Airborne observations of the effect of a cold front on the aerosol particle size distribution and new particle formation

Transcription

1 Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: , April 21 Part B Airborne observations of the effect of a cold front on the aerosol particle size distribution and new particle formation Justin R. Peter, a * Steven T. Siems, a Jørgen B. Jensen, b John L. Gras, c Yutaka Ishizaka d and Jörg M. Hacker e a School of Mathematical Sciences, Monash University, Clayton, Victoria, Australia b Earth Observing Laboratory, National Center for Atmospheric Research, USA c CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia d Hydrospheric Atmospheric Research Center (HyARC), Nagoya University, Japan e Flinders University of South Australia/Airborne Research Australia, Adelaide, South Australia *Correspondence to: Justin R. Peter, School of Mathematical Sciences, Monash University, Clayton, VIC, 38, Australia. justin.peter@sci.monash.edu.au The contribution of J. B. Jensen to this article was prepared as part of his official duties as a United States Federal Government employee. Airborne measurements of condensation nuclei (CN), aerosol particles (APs) and sulphur dioxide (SO 2 )weremadenearacoldfronttoexaminenewparticleformation and the effects of the front on CN and SO 2 concentrations and the AP size distribution and concentration. Measurements were made in the boundary layer (BL) and free troposphere (FT) both preceding and following the passage of the front. Statistical analyses of CN and AP concentrations in the air masses around the front show that new particle formation was prevalent in the pre-frontal and post-frontal FT, however the post-frontal FT contained higher concentrations of ultrafine condensation nuclei (UCN). Mixing diagrams of conserved thermodynamic quantities, total water content and wet equivalent potential temperature were constructed for horizontal aircraft transects in the pre-frontal and post-frontal FT regions; they show that many of the fluctuations in UCN concentrations can be explained by mixing of air masses with differing initial concentrations of UCN. However, regions of enhanced UCN were found to be associated with mixing between air masses of distinct thermodynamic properties, suggesting that mixing between air masses with large gradients in temperature and relative humidity may have promoted new particle formation. Furthermore, analysis of fluctuations in UCN concentrations during vertical soundings show that low pre-existing AP surface area was not a necessary requirement for the production of new particles but, rather, turbulent mixing was a major mechanism for new particle production. Observations also suggest that species other than SO 2 were required for new particle production to occur. Copyright c 21 Royal Meteorological Society Key Words: cloud-processing of aerosol; ultrafine condensation nuclei; extratropical cyclones Received 1 December 28; Revised 4 September 29; Accepted 14 September 29; Published online in Wiley InterScience 28 April 21 Citation: Peter JR, Siems ST, Jensen JB, Gras JL, Ishizaka Y, Hacker JM. 21. Airborne observations of the effect of a cold front on the aerosol particle size distribution and new particle formation. Q. J. R. Meteorol. Soc. 136: DOI:1.12/qj.515 Copyright c 21 Royal Meteorological Society

2 Processing of Aerosol by a Cold Front Introduction Atmospheric aerosol particles (APs) modify the planetary albedo, and hence climate, via the following mechanisms: (1) by altering short-wave light scattering and absorption (Charlson et al., 1992), and (2) by acting as cloud condensation nuclei (CCN), thereby altering the radiative properties and lifetime of clouds (Charlson et al., 1987; Albrecht, 1989). These effects of APs on climate, known as the direct and indirect effects respectively, are influenced by the AP size distribution and composition, which are, in turn, determined by AP sources and sinks. One possible source of condensation nuclei (CN) is new particle formation from gaseous precursor species. Formation of CN produces very little AP mass but a large number of particles, significantly altering the shape of the AP size distribution. Due to the importance of CN formation, many observational (Weber et al., 1999, 21; Birmili et al., 2) and numerical (Kulmala et al., 1995; Clarke et al., 1999a; Korhonenet al., 1999) studieshavebeen undertaken to assess the major mechanisms responsible for their generation, the exact details of which are still uncertain. Previous studies have found high levels of sulphuric acid (H 2 SO 4 ) in association with CN formation, suggesting that binary homogeneous nucleation of sulphuric acid and water (H 2 O) and subsequent condensation are the main source of particles in the atmosphere (e.g. Clarke et al., 1999a). However, observations of CN formation rates cannot be reconciled with theoretical models of H 2 SO 4 H 2 Obinary nucleation (Birmili and Wiedensohler, 2), because the measured H 2 SO 4 concentration is generally insufficient (Weber et al., 1999). Nucleation involving other precursor species, such as ammonia (NH 3 ), has been proposed (Coffman and Hegg, 1995; Korhonen et al., 1999) and shown to increase the rate of CN formation (Ball et al., 1999), especially during enhanced solar radiation (Birmili et al., 2). Several recent theoretical (Kulmala et al., 24, 26a) and laboratory (Kulmala et al., 26b) studies have proposed that soluble inorganic compounds play a second-order role in new particle formation and that waterinsoluble organic vapours contribute significantly to new particle formation. Another mechanism proposed to enhance CN formation is due to small fluctuations in temperature and relative humidity (RH). Such fluctuations can be generated under favourable atmospheric conditions such as breaking Kelvin Helmholtz waves (Bigg, 1997) or mixing processes in the atmosphere (Nilsson and Kulmala, 1998). Easter and Peters (1994) showed that a change in temperature of 2 3 C or 6 8% in RH could result in a factor 1 increase in the nucleation rate. Schröder and Ström (1997) observed frequent upper-tropospheric particle production in the vicinity of a cold front, which they attributed to dynamically induced mixing processes. Ion-induced nucleation has also been proposed as a mechanism for new particle formation (Yu and Turco, 2; Laakso et al., 22; Lee et al., 24). The CN formation rate predicted by the above mechanisms is dependent on several physical parameters: particulate surface area, precursor-gas concentrations, temperature, water vapour pressure and H 2 SO 4 vapour pressure. In the boundary layer (BL), Covert et al. (1992) attributed formation of CN to low ( 5 µm 2 cm 3 ) ambient AP surface area. In the free troposphere (FT), Clarke et al. (1998) observed particle production in the outflow of clouds when the AP surface area decreased to about 5 1 µm 2 cm 3. The decreased AP surface area was attributed to cloud processing (rain-out) of the APs, and was considered a necessary prerequisite to new particle formation (Perry and Hobbs, 1994). Twohy et al. (22) and Clement et al. (22a) observed extremely high concentrations of new particles in the outflow of a deep convective system, and concluded that cloud outflow regions could be a major source of new particles in the troposphere. Although the decreased AP surface area was attributed as the major parameter determining whether CN formation would occur, increased concentrations of precursor gases lofted from the BL to the upper troposphere were also hypothesised as a requirement for the presence of CN in cloud outflow. Enhanced actinic fluxes above clouds have been found to induce increased hydroxyl radical (OH) concentration (Mauldin et al., 1997) which will subsequently oxidise with SO 2 to form H 2 SO 4, which in turn condenses to form CN. Concomitant with other favourable conditions for CN formation (low surface area, high RH and low temperatures), the enhanced actinic flux above cloud results in favourable conditions for particle nucleation (Weber et al., 21). Hegg (1991) also observed that the increased enhanced actinic flux within cloud (Madronich, 1987) may be responsible for CN formation in the interior of clouds. Kulmala et al. (26b), however, showed via theoretical calculations and laboratory studies that the presence of new particles in cloud interiors did not originate from sulphuric acid nucleation but, rather, from insoluble organic vapours. In this article, we report airborne observations of AP size distributions and CN formation measured during the Asian Aerosol Characterization Experiment (ACE Asia). The observations are novel because a comparison of two research flights is made: a research flight preceding the cold front of an extratropical cyclone and one behind the cold front. The daily weather patterns throughout the middle and high latitudes are closely related to the passage of extratropical cyclones, consequently they have major implications for the formation, modification and removal processes of the ubiquitous AP populations at these latitudes. The air masses sampled are interpreted in terms of the conveyor-belt model of a front, to identify input and output airflow from the frontal clouds enabling us to determine the effect of a cold front on CN concentrations. Stability and turbulence parameters are examined to investigate possible enhancement of nucleation due to mixing processes, as well as correlations with ambient particle surface area, SO 2 and RH. The intentions of this paper are: (1) to present statistics of CN concentrations in the BL and FT of pre- and postfrontal regions, and (2) to examine mechanisms responsible for new particle formation. 2. Instrumentation The platform used for the Australian contribution to ACE Asia was the Airborne Research Australia (ARA) Beech B2T Super King Air. The aircraft was equipped with an extensive suite of instrumentation capable of measuring thermodynamics, three-dimensional wind components, Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

3 946 J. R. Peter et al. cloud and aerosol microphysics, radiation fluxes and trace gases. CN were sampled with an isokinetic inlet, and counted with separate CN counters able to resolve particles of two different size ranges; a TSI-325 resolved CN with a radius, r p 1.3 nm and a TSI-31 resolved CN with a radius, r p 6. nm. The minimum detectable radius for the TSI-325 (1.3 nm) is marginally smaller than the usual minimum quoted in the literature of 1.5 nm. This value was determined after intercomparison of several TSI-325 CN counters preceding the ACE Asia experiment (John Gras, personal communication). Larger AP were measured using a Particle Measuring Systems (PMS) Active Scattering Aerosol Spectrometer Probe (ASASP). The ASASP can measure AP in the radius range µm covered by 15 size bins; it has a heated inlet that dries the AP before size measurement. CN and ASASP measurements presented here are all taken in clear air, where clear air is defined to have a liquid water content (LWC), q l 1 3 gkg 1 as measured by a Commonwealth Scientific and Industrial Research Organisation (CSIRO) hot-wire probe (King et al., 1978). Exclusion of saturated regions ensures that dry, rather than ambient, AP size distributions are reported (Strapp et al., 1992) and that droplet break-up in cloudy regions did not result in spurious CN counts (Weber et al., 1998). The first and fifth size bins of the ASASP were subject to altitude-dependent detuning problems above approximately 4 m above sea level (asl). AP spectra sampled above 4 m asl have been modified by excluding the first bin and linearly interpolating the fifth size bin between the fourth and sixth size bins. Cloud droplets were measured with a PMS Forward Scattering Spectrometer Probe (FSSP- 1) capable of sizing and counting droplets in the radius range µm in 22 size bins. A PMS 2D-C optical array probe was used to measure droplet spectra in the radius range µm in 8 size bins. Temperature was measured with a Rosemount temperature sensor, wet-bulb temperature with a CSIRO wet bulb sensor. Liquid water content (LWC) was measured using the FSSP-1 and a CSIRO hot-wire probe. All thermodynamic measurements and ASASP data are 1 Hz averages of 64 Hz data, while the FSSP and 2D-C measurements are 1 Hz averages of 4 Hz data. The total concentration measured by the CN counters is referred to by their lower limit of size detection, N 1.3 for the TSI-325 and N 6 for the TSI-31. The concentration difference, N 1.3 N 6, is an indicator of nucleation of CN (e.g. O Dowd et al., 1998) and will be referred to as Ultrafine Condensation Nuclei (UCN) (sometimes also called the nucleation mode ). In addition, another parameter used as an indicator of new particle formation is the ratio UCN/N 6 (Warren and Seinfeld, 1985; Covert et al., 1992; Schröder and Ström, 1997; Young et al., 27). 3. Synoptic overview 3.1. Flight paths On 24 April 21, research flights 1424a and 1424b were conducted. Flight 1424a was conducted in the morning/early afternoon in pre-frontal air ahead of a surface cold front, while flight 1424b was flown in cold subsiding post-frontal air during the afternoon/early evening. Flights 1424a and 1424b will hereafter be referred to as the pre-frontal and post-frontal flights, respectively. A mean sea level pressure (MSLP) map (not shown) and satellite imagery (Figure 1) from 24 April 21, indicated the presence of an extratropical cyclone to the southwest of Kyushu. The visible spectrum satellite image of the frontal synoptic situation, shown in Figure 1 (obtained using the GMS-5/SVISSR satellite) was obtained at 1324 Japanese Standard Time (JST = UTC+9 h). The flight tracks for the pre-frontal and post-frontal flights are shown in Figure 2. Both flights were undertaken at about the same latitude and longitude, but advection of the front during the day resulted in the morning flight being flown in the pre-frontal air mass and the afternoon flight in the post-frontal air. The MSLP indicated that the front was advecting northeasterly at approximately 45 km hr 1. The research components of both flights are projected onto a vertical plane (in the horizontal flight direction) and shown in Figures 3 and 4 for the pre-frontal and post-frontal flights, respectively. Both flights consisted of upper-level legs conducted in the FT, followed by a descent sounding and then legs flown in the BL Vertical structure of the atmosphere Pre-frontal flight The King Air completed five legs in the upper troposphere, descending from 85 m to 65 m (Figure 3), to examine the upper-level outflow of the frontal clouds. A descent sounding was then made to approximately 1 m asl, during which time a thick (6 m) layer of stratocumulus was encountered, extending from 14 to 2 m. Following the sounding, a series of level legs were made below cloud base at 1 m and 5 m asl. At 1 km asl, convective clouds located east of the stratocumulus, were encountered. Finally, a level leg 15 m asl westward behind the frontal boundary was flown. During the penetration of the surface cold front, the forward-looking video revealed a ceasing of precipitation and clearing of the sky with cumulus in the distance, typical of a post-frontal BL (Cotton and Anthes, 199). The pre-frontal sounding (Figure 5), obtained during descent, reveals a mixed layer below 1 km asl and stable inversions located at 2 km and 3.8 km asl. The stratocumulus layer was confined below the inversion at 2 km, below which the sounding exhibits a humid structure, typical of warm fronts in the Japan region (Kurihara et al., 21). The presence of warm stable layers above cold air indicates a warm front located at 2 km asl. Appealing to the conveyorbelt model of a cold front, the upper-level legs and the sounding above 2 km were necessarily conducted in the warm conveyor belt (Houze, 1993). Furthermore, at higher altitudes the air consisted of air originating from the warm and dry conveyor belts. The height of the transition from the BL to the FT in the pre-frontal air mass as determined from the sounding (Figure 5) was also located at about 2 km. At this altitude there was a significant increase in temperature (about 1.5 C)andchangeofwinddirection, signifying a transition between different conveyor belts of the front. This transition is identified as the transition from the warm/dry conveyor-belt air to the underlying colder air of the cold conveyor belt. The height of the warm front and the direction of flow of the warm and dry conveyor belts are shown in Figure 3. 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4 14 E Processing of Aerosol by a Cold Front N 12 E 13 E 3 N Frontal boundary Figure 1. Visible satellite image at 424 UTC (1324 JST). A low pressure cell (and area of cyclogenesis) is located to the southwest of Kyushu. The boundary of the surface cold front is indicated, and outflowfrom the frontal clouds is seen extending eastward. The timing of the image is 1h after the research component of 1424a (pre-frontal flight) and 3 h before the research component of 1424b (post-frontal flight) Leg NW Dry conveyor belt Direction of motion of warm conveyor belt Leg 6 Leg 7 Leg 8 Leg 9 Leg 2 Leg 3 Leg 4 Leg 5 Sounding SE Figure 2. Flight tracks for 1424a (pre-frontal flight; solid line) and 1424b (post-frontal flights; dashed line). The relative positions of the surface cold and warm fronts during each flight are also shown Post-frontal flight Figure 3. Flight track for 1424a (pre-frontal flight) projected onto a vertical plane oriented along the horizontal flight direction, which is approximately northwest southeast (Figure 2). The leg numbers are labelled in the order undertaken from the first (Leg 1) to last (Leg 9). Leg numbers 1 5 were decreasing in altitude, but all were flown in the free troposphere. Legs 6 9 were flown in the boundary layer. The trajectories of the warm and dry conveyor belts, according to the Norwegian Conveyor Belt Model, are also shown. The position of the warm front was determined from the sounding and is indicated. Legs 1 5 were flown in the outflow of frontal clouds, which comprised air from the warm and dry conveyor belts. The afternoon flight was flown in the post-frontal air mass behind the surface cold front. The descent sounding (Figure 5) indicates the height of the BL extended to about 5 m at which point the wind shifts from easterly to near westerly which delineates the transition from the BL to the FT. Also evident is a strong temperature inversion located at about 2.2 km, which separates relatively warm dry air from cold dry air below, indicating where the aircraft penetrated the top of the surface cold front. Since the dry conveyor belt originates in the FT and passes over the surface cold and warm fronts, air above the inversion is most likely within the dry conveyor belt. Due to icing of the wetbulb temperature sensor, the sounding is unreliable below (above) 6 hpa (4 km). At an altitude of about 45 m, the aircraft penetrated deep convective clouds generated by convergence at the surface cold front. These clouds were precipitating heavily, typical of narrow cold-frontal rain bands. Approximately 25 km behind the surface cold front, a series of legs were flown within and below some lightly precipitating stratocumulus. The position of the cold front Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

5 948 J. R. Peter et al. Leg 1 of the warm conveyor belt. The two trajectories that start to the west of the end location are located above (at their end times) the height of the surface cold front and, therefore, are contained within the dry conveyor belt. 4. Observations W Dry conveyor belt Leg 2 Leg 3 Sounding Leg 6 Leg 4 Leg 5 Leg Figure 4. Flight track for 1424b (post-frontal flight) projected onto a vertical plane oriented along the horizontal flight direction, which is approximately west east (Figure 2). The leg numbers are labelled from highest in altitude (Leg 1) to lowest (Leg 7). The position of the cold front as determined from the sounding is indicated. The direction of the dry conveyor belt is also shown. The post-frontal flight was mostly undertaken in cold subsiding air behind the surface cold front and in the dry conveyor belt above the surface cold front. and the direction of motion of the dry conveyor belt are shown in Figure Back trajectories Back trajectories showing the history of air masses during the 72 h prior to the research flights, obtained using HYSPLIT (Draxler and Rolph, 1997), are shown in Figure 6. The meteorological data (Final Global Data Assimilation System run FNL) are on a horizontal grid of km and a vertical grid comprising 13 levels spaced from the surface to 2 hpa (Stunder, 1997). Although the resolution of the back trajectories is too coarse to explicitly resolve the cold front, they do reveal information about its air mass history and general conveyor-belt structure. The trajectories illustrate that air arriving at the sampling location had passed over densely populated areas of Japan and southwestern China. Therefore, the air masses were most likely influenced by anthropogenic emissions. Also the trajectory ending at 3 km agl had undergone substantial uplift during the previous 24 h due to the presence of the front. Upper-tropospheric air advected from northern India, maintaining a relatively constant altitude along its trajectory. Air at 1 km altitude had spent the previous 48 h within 1 km asl suggesting that air at this altitude acquired a marine component, subsequent to a continental influence obtained over Japan. The back trajectories are consistent with the wind directions shown in the soundings (Figure 5), and also illustrate the history of the dry and warm conveyor belts. The trajectory that originates to the east of the end location moves anticyclonically ahead of the approximate position of the surface cold front and descends for most of its history; however, in the final 3 h it undergoes substantial uplift, which indicates that this trajectory is representative E 4.1. Statistical properties of aerosol particles in the FT and BL Figures 7 and 8 are plots of the observed UCN (N 1.3 N 6 ) concentration with respect to the N 6 concentration; each point represents a 1 Hz sample during a horizontal leg in the BL or FT. The statistical properties of N 1.3, N 6,UCN and UCN/N 6, averaged over a flight leg, for the pre-frontal and post-frontal flights are summarized in Tables I and II, respectively. In the pre-frontal BL, N 6 concentrations range from about 8 to 35 cm 3 and average N 6 12 cm 3.TheUCN concentration ranges two orders of magnitude from about 1 to 1 cm 3,withanaverageN 6 36 cm 3.Inthe pre-frontal FT, N 6 concentrations span 1 3 cm 3 with an average N 6 13 cm 3, and UCN concentrations from about 1 to 6 cm 3, with an average of about 14 cm 3. The UCN/N 6 ratio was largest in the FT (mean 1.26) and smaller in the BL at.29. In the post-frontal BL, average N 6 values were about 132 cm 3, UCN concentrations averaged about 19 cm 3, and UCN/N 6 was.15. The UCN concentration and UCN/N 6 ratio in the post-frontal BL are about half their respective values in the pre-frontal BL. Substantial differences are also apparent between flights in the FT. In the post-frontal FT, N 6 and UCN concentrations show less variability and are about double the values for the pre-frontal flight; the FT UCN/N 6 ratio is lower, at approximately.9. The N 1.3 and N 6 BL concentrations agree within 1% between the pre-frontal and post-frontal air masses, which suggests that air in the warm conveyor belt and in the cold subsiding air behind the cold front have similar chemical sources. This is consistent with the back trajectories (Figure 6), which indicated that BL air, ahead of and behind the surface cold front, most likely contained an anthropogenic influence from Japan. In contrast, the postfrontal FT, which contains air located predominantly in the dry conveyor belt, has marked different AP properties from the pre-frontal FT. The pre-frontal FT contains a mixture of air from the dry and warm conveyor belts; air in the dry conveyor belt has passed over the surface cold front and mixed with air from the warm conveyor belt forced upwards by convergence at the leading edge of the surface cold front. The pre-frontal FT air has therefore been processed by clouds associated with the surface cold front. The postfrontal FT, unperturbed by convection, shows less variability in both N 6 and UCN concentrations than the pre-frontal BL, suggesting that mixing of air parcels has created regions both conducive and inhibitive for new particle formation in the FT. The UCN and N 6 concentrations are larger in the post-frontal FT which implies that frontal clouds are a net sink for UCN and N 6. However, the UCN/N 6 ratio is larger in the pre-frontal FT which has two explanations: (1) the pre-frontal FT is more conducive to UCN production than the post-frontal FT, which will increase the numerator in the UCN/N 6 ratio or, (2) precipitation scavenging of aerosol particles by the frontal clouds will preferentially scavenge Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

6 Processing of Aerosol by a Cold Front 949 Table I. Statistical properties of nucleation variables during the pre-frontal flight averaged over each leg as labelled in Figure 3. Leg number N 1.3 (cm 3 ) N 6 (cm 3 ) N 1.3 N 6 (cm 3 ) ( ) N1.3 N 6 N 6 Free troposphere ± 85 1 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 9 71 ± ± ± 1.31 Mean 276 ± ± ± ±.92 Boundary layer ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 3.23 ±.13 Mean 152 ± ± ± ±.23 Table II. As Table I, but for the post-frontal flight averaged over each leg as labelled in Figure 4. Leg number N 1.3 (cm 3 ) N 6 (cm 3 ) N 1.3 N 6 (cm 3 ) ( ) N1.3 N 6 N 6 Free troposphere 1 94 ± ± ± ±.12 Boundary layer ± ± ± ± ± ± ± ±.7 Mean 1515 ± ± ± ±.9 larger particles since it is the larger particles which activate to become cloud droplets, which will decrease the value of N 6 in the pre-frontal FT compared to the post-frontal FT. These observations do not preclude that UCN were recently formed near the frontal clouds (Weber et al., 21), only that scavenging of UCN was greater than their rate of formation. In summary, the following conclusions are evident from the statistical analysis of nucleation variables: (1) new particle formation, as measured by the difference of two CN counters operating at different minimum threshold radii, is most prevalent in the post-frontal FT, however the pre-frontal BL is more conducive to new particle formation than the post-frontal BL; (2) The UCN concentration in the pre-frontal FT was found to be about 14 cm 3 which was substantially smaller than in the post-frontal FT (about 44 cm 3 ) because UCN have been scavenged by frontal clouds; (3) The UCN/N 6 ratio is larger in the pre-frontal FT (1.26) than in the post-frontal FT (.15) which could have two explanations: UCN production is greater in the pre-frontal FT as compared with the post-frontal FT or, the frontal clouds have scavenged AP with radius larger than 6 nm preferentially over UCN Horizontal profiles We now focus on fluctuations in the UCN concentration for each of the horizontal legs. Figure 9 shows measured UCN, RH and temperature, wet equivalent potential temperature, sulphur dioxide and AP surface area (as measured by the ASASP). Wet equivalent potential temperature (θ q ) was defined as in Pointin (1984), which is an extension of the formulation employed by Paluch (1979) such that it is valid for saturated and sub-saturated air parcels. The following criteria have been been applied to the data: (1) LWC content, as measured by the CSIRO King probe, was less than gkg 1 ;(2)TheratioUCN/N 6 was greater than unity. The first condition is to prevent contributions to particle counts during cloud penetrations. The second criterion has been used in previous studies to identify regions of new particle formation in the upper troposphere (e.g. Schröder and Ström, 1997; Lee et al., 24; Young et al., 27). Note that criterion (2) means that samples shown in Figure 9 correspond to all points above the one-to-one line shown in Figure 7. Wet equivalent potential temperature is conserved for reversible adiabatic motions and provides a measure of the combined effects of fluctuations in water vapour content and temperature. The dew-point temperature sensor was subject to freezing during penetration of cloud during Leg 5 which resulted in unreliable humidity measurements. Therefore, temperature, RH and θ q are not shown for Leg 5. Regions of new particle formation, as defined by the above criteria, are evident throughout much of the horizontal extent of all legs. The UCN concentrations are relatively low compared to other studies made in the upper troposphere (e.g. Hermann et al., 23; Lee et al., 24; Young et al., 27), but are still significant in both number concentration and spatial extent, indicating the importance of the upper Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

7 95 J. R. Peter et al. Figure 5. Skew T log p plot of the soundings from the pre-frontal and post-frontal flights. Due to icing of the wet-bulb temperature sensor, the post-frontal sounding is inaccurate below 6 hpa (above 4 km). troposphere as a region conducive to new particle formation. Figure 1 shows the same measured quantities as Figure 9 but for the post-frontal flight. Again, new particle formation is prevalent throughout the horizontal extent of Leg 1, however, consistent with the statistical analysis of section 4.1, UCN concentrations are appreciably larger than measured during the pre-frontal flight. A comparison between the pre-frontal and post-frontal flights reveals information about the mechanisms that are promoting the formation of new particles. Sulphur dioxide concentrations are larger in the pre-frontal region, indicative of frontal convection lofting precursor gases from the BL to the upper troposphere. Despite larger SO 2 concentrations in the pre-frontal FT, UCN concentrations are lower than recorded during the post-frontal flight. Additionally, during the pre-frontal flight, SO 2 concentrations exhibit a general increase with decreasing altitude. All of these legs were flown in the outflow of the frontal clouds, so the increase of SO 2 with a decrease in height observed during the prefrontal flight is consistent with air parcels having spent decreasing amounts of time within the frontal rain clouds and undergone less aqueous scavenging. Despite the increase of SO 2 concentration with a decrease in altitude during the pre-frontal flight, there is no associated increase in the UCN concentration as the SO 2 concentration increases. Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

8 Processing of Aerosol by a Cold Front Pre-frontal free troposphere [N 1.3 ]-[N 6 ] (cm -3 ) [N 6 ] (cm -3 ) 1 4 Pre-frontal boundary layer [N 1.3 ]-[N 6 ] (cm -3 ) Figure 6. Back trajectories extending from 31. N, 129. E and ending at 18 JST. Final altitudes are 15 m, 3 m and 45 m. The coordinates and ending time are representative of the post-frontal air mass. The AP surface area, as measured by the ASASP, is shown in Figures 9(e) and 1(e) for the pre-frontal and post-frontal flights respectively. The total surface area is significantly larger during the pre-frontal flight, attaining values approaching 4 µm 2 cm 3,whereastheAPsurface area is an order of magnitude smaller during the post-frontal flight, remaining below 4 µm 2 cm 3.Thisisconsistentwith vertical transport of AP from the BL to the FT in the frontal clouds. The smaller AP surface area in the post-frontal FT region of the front may be a contributing factor to the elevated UCN concentrations compared to those measured in the pre-frontal FT. However, it is interesting that during the pre-frontal flight, the observed UCN concentrations do not immediately appear to be inversely correlated with fluctuations in total AP surface area. There are periods, evident during Leg 1 and Leg 4 where the UCN concentration is lower when the AP surface area reaches a maximum, however this is not obvious throughout the complete horizontal extent of each leg, or indeed all legs. For instance in Leg 2, at a horizontal distance of approximately 15 2 km and again between 3 and 4 km, both UCN and AP surface area attain near-maximum values. We now explore the possibility that fluctuations in RH and temperature are responsible for much of the observed new particle formation in the upper troposphere. During both flights, fluctuations in temperature of a few degrees K and large gradients in RH of up to 4% are present. The combined contribution of changes in RH and temperature lead to changes of up to 4 K in the traces of θ q. Figure 11 is a mixing diagram of θ q versus total water mixing ratio Q tot, for the pre-frontal (Figure 11) and post-frontal flights (Figure 11). These diagrams were [N 6 ] (cm -3 ) Figure 7. The concentration of ultrafine particle concentration (UCN) as a function of the condensation nuclei concentration with a radius, r p 6nm (N 6 ) observed during the horizontal legs of the pre-frontal flight in the free troposphere and the boundary layer. first utilised to investigate the entrainment process in cumulus clouds (Paluch, 1979; Jensen et al., 1985; Blyth et al., 1988). Since θ q is conserved for reversible adiabatic motions and total water mixing ratio is conserved if there is no precipitation, the mixing diagram can be used to identify the sources of entrained air in cumulus clouds. When used in this capacity, a clear-air sounding in the unperturbed environment near cloud, and the thermodynamic state of cloud base are also required. However, in the present case, since the frontal clouds were precipitating heavily we are unable to use the mixing diagram to evaluate how much of the air detrained from the frontal clouds has originated from the BL. We can, however, use the diagram to provide information about the thermodynamic properties and mixing processes occurring within and between detrainment regions from the frontal clouds. It is evident from Figure 11 that the thermodynamic characteristics along each horizontal leg (each at a nearconstant altitude) exhibit linear mixing when plotted on the mixing diagram. Clement et al. (22a) observed similar linear mixing patterns in the outflow of a mesoscale convective system over the continental United States, however they used gas phase species (CO 2 and NO y )as conserved tracers. The colour coding used for each level leg in Figure 11 is identical to that used in Figure 9. Several conclusions can be drawn from this mixing diagram. In general, there is an increase in water vapour Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

9 952 J. R. Peter et al. [N 1.3 ]-[N 6 ] (cm -3 ) [N 1.3 ]-[N 6 ] (cm -3 ) Post-frontal free troposphere [N 6 ] (cm -3 ) Post-frontal boundary layer [N 6 ] (cm -3 ) Figure 8. As Figure 7, but for the post-frontal flight. mixing ratio as altitude decreases. The mixing line for Leg 1 shows two distinct mixing lines, one portion that exhibits an increasing water vapour mixing ratio as θ q increases, and another portion where water vapour mixing ratio decreases as θ q increases. Leg 1 was undertaken at about 82 m, which is near the height of the thermal tropopause in midlatitude regions. It is therefore probable that the portion of the mixing line that exhibits a decrease of water vapour mixing ratio as θ q increases is a consequence of tropopause folding near the extratropical cyclone. Ozone measurements would help validate this observation, however, no ozone measurements were made. Nevertheless, although Leg 1 was flown at a near-constant altitude, air masses with quite distinct thermodynamic properties were encountered. The mixing lines of Leg 2 and Leg 3 are connected, which indicates that, despite the legs being separated by several hundred metres, mixing is occurring. This is a clear indication that the layers are coupled and that vertical mixing is occurring between layers in the outflow of the frontal clouds. The mixing line of Leg 4 is separate from the upper-level legs which indicates that it was undertaken at an altitude unaffected by vertical mixing. It is possible that much of the variation in UCN concentration during each leg is due to mixing between air masses with different particle concentrations (Clement et al., 22a). To assess this possibility, we evaluated the minimum and maximum (θ q, Q) pair for each mixing line and (nominally) assigned the maximum (θ q, Q) value as containing a fraction of air F equal to one. Similarly, the minimum (θ q, Q) wasgivenanf fractionequaltozero.in this manner, we can determine the degree of mixing for each (θ q, Q) pair along the mixing line by (Jensen et al., 1985) F 1 ( θ q θ q, + Q Q ), (1) 2 θ q,1 θ q, Q 1 Q wherethesubscriptsand1correspondtoanf-fraction of and 1, or in other words, the minimum and maximum (θ q, Q) pair, respectively. Therefore, if the fluctuations in the UCN concentration for each leg can be described solely by mixing of air masses with different particle concentrations, then the UCN concentration, as a function of the F- fraction, will also exhibit a linear relationship. Figure 12 shows plots of the UCN concentration against the F- fraction, as defined in Eq. (1). There is a general linear trend observed during all pre-frontal legs, suggesting that some of the observed fluctuations in UCN concentration were due to mixing between air masses with different initial UCN concentrations. For all of the pre-frontal legs, the trend is towards increasing UCN concentrations with decreasing F-fraction. Recalling that an F-fraction of zero corresponds with the smallest (θ q, Q) pair on the mixing diagram (Figure 11), then Figure 12 suggests that most UCN are produced in regions of lowest temperature and water mixing ratio. Theoretical parametrisations of new particle formation predict the nucleation rate to be a nonlinear, and particularly sensitive, function of temperature and RH (Vehkamäki et al., 22; Spracklen et al., 25). For instance, the Vehkamäki et al. (22) parametrisation predicts that a temperature decrease of 5K, or an RH increase of 2%, will result in an order of magnitude increase in the nucleation rate. Our observations (Figure 12) are consistent with particle nucleation being favoured in regions of lower temperature. However, there are many departures from linearity indicative of processes other than linear mixing being responsible for the fluctuations in UCN concentration. Of interest is the large change in UCN concentration over a narrow range of F-fraction (F.6) during Leg 1, which suggests that a localised mixing event has resulted in parcels which are more conducive to the formation of new particles. We now identify where, along the extent of Leg 1, this mixing event has occurred. Figure 13 summarises the information from Figures 9, 11 and 12 for Leg 1; the separate mixing lines have been distinguished with different colouring. It is apparent that the large range of UCN concentration over the narrow F-fraction range occurred where the dry intrusion was encountered. This is evidence that mixing of air, with different thermodynamic properties has been responsible for the burst of new particles encountered at a horizontal distance of 6 7 km during Leg 1. Similar bursts of new particle formation have recently been observed by Young et al. (27). In conclusion, new particle formation in clear air, as defined by the criteria that the ratio UCN/N 6 was greater than unity, was evident in both the pre-frontal and postfrontal regions. The presence of large AP surface area did not appear as a factor in controlling whether new particle formation would occur, however it did affect the number concentration of UCN; UCN concentrations were larger in the post-frontal region where AP particle surface area was smallest. Despite larger UCN concentrations in the post-frontal FT region (as compared to the pre-frontal FT), SO 2 concentrations were lower, suggesting that increased concentrations of SO 2 do not necessarily result in increased concentrations of UCN. Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

10 Processing of Aerosol by a Cold Front 953 UCN (cm -3 ) Leg 1 Leg 2 Leg 3 Leg 4 Leg Relative humidity Relative Humidity Temperature T (K) (c) θ q (K) (d) 1..8 SO 2 (ppbv) (e) Surface Area (mm 2 cm -3 ) Horizontal Distance (km) Figure 9. Measured UCN, relative humidity and temperature, (c) wet equivalent potential temperature, (d) SO 2 and (e) aerosol particle surface area as measured by the ASASP. The leg numbers are shown in Figure 3. Due to icing of the dew-point temperature sensor, relative humidity, temperature and wet equivalent potential temperature are not plotted for Leg 5. Some of the observed fluctuations in UCN concentration along level-flight legs could be explained simply by dilution due to mixing of air parcels with differing UCN concentrations, a result also found by Clement et al. (22). However, the major factor controlling the formation of UCN was found to be mixing of air parcels with differing thermodynamic characteristics. These results are in accord with the hypothesis of Bigg (1997), the modelling studies of Easter and Peters (1994) and the recent observations of Young et al. (27). Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

11 954 J. R. Peter et al. UCN (cm -3 ) Leg 1 Relative humidity Relative humidity Temperature 255 T (K) 6 25 (c) θ q (K) (d) 1..8 SO 2 (ppbv) (e) Surface Area (mm 2 cm -3 ) Horizontal Distance (km) Figure 1. As Figure 9, but for Leg 1 of the post-frontal flight Vertical profiles Pre-frontal flight Vertical profiles of measurements for the pre-frontal flight are shown in Figure 14. Total particle concentrations N 1.3 and N 6, are shown in Figures 14 and respectively. The profile of UCN and UCN/N 6 reveals a plume of newly formed particles, constrained below 5 m asl, which has been highlighted by horizontal shading. The region of new particle formation is located below a cloud layer and slightly stable layer at 1 km asl. Identification of regions of new particle formation by high concentrations of UCN is arbitrary and we have only included the obvious peak in UCN. For example, Weber et al. (23) considered areas of new particle formation to be regions where the UCN concentration exceeded 1 cm 3 which, if applied here, would extend the region of new particle formation to 1 km asl. Accumulation mode AP, as measured by the ASASP, had a maximum concentration of about 9 cm 3 and surface area 15 µm 2 cm 3. The SO 2 concentration was nearly constant at.5.6 ppbv. Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

12 Processing of Aerosol by a Cold Front 955 Q (g/kg) Q (g/kg) θ q (K) Leg 1 Leg 2 Leg 3 Leg θ q (K) Figure 11. Mixing diagram of conserved variables total water mixing ratio, Q, plotted against wet equivalent potential temperature, θ q, for each of the horizontal legs of the pre-frontal and post-frontal flights. Data points are 1 Hz samples. The leg numbers are shown in Figures 3 and 4 for the pre-frontal and post-frontal flights, respectively. The plume of new particles is in a region with large AP surface area; additionally, the AP surface area is constant throughout the vertical extent of the particle formation event. Model studies (e.g. Clement et al., 22b) have shown that a large AP surface area will suppress nucleation events, if the SO 2 concentration is less than a specific value, due to the competing mechanisms of gas-phase nucleation and coagulation scavenging to pre-existing AP. In a plume with large AP surface area such as this, Weber et al. (23) indicated that new particle formation would proceed if the SO 2 concentration was above about 2 ppbv (the measured SO 2 concentration was about.5 ppbv). However, recent calculations by Kulmala et al. (26a) have shown that only a minimal amount of SO 2 may be required for the formation of 3 nm (diameter) particles via cluster activation, as opposed to classical binary homogeneous nucleation. Nevertheless, the particles subsequently formed are subject to rapid Brownian diffusion to existing AP, so it appears that whichever nucleation mechanism is responsible, it is sufficient to overcome the quenching due to the large measured AP surface area. The large AP surface area and concentration between 3 and 37 m and between 75 and 8 m asl were revealed, by examination of 2D-C images, to be due to the presence of ice particles. Ice impacting on the ASASP shattered, resulting in a spurious increase in concentration and surface area. Between 3 and 37 m asl there is also an increase in SO 2 concentration from.5 to 1.7 ppbv. The back trajectories (Figure 6) indicate that air at this altitude had originated from the BL over mainland China, where it acquired a large anthropogenic SO 2 component. Despite the substantial increase in SO 2 concentration, there was no corresponding increase in the formation of UCN, indicating that increased levels of SO 2 are not necessarily a prerequisite for the nucleation of new particles. It is possible that no new particle formation was observed because the SO 2 concentration was below the threshold value (e.g. 2 ppbv) required for new particle formation to be detectable. To examine if spatial gradients and turbulent fluctuations of temperature and RH have played a role in the generation of new particles, stability and turbulence parameters have also been plotted. Figure 14(h) shows the stability of the pre-frontal atmosphere in terms of the virtual potential temperature gradient. If the virtual potential temperature gradient profile is positive, the atmosphere is stable; if negative, the atmosphere is unstable; if zero, the atmosphere is neutral. Figure 14(i) shows the gradient Richardson number Ri g, which represents the ratio of turbulence due to buoyancy relative to turbulence due to shear, and is given by Ri g = g θ v θ v z / { ( ) u 2 + z ( ) } v 2. (2) z TheRichardsonnumberisameasureofwhethertheflow is turbulent or laminar. Generally, when Ri g is larger than the critical Richardson number, Ri c, of.25, the flow becomes less turbulent and more closely resembles laminar flow (Jacobson, 1999). The region of new particle formation has a RH gradient of 1% (Figure 14(j)), turbulent flow, and a transition from a stable to neutral atmosphere, suggesting that turbulent eddies in the BL are responsible for conditions conducive for new particle formation. Between 7 and 77 m asl is another region of enhanced UCN concentration, especially compared to lower altitudes. Again, this region exhibits a gradient of a stable to a neutral atmosphere, large RH fluctuations (> 3%) and turbulent flow. Hegg et al. (1992) conducted a modelling study of particle production and concluded that mixing of air at the top of the BL down to the surface created conditions favourable for particle nucleation. Rather than appealing to temperature and RH fluctuation on the nucleation rate, he proposed that higher RH aloft resulted in larger particle surface areas than close to the surface. As dry AP sizes have been measured, it is possible that the 1% decrease in RH throughout the nucleation region has lowered the AP surface area such that it falls below a nominal threshold for particle nucleation Post-frontal flight In contrast to the pre-frontal vertical profile, the post-frontal profile contains significant UCN concentrations throughout the extent of the BL and FT (Figure 15). In addition, two regions of enhanced UCN concentration, indicated by horizontal shading, exist between 2 and 42 m asl. Of particular interest is the minimum in UCN concentration, located at about 27 m asl, that separates the maxima in UCN concentrations. The minimum is intriguing as it begs the question, What characteristics differentiate the Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)

13 956 J. R. Peter et al. 6 Prefrontal Leg 1 6 Prefrontal Leg UCN (cm -3 ) 3 UCN (cm -3 ) (c) F 6 Prefrontal Leg 3 (d) F 6 Prefrontal Leg UCN (cm -3 ) 3 UCN (cm -3 ) F (e) 12 Postfrontal Leg F 1 8 UCN (cm -3 ) F Figure 12. The number of UCN plotted against the F-fraction, as defined by Equation (1). Data points are 1 Hz samples. minimum from the areas of enhanced UCN concentration above and below? The largest UCN concentrations are where the ASASP integrated concentration and surface area reach maxima of approximately 5 cm 3 and 8 µmcm 3 respectively. This is a large integrated surface area, but is consistent with the back trajectories (Figure 6) which indicated that air at this altitude originated in the BL over China and acquired a substantial continental and anthropogenic influence. There is a negative correlation of UCN concentration with RH. Copyright c 21 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: (21)