February-March ronmental Research, Stockholm University, Sweden. The particle number concentration was measured by two condensation particle counters (CPC), one standard Thermo Systems Inc. (TSI) 3760 and one modified TSI 3010 particle counter, in which the temperature difference between the saturator and condenser tube was increased in order to reduce the 50% counting efficiency diameter of the instrument. Laboratory calibration using ammonium sulfate particles showed that the 50% counting efficiencies of the instruments were and /xm diameter, respectively. With these instruments the total number of particles with a diameter larger than (N6) and 0.018/xm (N 8) were measured with a time resolution of 0.1 s, from which 1 s average concentrations were calculated. These instruments were connected to a backward facing inlet, allowing only aerosol smaller than about 1/xm to be sampled [Schr6der and Str6m, 1997]. The difference between the two CPC instruments represents the nucleation mode or ultrafine particle number concentration (UCN), with a diameter between and 0.018/xm. This measurement is used as the first bin in the aerosol size distribution. focused on the aerosol distribution at higher altitudes in this polluted marine environment. In this paper we present aerosol number concentrations and A custom-built differential mobility analyzer (DMA) with a closed loop sheath air system [Jokinen and Miikela, 1997] was used to determine the particle size distribution for particles size distributions as a function of altitude and latitude from between and 0.15 /xm diameter. The DMA used was a near the surface to 12.5 km altitude. The latitude and altitude short Hauke type, with a sheath airflow of 5 L min-. Particles distribution of the trace gas species measured on board the Citation aircraft have been presented elsewhere [de Gouw et al., this issue]. Furthermore, by using a novel combination of satellite-derived cloud top temperatures, back trajectory information, and in situ aerosol measurements we try to infer the origin of the nucleation mode particles observed at high altiselected by the DMA were detected by a TSI 3010 condensation particle counter with a sample flow rate of 1 L min -. Every 20 s one scan over 10 size bins was performed. The measurements were performed at atmospheric pressure, which varied between 1000 hpa in the marine boundary layer to about 200 hpa at the highest measurement level. The mobility tudes over the northern Indian Ocean. distribution measured by the DMA instrument was inverted to a number distribution assuming a Fuchs charge distribution 2. Indian Ocean Experiment [Wiedensohler, 1988]. The inversion program accounts for the changes in pressure through the change of the mean free path The Indian Ocean Experiment (INDOEX) was carried out over the Indian Ocean during February-March The main with pressure. To account for double charged aerosols, the measurements of the optical particle counter were used. The goal of the measurements was to improve our understanding correction due to the presence of double charged particles was about aerosols, clouds, and chemistry-climate interactions in the Indian Ocean region, and assess the significance for global radiative forcing. With the Cessna Citation aircraft, 23 research flights were at most 12% for the largest size bin of the DMA (Dp = O. 13 /xm) and 1% for the smallest size bin (Dp = 0.02 /xm). An optical particle counter (OPC) particle measuring system passive cavity aerosol spectrometer probe (PMS PCASP) was completed between February 14 and March 21, corresponding used to determine the particle size distribution of particles with to a total flight time of about 100 hours. The measurements a diameter between 0.12 and 3.5/xm by classifying them in 32 were performed from within the marine boundary layer up to 12.5 km altitude. These flights included 160 level flight legs, with a length of at least 100 km, conducted at constant airspeed size bins using the scattering properties of the particles. This instrument was connected to a forward facing, near-isokinetic inlet, allowing particles larger than 1 /xm to be analyzed. The of about 150 m s-. The measurements were performed within total number concentration measured by this instrument, also a range of 1500 km of the operation base at Hulhule Airport (4ø11'N, 73ø31'E), the Maldives (see Figure 1). The aircraft was equipped to measure the aerosol number concentration and size distribution in the size range of to 3.5/xm diameter, as well as trace gases such as ozone, carbon monoxide, and volatile organic compounds. A standard set of meteorological and aircraft parameters was also simultaneously recorded, including temperature, pressure, altitude, wind direction, and wind speed. A short description of the referred to as accumulation mode particles (Nacc), is the number concentration of particles with a diameter between 0.12 and 3.5/xm. The data were recorded at 1 Hz. Combining the four instruments described above yields a particle size distribution for particles with a diameter between and 3.5/xm. An aerosol size distribution measurement is preferably obtained on isobaric flight levels since the DMA instrument requires a relatively constant operating pressure. Moreover, aerosol size distribution measurements were only instrumentation used to measure the aerosol size distribution calculated during flight levels lasting more than 10 min, which and number concentration is given below. corresponds to a spatial distance of about 100 km, in order to obtain good counting statistics Instrumentation To check the reliability of the size distribution measure- Aerosol size distribution and number concentration mea- ments, the ratio between the N 8 particle number concentrasurements were performed by the Institute for Applied Envi- tion and the integral of the aerosol size distribution was used.

3 DE REUS ET AL.: AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN 28,631 When the majority of the particles are larger than 20 nm diameter (the lower size of the DMA), this ratio should be ideally close to 1. This ratio was about 1 in the boundary layer, but increased with increasing altitude, which could be due to a decrease in charging efficiency with altitude. Another explanation could be the small gap between the 50% cutoff diameter of the CPC instrument and the first bin of the DMA instru- ment. This gap increases in size with altitude and contains most aerosols at higher altitudes. We defined a good measurement when the ratio was between 0.5 and 10. The uncertainty in the measured particle number concentrations by the CPC, OPC, and DMA instruments is dependent on counting statistics, and equal to the square root of the number of particles counted in a sampling period. Hence the uncertainty decreases with increasing particle number concentration and averaging time. For the 10 min averaged aerosol size distributions presented in this paper, the error due to counting statistics is less than 2% for the UCN and DMA size bins. For the OPC size bins the error increases strongly with particle size. For particles smaller than 0.40 tzm diameter the error is less than 1% below 4 km altitude, and less than 10% at higher altitudes. For the larger particles the error increases from 15% at the lowest level to 100% at the highest levels. The uncertainty in the flow rate of the different instruments is estimated to be about 5%. In the aircraft the instruments were placed close to the sample inlet and were operated with a flow rate of about 10 L min -1, which caused a transportime of less than 1 s in the sampling line, minimizing the diffusion losses for small particles. A comparison of the particle number concentration in the largest size bins of the OPC with data from a forward scattering spectrometer probe (FSSP) during the Second Aerosol Characterization Experiment (ACE 2) showed a good agreement, indicating no significant losses of large particles in the inlet system [de Re ts et al., 2000a]. The measured particle diameter is the diameter at instrument conditions, which at high altitudes is equal to the dry diameter due to the temperature increase of the sample air when entering the aircraft. Carbon monoxide (CO) was measured by the Max Planck Institute for Chcmistry, Mainz, Germany, using a tunable diode laser absorption spectrometer (TDLAS), with a time resolution of 1 Hz, an average precision of _+3.6%, and a calibration accuracy of 2.8%. A detailed description of the instrument is given by Wienhold et al. [1998] Meteorology The INDOEX intensive field phase was conducted during the dry monsoon season when northeasterly trade winds prevail in the marine boundary layer, bringing pollutants from the Indian subcontinent over the Indian Ocean toward the ITCZ. Ten-day back trajectories have been calculated for each level flight section by the Royal Netherlands Meteorological Institute (KNMI), using three-dimensional ECMWF wind fields [Scheelet al., 1996]. The boundary layer trajectories indicate that the air masses in the marine boundary layer originated from the Bay of Bengal during the first part of the campaign (February) and from the Arabian Sea during the second part (March). At higher altitudes the wind direction was much more variable. Between 3 and 8 km altitude, winds were both from northerly and southerly directions, while above 8 km altitude, winds were mainly from the southeast. At these altitudes the outflow of large convective clouds occurs, which were very frequently observed in the measurement region. An ex- tensive analysis of the meteorological conditions as well as the atmospheric transport during the INDOEX intensive field phase are presented by Verver et al. [this issue]. The measurements with the Citation were performed up to an altitude of 12.5 km. This is well below the tropopause in this region, which is typically located between 15 and 19 km altitude in the tropics. The observations reported here cover therefore the marine boundary layer and lower and middle free troposphere. The Intertropical Convergence Zone (ITCZ) was located between 5øS and 12øS during February and March 1999, which was mostly beyond the range of our aircraft. Only during one flight were we able to penetrate into the meteorological southern hemisphere. In this paper, only data obtained north of the ITCZ are presented. 3. Observations To provide an overview of the aerosol properties in the boundary layer and free troposphere over the Indian Ocean in February-March 1999, we will present vertical profiles and latitude gradients of the aerosol number concentration and size distribution. Moreover, the horizontal and vertical distribution of the CO mixing ratio will be presented in order to facilitate the interpretation of the aerosol data. All particle number concentrations presented in this paper are corrected for standard temperature and pressure (STP, hpa, K) Aerosol Number Concentrations: Altitude Profiles Figure 2 presents mean vertical profiles of the aerosol number concentration for different particle size ranges and the CO mixing ratio for the entire INDOEX campaign. The data are divided into altitude bins of 250 m, and for each altitude bin the median concentration is calculated. To show the variability in the observations, the 25 and 75 percentile are indicated as well. Figures 2a-2d show the submicron (N6), accumulation (Nacc), and ultrafine (UCN) particle number concentration and the CO mixing ratio, respectively. The median N 6 particle number concentration ranged between 400 and 4000 cm -3. High concentrations were found in the boundary layer and above 8 km altitude; however, the N 6 particle number concentration at high altitudes exceeded the boundary layer concentration by a factor 3 to 4. Between 2 and 4 km altitude the N 6 particle number concentration was relatively low. The variation in the data was small, as can be seen from the relatively small difference in 25, 50, and 75 percentile values, indicating similar altitude profiles of the N 6 number concentration throughout the entire region and campaign. This is especially valid for the layer between 2 and 6 km altitude. Most variable was the N 6 particle number concentration in the middle free troposphere between 8 and 12.5 km altitude. In this layer the outflow of large convective clouds occurred, as can be inferred from the observed CO profile (Figure 2d). CO is mainly produced by anthropogenic processes in the boundary layer, such as incomplete combustion, and has no large sources in the free troposphere. Therefore it normally shows high concentrations in the boundary layer and decreasing concentrations with increasing altitude. The median CO mixing ratio during INDOEX was about 160 ppbv in the boundary layer and decreased strongly above 2 km altitude. The high CO mixing ratio observed in the lowest 2 km of the atmosphere indicates transport of anthropogenic pollution,

4 28,632 DE REUS ET AL.: AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN 12- a 10-8 m 4g 2 I 0- i i i i i i l[ I i i i i i i io 3 N 6 (STP ½m -3) I 1 O0 150 Na (STP cm -3) UCN (STP cm -3) CO (ppbv) Figure 2. Median altitude profiles of the (a) submicron, (b) accumulation, and (c) ultrafine particle number concentration, and (d) CO mixing ratio. The thin lines indicate the 25 and 75 percentile values. I most probably originating from the Indian subcontinent. Enhanced concentrations were observed above 8 km altitude, indicating transport of CO-rich air from the boundary layer to the middle free troposphere. This enhancement is not as large as the observed increase in submicron particle number concentration at the same altitudes, but taking into account that the CO mixing ratio normally shows a steady decrease in the free troposphere, the enhancement is significant. Since the measurements were performed in a region with extensive convection, the CO was likely to be transported to the middle free troposphere through convective clouds. The decrease in CO mixing ratio from about 160 ppbv in the boundary layer to about 110 ppbv in the middle free troposphere indicates dilution of the boundary layer air when transported upward through the convective cloud. However, we cannot exclude that this is due to convective transport of air masses closer to the ITCZ, where the observed CO mixing ratio in the marine boundary layer decreased to about 110 ppbv. The N 6 particle number concentration was 3 to 4 times higher in the middle free troposphere compared to the boundary layer, which means that convective transport cannot simply explain all submicron aerosols observed at high altitude, indicating an additional source of submicron particles in the middle free troposphere. The accumulation mode particle number concentration (Figure 2b) showed a clear maximum of about 1000 cm -3 in the boundary layer and decreased strongly with increasing altitude. Note that the accumulation mode particle number concentration in the boundary layer is about the same as the N 6 particle number concentration in this layer, indicating that the major part of the aerosols are larger than about 0.12 / m diameter. Above 3.5 km altitude, very low concentrations of accumulation mode particles were observed, generally below 20 cm -3. The transition region, where the accumulation mode particle number concentration dropped below 20 cm -3, was on average located at 3.5 km altitude. On the individual flights, however, this transition region varied between 2.7 and 4.2 km and was generally found to coincide with a weak temperature inversion in the atmosphere. The observed temperature profiles from the individual flights generally showed two temperature inversions, one at about 1 km altitude and one at about 3.5 km altitude. A slight enhancement in the accumulation mode particle number concentration was observed above 6 km altitude. Assuming that these particles were transported to these altitudes by convective transport suggests that they may result from evaporating cloud droplets or were less hygroscopic particles which did not participate in the cloud processes. The ultrafine particle number concentration (Figure 2c) was very low in the lower part of the atmosphere, but increased sharply above 8 km altitude. The maximum UCN concentration was observed between 8 and 12 altitude, where the median concentration exceeded 1000 cm -3. Analogously to the N 6 particle number concentration, the variation in the UCN number concentration was largest in this layer. The presence of ultrafine particles in the atmosphere is an indication for recent particle production by gas to particle conversion [de Reus et al., 1999]. The enhanced UCN and N 6 particle number concentrations between 8 and 12.5 km altitude are hence a result of in situ new particle formation as opposed to transport from the boundary layer. We will further investigate the high ultrafine particle number observed in the middle free troposphere in section 4.1. Although the general pattern of the altitude profiles obtained during the individual flights was similar to the median profile shown in Figure 2, the high-resolution data included much higher concentrations. The 1-s averaged N 6 and UCN particle number concentration showed peak concentrations up to 104 cm -3, and the accumulation mode particle number concentration showed peak concentrations up to 3000 cm -3. From the temperature profiles for the individual flights and the mean vertical profiles of the aerosol number concentration we identified four distinct layers in the atmosphere, which we will use later to evaluate the observed particle size distributions. The lowest layer, between 0 and 1 km altitude, contained

5 DE REUS ET AL.: AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN 28,633 Table 1. Median, Mean, and Standard Deviation for the Observed Parameters in the Different Layers Described in Section km: Mixed Layer km: Transition km: Lower Free km: Middle Free Layer Troposphere Troposphere Median Mean s.d. Median Mean s.d. Median Mean s.d. Median Mean s.d. Pressure, hpa Temperature, øc Wind speed, m s Wind angle, deg N6, STP cm N 8, STP cm N... STP cm UCN, STP cm CO, ppbv very high concentrations of N 6 and Nacc particles and CO, while the UCN concentration was very low. The aerosol and trace gas concentrations in this layer were approximately constant, indicating a well mixed layer. In the remainder of this paper we will refer to this layer as the mixing layer or marine boundary layer. The top of the mixing layer coincides with the first temperature inversion. Above the mixing layer the N 6 particle number concentration as well as the accumulation particle number concentration decreased strongly to an altitude of about 3.5 km; however, still a relatively large amount of accumulation mode aerosols were present. The UCN particle number concentration remained very low. The upper boundary of this layer coincides with the second temperature inversion and varied between 2.7 and 4.2 km altitude on the individual flights. Since this layer connects the marine boundary layer with the free troposphere, we will refer to this layer as the transition layer. The transition layer could be subdivided into two layers, below and above 2 km altitude, which is most clear in the Nac and CO profiles and is probably connected to the presence of shallow convective clouds. However, since the aerosol size distributions were very entrainment of clean free tropospheric air, trace gases which are emitted over the Indian subcontinent will decrease in con- centration while moving south. The decrease in CO mixing ratio with latitude is shown in Figure 3c. This graph presents the average CO mixing ratio for each horizontal flight level in the mixing layer. The data are divided into two time periods, February and March, since the CO concentration decreased when the air mass origin changed from the Bay of Bengal to the Arabian Sea [Vetvet et al., this issue]. Although the absolute CO mixing ratios were lower in March, the latitude gradients were about the same, namely, 8.2 _+ 2.3 ppbv CO per degree latitude in February and 4.8 _+ 1.5 ppbv per degree latitude in March, indicating similar OH concentrations in these two time periods and/or similar entrainment rates. The aerosols, which are emitted over the Indian subcontinent, will decrease in concentration due to coagulation, dry deposition, cloud scavenging, and entrainment of free tropospheric air. On the other hand, the aerosol concentration in the marine boundary layer increases through the injection of seasalt aerosols or by in situ particle formation via gas to particle conversion. Because of the low wind speed observed in the similar in these two layers, we did not make this distinction in this study. The next layer, between 3.5 and 8 km altitude, showed low N... UCN, and CO concentrations, while the N 6 particle number concentration increased again with altitude. This layer showed the least variation in the aerosol data and hence apmarine boundary layer, the source of sea-salt particles is low compared to the total particle number concentration (see section 4.2). Also, the UCN particle number concentration in the mixing layer is very low compared to the prevailing aerosol number concentration (see Figure 2c), indicating that particle formation via gas to particle conversion is not an important pears to be a very stable layer, remote from or disconnected source for aerosols in the mixing layer. Hence, using the defrom pollution sources. In the remainder of this paper we will refer to this layer as the lower free troposphere. The middle free troposphere, between 8 and 12.5 km altitude, where the outflow of large convective clouds occurred, can be characterized by slightly enhanced CO and Na concentration and strongly enhanced N 6 and UCN concentracrease of the aerosol number concentration with latitude, an aerosol loss rate can be determined. To do this, the average N 6 and N c particle number concentration for each horizontal flight level in the mixing layer are calculated and plotted against latitude in Figure 3. For this analysis, three data points with very low accumulation mode tions. The mean aerosol number concentrations and CO mix- concentrations were removed, since they were directly influing ratio in the different layers together with some enced by rain. These data points are shown in Figure 3b as meteorological parameters are presented in Table 1. The en- stars. The standard deviation of the measurements correhancement in CO mixing ratio at high altitude is not apparent in this table, which is due to the large altitude range which is sponds approximately to the marker size in Figure 3. As is done for CO, the particle measurements are divided into two used to calculate the mean concentrations. time periods, February and March. Both the N 6 and N particle number concentration de Aerosol Number Concentrations: Latitude Gradients creased while moving toward the ITCZ. In February the accu- The northeast trade wind in the marine boundary layer over the northern Indian Ocean transports pollution from the Inmulation mode particle number concentration decreased with 99 _+ 19 cm -3 per degree latitude, and the submicron particle dian subcontinent over the ocean toward the Maldives islands number concentration decreased with 127 _+ 16 cm -3 per deand the measurement region. Owing to chemical reactions and gree latitude. In March the gradients were 83 _+ 21 and 125 _+

6 28,634 DE REUS ET AL.: AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN and accumulation mode particle number concentrations are shown as thin dashed lines in Figures 3a and 3b, respectively The submicron particle number concentration decreased only significantly in the first few hours of the simulation, due to coagulation of the smallest particles. During this time also the most significant change in the shape of the aerosol size distri bution took place. After this time, no strong decrease in the N particle number concentration occurred. The accumulation mode particle number concentration did not decrease signifi- O- cantly during the 5 days of simulation. Hence the observed decrease in submicron and accumulation mode particle num ber concentration with latitude cannot be entirely explained by coagulation Dry deposition of particles with a diameter larger than about 10 / m is governed by gravitational settling, while molecular diffusion and Brownian motion dominate the deposition processes of particles smaller than 0.1 / m diameter. These pro cesses are not very effective for particles between 0.1 and 1/ m diameter, and therefore the deposition velocity shows a mini- O- mum for particles in this size range. Wind tunnel measurements show that the dry deposition velocity over a water sur face is not strongly size-dependent for particles between 0.1 and 1/ m, and is estimated to be about 10-2 cm s - [Seinfeld and Pandis, 1998]. Lo et al. [1999] calculated the dry deposition velocity over an ocean surface for different wind speeds and relative humidities. They found a minimum dry deposition velocity of 10-3 cm s - for particles of about 0.08/ m diame ter, while the deposition velocity was below 10-2 cm s - for all particles between and 1 / m at low wind speeds. The observed accumulation mode aerosol loss rate over the Indian Ocean was 16 cm -3 h -. If the gradient in the particle Latitude number concentration with latitude was only caused by dry Figure 3. Latitude gradient for the (a) submicron and (b) deposition, the particle flux to the ocean surface should have accumulation mode particle number concentration, and (c) been 444 particles cm -2 s -, considering a mixed layer height CO mixing ratio. The solid symbols mark the start of the of 1 km. For the average observed particle number concentrameasurement campaign (February 1999), and the open sym- tion in the mixed layer (1000 cm -3) this would mean a dry bols mark the end (March 1999). The thin-dashed lines present deposition velocity of about 0.4 cm s -1, which is a factor 10 to the decrease in N 6 and Nac c particle number concentration by 100 higher as the dry deposition velocities discussed before. coagulation. The three points marked by stars in Figure 3b Dry deposition alone can therefore not explain the observed were occasions where rain was observed. decrease in particle number concentration with distance from the Indian subcontinent. However, it probably contributes slightly to the observed aerosol gradient in the marine bound- 22 for Nac c and N6, respectively. Hence, although the total ary layer. particle number concentration in March was lower, the latitude The most effective mechanism to remove particles with a gradients were approximately the same. The north-south wind diameter between 0.1 and 1 / m from the atmosphere is by component in the marine boundary layer is estimated to be precipitation. When air masses are exposed to precipitation, about 3.8 _+ 0.5 m s -1, corresponding to 8.1 +_ 1.1 hours per the accumulation mode decreasestrongly in concentration or degree latitude [de Gouw et al., this issue], resulting in an even disappears completely, as can be seen in Figure 3b. For average aerosol loss rate of 11 cm -3 h - for the accumulation safety reasons, we tried to avoid passing through large convecmode particles and 16 cm -3 h - for the submicron particles. tive systems during the measurement flights, and therefore Note that the transport time of an air mass in the marine only a few data points are directly influenced by rain. These boundary layer from the Indian subcontinento the ITCZ was occasions occurred close to the airport during takeoff and about 5 days. landing, when passing below or through a large convective In the following section, we compare the observed aerosol system was unavoidable. Hence the occurrence of rain events loss rates to coagulation and dry deposition rates for the ac- in the data does not depict the actual occurrence of convective cumulation and submicron particles. The change in accumula- precipitation in the measurement area. tion and submicron particle number concentration, due to co- Large convective clouds were actually very abundant over agulation, has been calculated using a coagulation model the northern Indian Ocean. Air masses coming from the Indian [Strdm et al., 1992]. This model calculates the change in the subcontinent are therefore likely to meet convective clouds initial size distribution with time due to Brownian coagulation during their transport toward the ITCZ, and when exposed to only. The model was initialized with the observed particle convective precipitation, they lose most of their accumulation number size distribution in the mixing layer (see section 3.2) mode aerosols. This reduction in accumulation mode particle and run for a time period of 5 days. The resulting submicron number concentration is, however, very localized, and after a

7 . DE REUS ET AL.: AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN 28,635 relative short time, the particle number concentration increases again due to rapid mixing with other air masses which were not in contact with the convective system. This does result in a general decrease of the accumulation particle number concentration. Convective clouds were observed over the en- tire measurement region, so the farther the air mass travels from the Indian subcontinent toward the ITCZ, the more it is likely to interact with convective clouds and the larger the decrease in the accumulation mode particle number concentration. The same yields for the submicron particle number concentration since most submicron aerosols belong to the accumulation mode size range in the mixing layer. Entrainment of clean free tropospheric air can also cause a reduction of the particle number concentration in the marine boundary layer, simply by dilution. De Gouw et al. [this issue] discussed the latitude gradient of different trace gases measured on the Citation aircraft. On the basis of the fact that no significant latitude gradient in the mixing ratio of long-lived trace gases was observed, they concluded that dilution of the boundary layer air during its transport toward the ITCZ was negligible. However, as will be further discussed in section 3.4, we believe that a combination of convective precipitation and entrainment of free tropospheric air determined the aerosol gradient over the northern Indian Ocean. Latitude gradients for the submicron and accumulation mode particle number concentration at higher altitudes were also investigated; however, no clear gradients were found at these altitudes Aerosol Size Distributions: Altitude Profile Figure 4 shows the observed aerosol size distributions for the four different layers, which are described in the previous section, with the highest layer in the top panel and the lowest layer in the bottom panel. To separate the aerosol size distributions observed in the transition layer and the lower free troposphere, the height of the transition layer on the individual flights was used, since this height varied from flight to flight. For the other layers, no large difference in the height of the layer during the individual flights was found, and the average altitude of the layer was used to divide the aerosol size distributions over the different altitude bins. From Figure 4 it becomes evident that the observed aerosol size distributions within a layer show many similarities. Although the total aerosol concentration changed between the individual size distribution measurements, the peaks in the size distribution were always located at a similar particle diameter within a single layer. All aerosol size distributions in the mixing layer showed a maximum at a particle diameter of about 0.2 tzm. However, local maxima could also be detected at about 0.05 and 1 tzm and below 0.01 tzm. Similar aerosol size distributions were observed in the transition layer, between 1 and 3.5 km altitude, although the concentration of accumulation mode particles was slightly lower than in the mixing layer. Even if the measurements presented in this paper extend to much smaller particle sizes, the observed particle size distribution for the larger particles compares reasonably well with the columnar size distributions retrieved from radiometer measurements during the winter monsoon period in 1996, when maxima in the number size distribution were found at and 1-2 tzm diameter [Moorthy et al., 1999; Satheesh et al., 1998]. Aerosol size distributions observed in the marine boundary layer over other large ocean surfaces generally show a bimodal structure, with a minimum at about 0.1 tzm diameter [e.g., Clarke et al., lo o ø lo o o 0.01 ' ' ' ' '''1 ' ' ' ' ' ''' km Op (,um) km Figure 4. Observed aerosol size distributions, subdivided into four layers: mixed layer (0-1 km), transition layer (1-3.5 km), lower free troposphere (3.5-8 km), and middle free troposphere ( km). 1998; Covert et al., 1996], which is indicative for cloud processing. The aerosol size distribution in the marine boundary layer over the northern Indian Ocean, however, does not show such a pronounced bimodal structure. This is probably due to the high pollution level of the marine boundary layer over the northern Indian Ocean, which masks the effects of rainout and cloud processing. The maximum in the aerosol size distribution shifted to smaller sizes with increasing altitude. In the lower free troposphere it was located at about 0.06 tzm diameter and in the middle free troposphere was located below 0.01 tzm diameter. The concentration of accumulation mode particles decreased with altitude; however, more accumulation mode particles were found in the middle free troposphere than in the lower free troposphere. Since the observed size distributions were so uniform within each layer, we will use the median aerosol size distribution to analyze different processes regulating the aerosol size distribution. Moreover, in order to facilitate the use of these size distributions in model studies, three to four modal lognormal distributions are fitted through the median observed size distributions. The median aerosol size distributions together with

8 28,636 DE REUS ET AL.: AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN lo km i i i i i i 11 i. 8 km km Table 2. Lognormal Fitting Parameters to the Size Distributions Shown in Figure 5 a N Dp 0-1 km km km km O athe mean mode diameter (Dp) is given in m, the total number of particles in each mode (N) is given in STP cm-3; cr is the standard deviation of the lognormal size distribution ,, ,' 0.01 ß ß ß ß ß ß ß ß ß. ß,, i i I,,, I 0.1 Dp m) 0-1 km I i i, i [ i i I [ Figure 5. Median observed aerosol size distributions in the four different layers, together with the fitted lognormal distributions. The lognormal fitting parameters are presented in Table 2. The diamonds represent the differential measurement of the two CPC instruments, the squares denote the measurements of the DMA, and the triangles are those of the OPC. diameter of 0.01/ m, hence the fitted mode at p m is only based on one data point and is therefore uncertain in place and sigma. Transferring the observed particle number size distributions into volume/mass size distributions reveals a three modal dis- tribution. The mas size distributions in all layers show a maximum for particles larger than 1 / m diameter. In addition, a maximum at about 0.08 and 0.3/ m is observed in the boundary layer and in the lower free troposphere and at 0.07 and 0.4 / m in the middle free troposphere, The total aerosol surface area, volume, and effective radius for the median size distributions presented in Figure 5 are given in Table Aerosol Size Distributions: Latitude Gradient A large change in the shape of the size distribution with latitude was not observed in the mixing layer, only a decrease in total particle number concentration. However, the ratio between Nacc and N 6 particle number concentrations increased while moving toward the ITCZ, indicating less small particles at larger distance from the Indian continent. This change, seen as a trend in the Nacc/N 6 ratio, is, however, too small to be clearly seen in the size distribution measurements. How can the aerosol size distribution show such small changes over a time period of 5 days, although the aerosols are exposed to coagulation, condensation, dry deposition, and cloud pro- cesses? the fitted curves are presented in Figure 5, and the lognormal fitting parameters are presented in Table 2. The median aerosol size distributions in the mixing and transition layer can be described with a similar four modal lognormal distribution. Only the aerosol number concentration in the different modes is lower in the transition layer. The modes have a mean mode diameter of 0.008, 0.06, 0.16, and 0.9 / m, respectively. in the lower free troposphere a three modal lognormal distribution with mean mode diameters of 0.008, 0.06, and 0.18/ m is sufficient to describe the observed aerosol Table 3. size distribution. The middle free troposphere shows an openended size distribution at the small particle sizes with its major mode at about 0.008/ m diameter. Additional modes are observed at 0.04, 0.20, and 0.90/ m diameter. Note that the first bin of the aerosol size distribution has a The effect of dry deposition on the particle size distribution is already discussed in section 3.2. Since the dry deposition velocity is very similar for the observed Aitken and accumulation mode size range [Lo et al., 1999], dry deposition will not largely change the particle size distribution in the mixing layer. To study the effect of coagulation on the observed aerosol size distribution in the mixing layer, model calculations are performed with the coagulation model which is also used in Aerosol Surface Area, Volume, and Effective Radius (Reff) for the Median Aerosol Size Distributions Presented in Figure 5 Surface Area, Volume, STP STP gm 2 cm -3 m 3 cm -3 Reft, 0-1 km km km km

9 DE REUS ET AL.: AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN 28,637 section 3.2. The model is initialized with the observed particle size distribution in the mixing layer and run for a time period of 5 days. This simulation shows that the aerosols in the Aitken mode size range rapidly coagulate with the accumulation mode aerosols in the mixing layer. Within 1 day the Aitken mode decreased in concentration by about 12%, and the mean mode diameter shifted from 0.06 txm to about 0.07 txm. The accumulation mode did not change significantly. When the model was initialized with a lower total particle number concentration, as observed closer to the ITCZ, the Aitken mode particle number concentration also decreased with about 15% in 1 day; however, the mean mode diameter of the Aitken mode remained about the same. In the previous section we showed that when an air mass is subjec to convective precipitation, the major part of the accumulation mode particles are removed, while the Aitken mode particles remain. Therefore an additional model run is performed which is initialized with the observed aerosol size dis- tribution in the mixing layer from which the accumulation mode is removed. In this simulation, coagulation did not change the Aitken mode, even after 5 days of simulation time, indicating that the self-coagulation of Aitken mode particles is very slow. In order to maintain the observed Aitken mode in the ma- rine boundary layer at a large distance from the Indian continent, air masses with low accumulation mode particle number concentration should have been abundant over the northern Indian Ocean. When such an air mass is mixed with an air mass that did not have recent contact with a convective cloud, the size distribution in the mixed air mass will include an Aitken mode, from the air mass which experienced convective precipitation, and an accumulation mode, from the air mass that did not have contact with clouds recently. At the same time, the total concentration of accumulation and Aitken mode particles will decrease, as has been shown in section 3.2. Entrainment of free tropospheric air into the boundary layer will have a similar effect on the particle size distribution since free tropospheric air contains very few accumulation mode particles. The free troposphere includes even more Aitken mode particles than the marine boundary layer, so that entrainment of a small amount of free tropospheric air already has a large effect on the observed particle size distribution in the marine boundary layer. This would also explain the similar characteristics of the Aitken mode in the boundary layer and lower free troposphere. Since a large abundance of convective precipitation is necessary to explain the stable size distribution in the mixing layer, we believe that a combination of entrainment of free tropospheric air and convective precipitation caused the decrease in particle number concentration and the uniform aerosol size distribution observed in the mixing layer. Dry deposition only contributed slightly to the decrease in particle number concen- tration. 4. Discussion 4.1. New Particle Formation in the Middle Free Troposphere In this section we will make a more detailed analysis of the high ultrafine particle number concentration observed in the layer between 8 and 12.5 km altitude. The presence of ultrafine particles is an indication for recent particle production since these particles are so small that they have to be produced in situ in the atmosphere [de Reus et al., 1999]. The formation of new particles is generally attributed to homogeneous binary nucleation of sulfuric acid and water vapor. However, evidence is growing that also ammonia plays an important role in the nucleation process [Weber et al., 1998; Korhonen et al., 1999], by either reducing the vapor pressure of the nucleatin gases or by participating in ternary nucleation of the H2SO4-NH3-H20 system. The rate of production of new particles is hence strongly dependent on the concentration of H2504, H20, and possibly NH3. The high relative humidity in the vicinity of clouds makes this region a favorable environment for new particle formation. Evidence for new particle formation near marine cumulus clouds has been observed [e.g., Hegg et al., 1990; Perry and Hobbs, 1994; Clarke et al., 1998], but also the outflow of larger convective clouds has been recently suggested to be a source of new particles [Wang et al., 2000; Clarke et al., 1999]. In section 3.1 we showed that the CO mixing ratio and the ultrafine particle number concentration exhibit an enhancement at the same altitude in the middle free troposphere. We attributed the CO enhancemento convective transport and attributed the enhancement in ultrafine particle number concentration to in situ new particle formation. The question is if the process of new particle formation is connected to the presence of convective clouds, or if it was just coincidence to find the enhancement in CO and UCN at the same altitudes over the northern Indian Ocean. Using a novel method of combined trajectory calculations and cloud top temperatures retrieved from satellite images, we have investigated if it is possible to directly couple the presence of UCN particles to the occurrence of convection. Five-day back trajectories were calculated by the Royal Netherlands Meteorological Institute (KNMI) with the TRA- JKS model, using three-dimensional ECMWF wind fields [Scheele et al., 1996], for each horizontal flight level in the middle free troposphere. Along the track of the trajectory the cloud top temperature was computed, using the infrared channel of the NOAA polar satellite. These calculations are based on Planck's radiation equation and are further explained in the Polar Orbiting Users Guide, section 3.3 ( www2.ncdc.noaa.gov/docs/podug/). Every hour the satellite image is updated, so that the retrieved cloud top temperature corresponds approximately to the time that the air mass passes over a certain location. Comparing the temperature along the trajectory with the cloud top temperature at the same position and time provides information about whether the air mass has been in contact with clouds during the last 5 days of its transport toward the measurement area. Since the free troposphere in this region with extensive convection has proven to be a difficult region for calculating back trajectories, the reliability of the trajectory is checked using the observed wind direction at the starting point of the trajectory. Only when the trajectory wind direction was within 90 ø of the average observed wind direction during the level flight is the trajectory used for the analysis below. This reliability check excluded 10% (5 out of 44) of the data. We assume that the trajectory information is reliable back to the point where convection is observed, and it is this part of the trajectory which is used in this study. For each trajectory the difference in cloud top temperature and the temperature along the trajectory is used to determine if the air mass has been in contact with clouds during the last 2 days of transport. Two days are chosen because we believe that this is the maximum lifetime of an ultrafine particle in the

10 28,638 DE REUS ET AL.: AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN o 0-12o Time (hours) Figure 6. Temperature (solid line) and cloud top temperature (gray circles) along four 5-day back trajectories. (a and b) An air mass which has not been in contact with clouds within the last 2 days. (c and d) Air masses which very likely have been in contact with clouds. middle free troposphere [de Reus et al., 2000b]. Figure 6 shows four examples of trajectory temperatures compared to cloud top temperatures along a 5-day back trajectory. Figures 6a and 6b show examples of no recent cloud contact, while the air mass in Figures 6c and 6d has been in contact with clouds recently. The occurrence of new particle formation is determined using the ratio of the UCN over the N 6 particle number concentration, indicating the percentage of ultrafine particles to the total amount of particles. This ratio is averaged over the horizontal flight level and ranged from 0 to 0.8 in the middle free troposphere. We will only consider those cases where very few or no particle formation occurs (UCN/N 6 < 0.1) or where a lot of new particles are formed (UCN/N 6 > 0.3). Since the measurement area was close to the ITCZ, lots of convection was observed, and not many level flights with a very low UCN particle number concentration were observed. There were only five flight levels in the middle free troposphere where a UCN/N 6 ratio below 0.1 was observed and all necessary data for the trajectory analysis were available. This corre- sponds to 13% of the total amount of flight levels available for this analysis. In all five cases (100%), no contact of the air mass with clouds was indicated by the trajectory temperature and cloud top temperature (as in Figures 6a and 6b). High concentrations of ultrafine particles were observed at considerably more flight levels. A ratio larger than 0.3 was found on 13 level flights (33% of the total amount of flight levels), and of these 13 level flights 11 (85%) were considered to have been in contact with clouds within the last 2 days, based on the comparison of the temperature and cloud top temperature along the trajectory (as in Figures 6c and 6d). For UCN/N 6 ratios between 0.1 and 0.3, no clear trend could be found. Although there is a large transition zone where the ultrafine particle number concentration could not be coupled to the presence of convective clouds, in the extreme situations of very low and very high ultrafine particle number concentrations, we succeeded reasonably well in predicting if the air mass had been in contact with clouds or not (100 and 85 %, respectively) Convective Transport and Aerosol Size Distribution In an attempt to link the observed particle size distributions at different altitudes, the median size distributions observed in the different layers discussed in section 3.3 are compared in Figure 7. The size distribution of the transition layer is omitted here, because it is similar to the distribution in the mixing layer. In all layers a mode with a mean mode diameter of 0.008/am was observed, the nucleation mode. This mode was largest in the middle free troposphere, where the ultrafine particles contributed significantly to the total aerosol number concentration, as discussed in the previous section. Also in the mixed layer a small nucleation mode was observed. These particles may have been formed when the accumulation mode particle number concentration was low after the air mass passed through convective precipitation. In the Aitken mode size range a mode at 0.06/am diameter could be found in the boundary layer and lower free troposphere, and a mode at 0.04/am diameter could be found in the middle free troposphere. Since the Aitken mode particle number concentration in the free troposphere is higher than in the boundary layer, these particles are not simply transported from the boundary layer. The Aitken mode particles in the middle free troposphere likely originate from the growth of ultrafine particles, which are formed by gas to particle conversion in the vicinity of large convective clouds. This growth is caused by coagulation and condensation. In the lower free troposphere the Aitken mode aerosols may have been transported over a long distance and originated far away from the measurement area. From the low variability in the aerosol and trace gas concentrations in this layer we deduce that it is an aged layer, which is not interacting strongly with the layer above or below. Large convective clouds, however, probably entrained air into this layer since shallow layers of thin clouds, mostly at about 5 km altitude, connected to large convective cells, were regularly observed over the measurement area. In the accumulation mode size range a mode was observed at 0.16/am in the boundary layer size distributions, at 0.18 in the lower free troposphere, and at 0.2/am in the middle free troposphere. The number of aerosols in this mode in the free troposphere was very small. Assuming that the air was transported upward through convective clouds, the shift in the mean mode diameter from 0.16/am in the boundary layer to 0.18/am in the lower free troposphere and 0.2/am in the middle free

11 DE REUS ET AL.: AEROSOL MEASUREMENTS OVER THE INDIAN OCEAN 28, krn E _o 10 _ krn 10 _ Dp (p,m) Figure 7. Median observed aerosol size distributions for the marine boundary layer and the lower and middle free troposphere. troposphere can be explained by cloud processing. The aerosols which were transported from the boundary layer through the convective cloud grow by the condensation of SO2 and other soluble gases on the aerosols and by scavenging of smaller particles [Svenningsson et al., 1997]. At the same time, the concentration of accumulation mode particles decreased strongly with altitude, indicating that a large part of the accumulation mode particles acted as condensatio nuclei or were scavenged by cloud droplets, and were removed during precipitation. The shift in mean mode diameter is highlighted by the gray rectangles in Figure 5. This shift is even more pronounced in the aerosol mass distributions, which are presented in section 3.3. The mean mode diameter of the accumulation mode in the aerosol mass distribution shifted from about 0.3/,m in the marine boundary layer to 0.4/,m in the middle free troposphere. In the boundary layer a small amount of coarse mode aerosols with a mean mode diameter of 0.9 /,m were observed. These could be sea-salt aerosols, formed through bursting of bubbles of air from whitecaps at the ocean surface generated at moderately and high wind speeds [O'Dowd and Smith, 1993], or aerosols produced over the continent which were transported with the trade winds toward the measurement area. Even if all these particles were sea-salt aerosols, their concentration was very low, which might be due to the low wind speed prevailing in the marine boundary layer during the INDOEX experiment (on average, 5 m s- ). The ocean is hence not a significant source for the number concentration of aerosols over the northern Indian Ocean during this season, in comparison with the number of aerosols transported over the ocean by the trade winds. A coarse mode was also present at higher altitudes. If we assume that these particles were transported to the middle fre'e troposphere through convection, the aerosols either result from evaporated cloud droplets or were less hygroscopic aero- sols which did not act as condensation nuclei and are trans- ported unchanged through the convective cloud. 5. Summary and Conclusions Horizontal and vertical distributions of the aerosol number concentration and size distribution and CO mixing ratio over the northern Indian Ocean are presented. These data were obtained during 23 research flights with the Cessna Citation aircraft during the INDOEX intensive field phase in February- March The observed vertical profiles of the aerosol number concentration for different particle size ranges and the CO mixing ratio observed on the individual flights show many similarities over the entire measurement region and time pe- riod. High CO and aerosol number concentrations were observed in the lowest 4 km of the atmosphere over the northern Indian Ocean. This is a result of transport of anthropogenic pollution, mainly originating from the Indian subcontinent, with the northeasterly trade winds. A gradual decrease in submicron and accumulation mode particle number concentration was observed in the marine boundary layer over the northern Indian Ocean with distance from the Indian subcontinent. This decrease could not be explained by coagulation or dry deposition, but was likely to be due to the mixing of air which has been affected by rain and air which has not been affected by rain recently, and by the entrainment of free tropospheric air. This process can also explain why the size distribution did not change significantly with distance from the continent. How the aerosol is able to retain the shape in the size distribution even after 5 days is not entirely clear. However, dilution of the boundary layer air by entrainment of air from the free troposphere may be larger than inferred from the trace gas measurements. Between 4 and 8 km altitude a layer with very low variability in aerosol and trace gas concentrations was observed, indicating an aged air mass. The aerosols observed in this layer might have their origin far away from the measurement area. Above 8 km altitude an enhancement in the CO mixing ratio is observed, indicating convective transport from the boundary layer. The submicron particle number concentration shows an enhancement at the same altitudes; however, the particle number concentration in the middle free troposphere is much higher than in the boundary layer. Convective transport of boundary layer air can therefore not explain the major part of the submicron aerosols observed in the middle free troposphere. Ultrafine particles were almost only observed in the middle free troposphere at the same altitude as the enhance-

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