Marine particle nucleation: Observation at Bodega Bay, California

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2005jd006210, 2006 Marine particle nucleation: Observation at Bodega Bay, California Jian Wen, 1 Yongjing Zhao, 1 and Anthony S. Wexler 1,2,3 Received 12 May 2005; revised 14 October 2005; accepted 26 January 2006; published 29 April [1] A TSI nano-smps was installed in a lab at Bodega Bay, about 50 m from the coastline and 5 m above sea level. On the basis of measurements conducted from June to December 2001 and from January to June 2003, we have observed two kinds of nucleation events, i.e., long-term (a few hours) and short-term (a few minutes) particle bursts. The long-term events mostly occur during daytime in the summer, lasting from 0.5 to 8 hours. Narrow spikes (short-term events) that occur year-round, both day and night, last only a few minutes to a half hour but contain particle number concentrations comparable to some of the long-term events. Wind direction and speed affect the occurrence and intensity of the particle burst. Nucleation mostly takes place during northwesterly onshore wind for both long- and short-term events, and the probability of nucleation occurrence is higher at higher wind speed. However, in contrast to what has been observed at Mace Head, Ireland, nucleation at Bodega Bay does not correlate with tidal height. Instead, the seasonal and interannual variations of ultrafine particle number concentration N 3 10nm appear to correlate with ocean upwelling, a characteristic of currents along the west coast of the United States that brings up nutrients from subsurface waters, promoting plant productivity. Simultaneous measurements of nucleation at the coast and 1.6 km out suggest that nucleation is a coastal phenomenon, supporting the contention that it is related to direct or biogenic emission of precursor gases from the coastal area during the sea upwelling periods. Citation: Wen, J., Y. Zhao, and A. S. Wexler (2006), Marine particle nucleation: Observation at Bodega Bay, California, J. Geophys. Res., 111,, doi: /2005jd Introduction [2] Aerosol particles are ubiquitous in the atmosphere. They influence the quality of life in many ways through their climatic and health effects and by affecting visibility. Tropospheric particles influence global climate by directly scattering solar radiation and indirectly by affecting cloud formation when enhanced particle concentration increases the number of cloud droplets by acting as cloud condensation nuclei. Better understanding of these effects, especially their role in climate change, requires knowledge of the mechanism by which new particles nucleate and grow in the atmosphere. [3] Nucleation has been observed in several studies in different surroundings, such as in the marine boundary layer [Covert et al., 1992], in the free troposphere [Weber and McMurry, 1999], at coastal sites [O Dowd et al., 2002a], in the Arctic [Pirjola et al., 1998; Wiedensohler et al., 1996], and over boreal forests where volatile organic compound (VOC) emissions are high [Makela et al., 1997]. Long-term measurements have been conducted on a high-alpine site 1 Department of Mechanical and Aeronautical Engineering, University of California, Davis, Davis, California, USA. 2 Department of Civil and Environmental Engineering, University of California, Davis, Davis, California, USA. 3 Department of Land, Air and Water Resources, University of California, Davis, Davis, California, USA. Copyright 2006 by the American Geophysical Union /06/2005JD [Weingartner et al., 1999] and at a boreal forest site in southern Finland [Komppula et al., 2003; Boy et al., 2003]. Bursts of charged nanometer particles have also been observed during measurements of air ions [Horrak et al., 1998]. Though many new particle formation events have been observed, the key questions, such as what molecules form the particles and what triggers nucleation and subsequent growth, remain unresolved. The objective of this study is to identify and characterize the nucleation events by long-term measurement of size distribution of nano size particles at Bodega Bay, California, a North Pacific coastal site. 2. Measurements [4] The sampling station is located in the Horseshoe Cove of Bodega Bay ( N, W), approximately 50 m from the coastline and 5 m above the sea level, as shown in Figure 1. The neighboring town is sparsely populated and has no industrial activity. It was selected as the measurement site because it has relatively clean air and the site of the University of California, Davis, Bodega Marine Laboratory (BML) includes a rich mix of coastal habitats such as marine algae, invertebrates, extensive lagoon mudflats and sand flats, and tidal salt marsh. A TSI model 3936 scanning mobility particle sizer (SMPS) with nano differential mobility analyzer (DMA) and 3025 condensation particle counter (CPC) is installed in a small building located to the south of the main Marine Laboratory. Particle diameters ranging from 3 to 90 nm are measured over 6 min scan 1of11

2 Figure 1. Aerial view of Bodega Bay, about 50 miles north of San Francisco. The sampling station is at Horseshoe Cove, approximately 50 m from the coast and 5 m above the ground. cycles. BML routinely measures meteorological parameters, such as air and seawater temperatures, humidity, photosynthetically active radiation, wind direction and speed. 3. Background Concentration [5] The background particle concentration is defined as the total particle concentration when there is no nucleation burst. For the clean marine air mass at Bodega Bay, westerly or northwesterly or southwesterly wind, the background concentration is about cm 3. When the wind is easterly from inland, the background concentration is about cm 3. A convenient indicator of nucleation is a significant increase in the nuclei mode particle count, N 3 10nm defined in our study as particle sizes from the lower detection Figure 3. (a) Time series plot of total number concentrations for particle diameter from 3 to 10 nm in January (b) Time series plot of total number concentrations for particle diameter from 3 to 10 nm in June Figure 2. Frequency distributions of N 3 10nm for the months with measurement. There are two modes in the distribution, one at concentration less than about 1000 cm 3 and the second at higher concentration. 2of11

3 Figure 4. (a) Wind rose at Bodega Bay during (b) Wind rose with nucleation events during (c) Wind rose during (d) Wind rose with nucleation events during The radial coordinate represents the total number of observations of nucleation events. limit of 3 nm up to 10 nm. Using the method of McGovern [1999], which displays the percentage distribution of concentration occurrences, we calculated the frequency of concentration occurrence for particle diameter from 3 to 10 nm. As shown in Figure 2 the concentration is bimodal with one mode roughly below 1000 cm 3 and a second mode above 1000 cm 3. For the rest of our analysis, we will focus on the N 3 10nm concentration higher than 1000 cm Identification of Nucleation and Its Correlation With Meteorological Parameters [6] From June 2001 to June 2003, except for sporadic instrument failures, we observed many nucleation events with the highest particle concentrations N 3 10nm = cm 3. The burst of new particles has a seasonal trend; the total particle concentration gradually increases in spring, peaks in summer and then declines in fall with almost no nucleation in winter. Comparing Figures 3a and 3b, the intensity of N 3 10nm in June is ten times higher than that in January. [7] The wind direction at Bodega Bay is mostly northwesterly, clean marine air, but in the month of January, February, November and December, the wind is mostly from inland, easterly or southeasterly. Figures 4a and 4b show the wind direction frequency and its frequency during nucleation events. Although the wind blows from southeasterly to southwesterly directions a fair amount of the 3of11

4 Figure 5. Scatterplot of N 3 10nm (cm 3 ) versus wind direction. The radial coordinate represents the magnitude of N 3 10nm, and the angle shows the wind direction. year, nucleation mostly occurs during northwesterly wind; however, in 2003 a few nucleation events occurred at wind direction of approximately 180, which represents air advected briefly over Bodega head. Northwesterly nucleation bursts are about three times more intense than the southwesterly ones, as shown in Figure 5. [8] The wind at Bodega Bay varies from calm to occasionally higher than 20 m/s. By defining F as the ratio of wind speed occurrence with nucleation to the overall wind speed occurrence frequency, we see in Figure 6 that nucleation is more favorable at higher wind speed. To investigate how sensitive the setting of N 3 10nm cut point to this analysis, we also investigated the cases of N 3 10nm higher than 2000 and 4000 cm 3. Even though the criterion is more stringent, the trend of F increasing with wind speed remains the same. A potential cause for this correlation is discussed in section 8. [9] The air temperature has no effect on the nucleation observed at Bodega Bay; as shown in Figure 7, nucleation is mostly observed at air temperature of 10 to 13 C, which is also the temperature typically observed at Bodega Bay. [10] Though nucleation is found to be correlated with low tide at Mace Head [O Dowd et al., 2002a], nucleation at Bodega Bay is not associated with tidal height. 5. Characteristics of Nucleation Events [11] The nucleation events at Bodega Bay can be categorized as long-term daytime, long-term nighttime, short-term daytime, and short-term nighttime events primarily on the basis of the duration of the particle burst and the time that the burst occurred. The long- and short-term events are classified on the basis of the number of consecutive scans of SMPS with N 3 10nm >1000cm 3. Figure 8 shows minimum event duration near 3 4 consecutive scans (18 24 min), so we use this as a cut point between short-term and long-term events. Hence we define short-term nucleation events to be those lasting less than 24 min (4 consecutive scans), and those lasting more than 24 min, long-term events. Events are termed daytime when instantaneous photosynthetic active radiation (PAR) is higher than 20 mmol/m 2 s; otherwise they are classified as nighttime events. Particle number concentration of the long-term events generally increases as the particle size decreases down to the instrument detection limit of 3 nm. About 30% of the short-day and 40% of the short-night events have a size distribution that peaks at 6 to 8 nm. Figure 9 shows the time series plot of N 3 10nm on a typical day with long-term daytime ( local time (LT)) and short-term nighttime (near 0200 LT) events. The frequency of occurrence of aforementioned long- and short-term and day and night events is summarized in Table 1. The directional occurrence of the four types of events are shown in Figure 10 with the radius corresponding to the frequency of occurrence of the nucleation event and the angle representing the wind direction. For all four types of events, nucleation almost exclusively occurs during northwesterly wind. It is found that long-term events primarily occur during the daytime in May and June and only occasionally at night, but the short-term events take place both day and night throughout the year, suggesting that the longterm events are more likely to be photochemical, but not the short-term event. Observation of I 2, OIO, IO and NO 3 at Mace Head, Ireland, led Saiz-Lopez and Plane [2004] to posit that IO and OIO, formed at night from reaction of I 2 and O 3 with NO 3, are the nucleation precursors. 6. Location of Nucleation Precursors [12] We hypothesize that (1) the long-term events are due to biogenic emissions of precursors at the coast that subsequently react in the atmosphere and (2) the short-term events are also coast related, but caused by chemical reactions that are not photosynthetic. [13] To explore our hypothesis that nucleation is a coastal phenomena, we have conducted a simultaneous nano particle measurement with two sets of nano SMPSs, one on board a research vessel over the open ocean and another at the coastal sampling station. During the measurement on Figure 6. Probability of nucleation occurrence as a function of wind speed. F is the ratio of wind speed occurrence with nucleation to the overall wind speed occurrence frequency. 4of11

5 Figure 7. Frequency of air temperature occurrence with and without nucleation. boat, the engine was turned off, the nano SMPS was powered by a battery patch, and the boat was floating at the position of about 1.6 km upwind of the lab. Comparing N 3 10nm on boat and in the lab on two separate days in Figure 11, it is clear that no particle bursts were observed over the open ocean, while nucleation bursts occurred in the lab on both days, suggesting that the source of nucleation is more related to the local activity at coastline rather than from the open ocean. 7. Yes, We Have No Banana! [14] The daily particle number distribution shown in Figure 12 illustrates a typical particle growth profile. The color scale shows the particle number concentration at each size bin (in dn/dlogdp). The highest concentration occurs at particle diameter below 5 nm, however, no obvious particle growth path can be seen compared with those nucleation observed at boreal forest [Lihavainen et al., 2003] and urban area [Vakeva et al., 2000]. The classic banana-shaped growth curve is observed when nucleation occurs regionally; the concentrations are spatially uniform so air parcel motion does not disrupt measurement of the growth dynamics. In Bodega Bay, the classic banana-shaped particle growth curve is not observed and we conclude that this is because nucleation only occurs in the near coastal region so the particles observed at the coastline have all grown roughly the same amount. These particle growth patterns are similar to the Type I events observed at Mace Head that are associated with clean marine air [O Dowd et al., 2002b]. [15] It appears that the seasonal variation of N 3 10nm has similar patterns as ocean upwelling, a characteristic of currents along the west coast of the United States that brings up nutrients from subsurface waters promoting plant productivity. Upwelling occurs along the California coast when the wind is northwesterly, and it is generally affected by wind stress, wind curl and ocean stability. From late spring to summer, the wind at Bodega Bay is typically very strong and northwesterly, and that is the period when higher upwelling is observed. A detailed description of upwelling Figure 8. Number of consecutive scans of SMPS with nucleation, used to define long- and short-term nucleation events. Each scan is 6 min long. 5of11

6 of new particles via nucleation. The concentration of a nucleating and condensing precursor compound in the air can be expressed as [Wexler et al., 1994] dc i dt C i ¼ P i V d t cond;i L C i L nucleation ð1þ Figure 9. Time series plot of N 3 10nm on a typical day showing both long- and short-term nucleation events. The short-night event occurred near 0200 LT; the long-day event occurred from 0800 to 1800 LT. index computation can be found at products/pfel/modeled/indices/upwelling/na/how_computed.html. Figure 13 shows a scatterplot of upwelling index versus daily averaged N 3 10nm. The correlation between N 3 10nm and ocean upwelling suggests that upwelling is one of the factors affecting nucleation. During the upwelling season, the surface water generally has much greater concentrations of nutrients such as nitrates, phosphates and silicates that are key to sustaining biological production. The biogenic productivity can be evaluated by ocean chlorophyll concentration, as shown in Figure 14. The color bar corresponds to chlorophyll concentration (in unit of mg/m 3 ), it is evident that chlorophyll concentration is very high in June compared to January with the highest concentration near the shore. [16] The studies at Mace Head, Ireland [Makela et al., 2002], show that iodine species could be the precursor species leading to the new particle formation in the coastal atmosphere. The results presented here, though insufficient to pinpoint a chemical species, nevertheless suggest that nucleation is correlated to the coastal biogenic activity. 8. Effect of Wind Speed on Nucleation [17] Consider the effect of wind speed on nucleation at low wind speed, (1) dilution by vertical diffusion and turbulence is less, (2) more time is available for chemical reactions to form nucleation precursors, and (3) sea spray particle surf zone emissions are lower providing less condensational sink, all tending to prefer nucleation and high nuclei concentrations. However, at low wind speed, the seato-air gas transfer is suppressed, and the dilution of preexisting particles is less, tending to suppress nucleation. To understand the wind speeds that favor nucleation, we can look at the competing process between gaseous precursor production and its loss due to condensation. Gas phase compounds such as sulfuric acid, iodine oxides or condensable organic species in the atmosphere are produced by gas phase chemistry and may be removed by condensation on preexisting aerosols, deposition to the surface, or formation where P i is the production rate of gas i, t cond,i is the characteristic time for transport between the gas and aerosol phase of i, L is the ratio of the volume of the parcel to its deposition surface area, V d is the deposition velocity, and L nucleation is the precursor loss rate due to nucleation. The production rate P i depends on the gas phase concentration of precursor species and their chemical transformation rates. [18] As an estimate, assume (1) gas phase concentration of precursor species is in steady state, i.e., dc i /dt = 0, and (2) deposition and nucleation loss are insignificant compared to condensation for precursor gas, we obtain the steady state ambient precursor concentration C ss;i ¼ P i t cond;i As shown in equation (2), when the condensation sink is very high, i.e., t cond,i is very low, the nucleating precursor gas concentration may not reach the threshold level to induce nucleation. The characteristic time t cond,i can be given as [Wexler et al., 1994] t cond;i ¼ C i ¼ dc i dt Z 1 0 ð2þ 1 2pD p D i 1 þ b nd ð3þ p ddp where b =8l/a i D p, D p is particle diameter, D i is precursor gas diffusivity, l is air mean free path, a i is the accommodation coefficient, and n(d p ) particle size distribution. The condensational sink of sea spray particles can be calculated on the basis of the sea spray particle size distribution measured on the California coast at wind speed of 1, 3 and 8 m/s [Vignati et al., 2001]. Because of very limited size distribution data at surf zone, we are only able to estimate the characteristic time of condensational sink at 3 different wind speeds. Table 1. Frequency of Nucleation Occurrence for Long, Short, Night, and Day Events a Long Events Short Events Month and Year Daytime Nighttime Daytime Nighttime June /69% 23/2% 214/20% 90/9% July /33% 20/3% 184/32% 184/32% Aug /32% 48/6% 320/43% 142/19% Sept /27% 60/5% 506/43% 293/25% Oct /30% 10/1% 459/41% 323/29% Nov /39% 99/10% 247/26% 230/24% Dec /17% 11/7% 57/39% 55/37% Jan /0% 0/0% 0/0% 8/100% Feb /12% 20/5% 145/39% 163/44% March /44% 33/4% 250/29% 195/23% April /58% 146/10% 256/17% 242/16% May /78% 141/7% 158/7% 174/8% June /62% 57/5% 208/19% 151/14% a Frequency of nucleation occurrence is defined as the total number of SMPS scans during which N 3 10nm is higher than 1000 cm 3. Each scan is 6 min long. 6of11

7 Figure 10. Wind roses for long- and short-term, day and night nucleation events during January through June of Notice scale change for long-day wind rose. The radial coordinate represents the total number of SMPS scans with N 3 10nm higher than 1000 cm 3. [19] Many chemical compounds have been implicated in nucleation events, most notably sulfuric acid and iodine compounds [Kulmala, 2003]. In the marine environment, gas phase sulfuric acid is formed by oxidation of biogenic DMS or anthropogenic SO 2, and nucleation events due to sulfuric acid should therefore occur throughout the marine boundary layer under suitable conditions. We have deduced previously that the nucleation precursor and nucleation processes occur near the coast only, so it is unlikely that sulfuric acid is the precursor. Field measurements at Mace Head, Ireland, and lab studies indicate that CH 2 I 2 [Makela et al., 2002; Hoffmann et al., 2001; Jimenez et al., 2003] or I 2 [Saiz-Lopez and Plane, 2004; McFiggans et al., 2004] out gassed from the ocean may be precursors for nucleation. Mass transfer of precursor across the sea-to-air interface can be expressed by Flux = K w (C w C a /H), where 1/K w =1/k w + 1/Hk a and 1/K w is the overall transfer resistance across the sea-air interface, which is the sum of seaside resistance (1/k w ) and the airside resistance (1/Hk a ). H is the dimensionless Henry s law constant (the concentration in air divided by that in water at equilibrium), and C a /H is the solute concentration in the liquid phase corresponding to C a in bulk air. For both diiodomethane and molecular iodine, relatively insoluble gases, 1/Hk a 1/k w [Moore et al., 1995; Palmer et al., 1985], the flux is controlled by seaside sublayer mass transfer. Wanninkhof and McGillis [1999] found k w ¼ 0:0283u 3 10 ðsc=660þ 1=2 ð4þ 7of11

8 where k w is in unit m/s, u 10 (in unit m/s) is the wind speed at 10 m, the Schmidt number is Sc = n/d and 660 is the value of Sc for CO 2 in seawater, n is the kinematic viscosity of water and D is the molecular diffusivity of the gas in water. The Schmidt number may be estimated from [Khalil et al., 1999] Sc ¼ 335:6M 1=2 1 0:0065T þ 0:002043T 2 2: T 3 ð5þ where M is the molecular weight, and T is the seawater temperature. Taking the concentrations in the air and sea as 0.12 ppt and 0.52 pmol/l for CH 2 I 2 [Carpenter et al., 2001], and 25 ppt [Saiz-Lopez and Plane, 2004] and 1 nmol/l [Moller et al., 1996] for I 2, respectively, the emission rates of CH 2 I 2 and I 2 across the sea-air interface are calculated, as shown in Figure 15. It should be noted that equation (4) is derived from open-ocean gas transfer. Wave breaking near shore will likely enhance the sea-air gas transfer [Farmer et al., 1993], though there is no well-defined equation to quantify this effect. If taking into account of the wave breaking enhanced mass transfer, the slopes of sea-to-air flux will be even steeper, presenting a stronger correlation of wind speed with mass transfer. [20] One hypothesis is that CH 2 I 2 or I 2 will react chemically to produce nucleation precursors [Hoffmann et al., 2001; Saiz-Lopez and Plane, 2004; McFiggans et al., 2004]. The production rate of these precursors (P i ) is assumed to be P i = k[c CH2 I 2 ]orp i = k[c I2 ], where k is the Figure 11. (a) Comparison of N 3 10nm measured over the open ocean and in the coastal lab on 1 July (b) Comparison of N 3 10nm measured over the open ocean and in the coastal lab on 12 August rate constant, and gas concentration of C CH2 I 2 or C I2 is approximated as the sea-to-air flux divided by the wind speed. Therefore, using equation (2), the nucleating precursor concentration at steady state can be expressed by C SS / k(flux/u w )t cond. The sea-to-air flux is proportional to the third power of wind speed (see equation (4)), whereas the characteristic time of condensational sink is relatively flat with respect to wind speed (see Figure 15), so the nucleating Figure 12. Particle growth of long-day and short-night nucleation events on 22 May of11

9 Figure 13. Daily averaged N 3 10nm as a function of ocean upwelling index. precursor concentration at steady state increases as wind speed increases, preferring nucleation at higher wind speed as observed at Bodega Bay (see Figure 6). 9. Conclusions [21] Observation of nucleation events at Bodega Marine Laboratory, on the coast of North Pacific Ocean, was conducted from June to December 2001 and January to June Nucleation bursts mostly occurs during northwesterly winds and during higher wind speeds. The observed nucleation events can be categorized as longterm and short-term events primarily on the basis of the duration of the burst. The long-term nucleation events mostly occurred and intensified during daytime in summer, whereas the short-term events occurred both day and night, throughout the year. We hypothesis that the longterm events are due to biogenic emission of precursor gases that subsequently react in the atmosphere, and the short-term events are caused by some other chemical reactions which are not photosynthetic. The seasonal variation of particle concentration correlates with ocean upwelling, a coastal effect driven by northwesterly wind that promotes biogenic plant productivity and sea-to-air gas transfer. The comparison of particle nucleation over the open ocean and in the coastal lab implies that the key Figure 14. Ocean chlorophyll concentration (in units of mg/m 3 ) in January and June 2001 along the California coast. 9of11

10 Figure 15. Condensational sink characteristic time and sea-to-air flux as a function of wind speed. biogenic activity is coastal. Reviewing all the data measured at Bodega Bay, nucleation events behave differently here than at Mace Head, though the coastal biological activity could provide the nucleation precursors at both sites. [22] Acknowledgments. The authors thank Ian Faloona, K. Max Zhang, and Doug Day for their help and comments. Our thanks also go to Kitty Brown and the staff at Bodega Marine Laboratory (BML) for their generous help. This work is funded by National Science Foundation (NSF) CHE References Boy, M., U. Rannik, K. E. J. Lehtinen, V. Tarvainen, H. Hakola, and M. Kulmala (2003), Nucleation events in the continental boundary layer: Long-term statistical analyses of aerosol relevant characteristics, J. Geophys. Res., 108(D21), 4667, doi: /2003jd Carpenter, L. J., K. Hebestreit, U. Platt, and P. S. 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11 Wiedensohler, A., D. S. Covert, E. Swietlicki, P. Aalto, and J. Heintzenberg (1996), Occurrence of an ultrafine particle mode less than 20 nm in diameter in the marine boundary layer during summer and autumn, Tellus, Ser. B, 48, Weingartner, E., S. Nyeki, and U. Baltensperger (1999), Seasonal and diurnal variation of aerosol size distribution (10 < D < 750 nm) at a high-alpine site, J. Geophys. Res., 104, 26,809 26,820. Wexler, A. S., F. W. Lurmann, and J. H. Seinfeld (1994), Modelling urban and regional aerosols: 1. Model development, Atmos. Environ., 28(3), J. Wen, A. S. Wexler, and Y. Zhao, Department of Mechanical and Aeronautical Engineering, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA. (janwen@ucdavis.edu) 11 of 11