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[1] Biomass burning is an important source of particulate organic carbon (OC) in the atmosphere. Quantifying this contribution in time and space requires a means of routinely apportioning contributions of smoke from biomass burning to OC. Smoke marker (for example, levoglucosan) measurements provide the most common approach for making this determination. A lack of source profiles for wildfires and prescribed fires and the expense and complexity of traditional smoke marker measurement methods have thus far limited routine estimates of these contributions to ambient aerosol concentrations and regional haze. We report here on the collection of source profiles for combustion of numerous wildland fuels and on the development of an inexpensive and robust technique for routine smoke marker measurements. Hi-Volume filter source samples were collected during two studies at the Fire Science Laboratory in Missoula, MT in 2006 and 2007. Levoglucosan (and other carbohydrates) were measured in these samples using high-performance anion-exchange chromatography with pulsed amperometric detection. Results of this analysis along with water-soluble potassium, OC, and elemental carbon are presented. The results show that emissions of levoglucosan are fairly correlated with OC with an average ratio of 0.031 μg C/μg C. Further, there was a definite pattern that emerged based on fuel component burned with the typical levoglucosan/OC ratio of branches > straw > needles > leaves. Additionally, this carbohydrate measurement method appears to provide fingerprint information about the type of fuel burned that could help constrain profiles chosen for aerosol source apportionment and lead to a better determination of source contributions from biomass burning.

[1] In this laboratory closure study, we compare sub- and supersaturated water uptake properties for aerosol particles possessing a range of hygroscopicity. Measurements for water sub-saturated conditions used a hygroscopic tandem differential mobility analyzer (HTDMA). Simultaneously, measurements of particle critical supersaturation were conducted on the same sample stream with a continuous flow cloud condensation nuclei (CCN) counter. For these experiments, we used filter-collected samples of biomass smoke generated in the combustion of two common wildland fire fuels, western sagebrush and Alaskan duff core. Extractions of separate sections of the filter were performed using two solvents, ultrapure water and methanol. The extracts were subsequently atomized, producing aerosols having a range of hygroscopic responses. HTDMA and CCN measurements were fit to a single-parameter model of water uptake, in which the fit parameter is denoted κ, the hygroscopicity parameter. Here, for the four extracts we observed mean values of the hygroscopicity parameter of 0.06 < κ < 0.30, similar to the range found previously for numerous pure organic compounds. Particles generated from the aqueous extracts of the filters had consistently larger κ than methanol extracts, while western sagebrush extract aerosols κ exceeded those from Alaskan duff core. HTDMA- and CCN-derived values of κ for each experiment agreed within approximately 20%. Applicability of the κ-parameterization to other multicomponent aerosols relevant to the atmosphere remains to be tested.

Fog, aerosol, and gas samples were collected during the winter of 1986 at Riverside, California. The dominant components of the aerosol were NH_4^+, NO_3^−, and SO_(42−). Gaseous NH_3 was frequently present at levels equal to or exceeding the aerosol NH_4^+. Maximum level were 3800, 3100, 690 and 4540 neq m^(−3) for NH_4^+, NO_3^(2−) and NH_(3(g)), respectively. The fogwater collected at Riverside had very high concentrations, particularly of the major aerosol components. Maximum concentrations were 26,000 29,000 and 6200 μM for NH_4^+, NO_3^− and SO_4^(2−), respectively. pH values in fogwater ranged from 2.3 to 5.7. Formate and acetate concentrations as high as 1500 and 580 μM, respectively, were measured. The maximum CH_2O concentration was 380 μM. Glyoxal and methylglyoxal were found in all the samples; their maximum concentrations were 280 and 120 μM, respectively. Comparison of fogwater and aerosol concentrations indicates that scavenging of precursor aerosol by fog droplets under the conditions at Riverside is less than 100% efficient. The chemistry at Riverside is controlled by the balance between HNO_3 production from NO_x emitted throughout the Los Angeles basin and NH3 emitted from dairy cattle feedlots just west of Riverside. The balance is controlled by local mixing. Acid fogs result at Riverside when drainage flows from the surrounding mountains isolate the site from the NH_3 source. Continued formation of HNO_(3(g)) in this air mass eventually depletes the residual NH_(3(g)). A simple box model that includes deposition, fog scavenging, and dilution is used to assess the effect of curtailing the dairy cattle feedlot operations. The calculations suggest that the resulting reduction of NH_3 levels would decrease the total NO_3^− in the atmosphere, but nearly all remaining NO_3^− would exist as HNO_3. Fogwater in the basin would be uniformly acidic.

The chemical composition of cloudwater in the Sierra Nevada is dominated by NO_3^−, SO_4^(2−), and NH_4^+. Cloudwater pH is determined largely by the balance between the concentrations of these three species, although inputs of formic and acetic acid also are believed to be important, particularly when anthropogenic inputs are small. Cloudwater samples collected in Sequoia National Park (SNP) exhibited pH values ranging from 3.9 to 6.5; Yosemite National Park (YNP) cloudwater samples had pH values ranging from 3.8 to 5.2. Samples collected at YNP were more acidic than those collected at SNP. The difference in pH between the two regions appears to be due to relatively small differences in inputs of NO_3^−, SO_4^(2−), and NH_4^+. In the absence of inputs of NH_3, cloudwater pH values in the Sierra may fall below 3. Over 250 h of cloud interception were observed during a 12 month period at a cloud monitoring site at 1856 m elevation in SNP. Estimates of cloudwater deposition of NO_3^−, SO^4^(2−), and NH^4^+ indicate that cloud interception contributes significantly to regional acid deposition for closed forest canopies. Cloud interception may be the dominant deposition mechanism for isolated conifers and ridgetop canopies, where wind speeds are higher and cloudy air parcels can impact directly on foliar surfaces.

Measurements of inorganic aerosol and gas phase species are presented for three sites in central California during a 4 day period in April 1988. The measurement sites were located along an east-west transect at Visalia, Ash Mountain, and Lower Kaweah, with elevations of 90, 550 and 1900 m, respectively. Aerosol compositions were nearly neutral at all locations, however large concentrations of NH_3 at Visalia contributed significant excess alkalinity to the air mass sampled there. Concentrations of all major species were observed to decrease with elevation during most of the sampling periods. Concentrations at the upper two sites exhibited diurnal fluctuations, with peaks in the late afternoon, consistent with the transport of pollutants from San Joaquin Valley sources by daytime upslope winds. Concentrations of most of these species reached a maximum at the elevated sites on 28 April, as a weak cold front approached, reducing the atmospheric stability over the valley floor. Concentrations at Visalia on this day were somewhat lower than those observed earlier in the week. Clouds intercepting the mountain slopes on 28 April were sampled at two locations. The coudwater pH at both sites was observed to fall throughout the event, dropping as low as 4.34. Precursor concentrations of aerosol NO_3^−, SO_4^(2-) and NH_4^+, and gas phase HNO_3 and NH_3, were sufficient to account for the observed cloudwater loadings of NO_3^−, SO_4^(2-) and NH_4^+. In-cloud measurements made near the cloud base indicated a considerable S(IV) oxidation potential in the form of H_2O_2, but only low S(IV) concentrations. Cloudwater concentrations of formic acid were approximately three times acetic acid concentrations. Carbonyl concentrations were dominated by formaldehyde and glyoxal.

A side-by-side comparison of the Rotating Arm Collector (RAC) and the Caltech Active Strand Cloudwater Collector (CASCC) was conducted at an elevated coastal site near the eastern end of the Santa Barbara Channel in southern California. The CASCC was observed to collect cloudwater at rates of up to 8.5 ml min^(−1). The ratio of cloudwater collection rates was found to be close to the theoretical prediction of 4.2:1 (CASCC:RAC) over a wide range of liquid water contents (LWC). At low LWC, however, this ratio climbed rapidly, possibly reflecting a predominance of small droplets under these conditions, coupled with a greater collection efficiency of small droplets by the CASCC. Cloudwater samples collected by the RAC had significantly higher concentrations of Na^+, Ca^(2+), Mg^(2+) and Cl^− than those collected by the CASCC. These higher concentrations may be due to differences in the chemical composition of large vs small droplets. No significant differences were observed in concentrations of NO_3^−, SO_4^(2−) or NH_4^+ in samples collected by the two instruments.

Microfluidic devices have gained significant scientific interest due to the potential to develop portable, inexpensive analytical tools capable of quick analyses with low sample consumption. These qualities make microfluidic devices attractive for point-of-use measurements where traditional techniques have limited functionality. Many samples of interest in biological and environmental analysis, however, contain insoluble particles that can block microchannels, and manual filtration prior to analysis is not desirable for point-of-use applications. Similarly, some situations involve limited control of the sample volume, potentially causing unwanted hydrodynamic flow due to differential fluid heads. Here, we present the successful inclusion of track-etched polycarbonate membrane filters into the reservoirs of poly(dimethylsiloxane) capillary electrophoresis microchips. The membranes were shown to filter insoluble particles with selectivity based on the membrane pore diameter. Electrophoretic separations with membrane-containing microchips were performed on cations, anions, and amino acids and monitored using conductivity and fluorescence detection. The dependence of peak areas on head pressure in gated injection was shown to be reduced by up to 92%. Results indicate that separation performance is not hindered by the addition of membranes. Incorporating membranes into the reservoirs of microfluidic devices will allow for improved analysis of complex solutions and samples with poorly controlled volume.

[1] Samples of marine cloud water collected over the eastern Pacific Ocean have been analyzed for pH, major ions (Cl−, NO3−, SO42−, Na+, NH4+, K+, Ca2+, and Mg2+), hydrogen peroxide, formaldehyde, S(IV), Fe, Mn, and total organic carbon. The samples were obtained during the Dynamics and Chemistry of Marine Stratocumulus, Phase II (DYCOMS-II) field project in July 2001. These measurements represent a unique in situ description of cloud water composition for stratocumulus clouds residing in the remote marine boundary layer, a regime for which there are few published cloud water composition data. A wide range of concentrations was observed for each species of interest, including sodium (36–2784 μN), sulfate (15–905 μN), nitrate (3–205 μN), hydrogen peroxide (37.8–283.2 μM), formaldehyde (2.2–8.7 μM) and Fe (

The ability to monitor and quantify anionic components of aerosols is important for developing a better fundamental understanding of temporal and spatial variations in aerosol composition. Of the many methods that can be used to detect anions, capillary electrophoresis is among the most attractive ones because of its high separation efficiency, high resolving power for ionic compounds, and ability to be miniaturized for in-field monitoring. Here we present a method to baseline resolve common aerosol components nitrate, sulfate, chloride, and over two dozen organic acids in a single separation. A capillary electrophoresis separation utilizing a pH 5.78 piperazine buffer with 1,5-naphthalenedisulfonic acid as a probe for indirect UV absorbance detection was developed for this analysis. Previously, two different buffers were required to adequately separate all of these compounds. Electrophoretic mobilities, limits of detection, and migration time reproducibilities were measured for 38 organic and 8 inorganic anions. For solutions of low conductivity, detection limits for electrokinetic injections were found to be up to two orders of magnitude lower (0.2–0.4 μM) than those for pressure injection (1–45 μM). This separation was optimized and used for routine analysis of aqueous extracts of ambient atmospheric aerosols, but may be extended to other samples containing similar mixtures of anions.

We describe a microchip capillary electrophoresis method for the analysis of nitrate and sulfate in ambient aerosols. Investigating the chemical composition of ambient aerosol particles is essential for understanding their sources and effects. Significant progress has been made towards developing mass spectrometry-based instrumentation for rapid qualitative analysis of aerosols. Alternative methods for rapid quantification of selected high abundance compounds are needed to augment the capacity for widespread routine analysis. Such methods could provide much higher temporal and spatial resolution than can be achieved currently. Inorganic anions comprise a large percentage of particulate mass, with nitrate and sulfate among the most abundant species. While ion chromatography has proven very useful for analyzing extracts of time-integrated ambient aerosol samples collected on filters and for semi-continuous, on-line particle composition measurements, there is a growing need for development of new compact, inexpensive approaches to routine on-line aerosol ion analysis for deployment in spatially dense, atmospheric measurement networks. Microchip capillary electrophoresis provides the necessary speed and portability to address this need. In this report, on-column contact conductivity detection is used with hydrodynamic injection to create a simple microchip instrument for analysis of nitrate and sulfate. On-column contact conductivity detection was achieved using a Pd decoupler placed upstream from the working electrodes. Microchips containing two Au or Pd working electrodes showed a good linear range (5-500 microM) and low limits-of-detection for sulfate and nitrate, with Au providing the lowest detection limits (1 microM) for both ions. The completed microchip system was used to analyze ambient aerosol filter samples. Nitrate and sulfate concentrations measured by the microchip matched the concentrations measured by ion chromatography.

[1] Physical and optical properties of inorganic aerosols have been extensively studied, but less is known about carbonaceous aerosols, especially as they relate to the non-urban settings such as our nation's national parks and wilderness areas. Therefore an aerosol characterization study was conceived and implemented at one national park that is highly impacted by carbonaceous aerosols, Yosemite. The primary objective of the study was to characterize the physical, chemical, and optical properties of a carbon-dominated aerosol, including the ratio of total organic matter weight to organic carbon, organic mass scattering efficiencies, and the hygroscopic characteristics of a carbon-laden ambient aerosol, while a secondary objective was to evaluate a variety of semi-continuous monitoring systems. Inorganic ions were characterized using 24-hour samples that were collected using the URG and Interagency Monitoring of Protected Visual Environments (IMPROVE) monitoring systems, the micro-orifice uniform deposit impactor (MOUDI) cascade impactor, as well as the semi-continuous particle-into-liquid sampler (PILS) technology. Likewise, carbonaceous material was collected over 24-hour periods using IMPROVE technology along with the thermal optical reflectance (TOR) analysis, while semi-continuous total carbon concentrations were measured using the Rupprecht and Patashnick (R&P) instrument. Dry aerosol number size distributions were measured using a differential mobility analyzer (DMA) and optical particle counter, scattering coefficients at near-ambient conditions were measured with nephelometers fitted with PM10 and PM2.5 inlets, and “dry” PM2.5 scattering was measured after passing ambient air through Perma Pure Nafion® dryers. In general, the 24-hour “bulk” measurements of various aerosol species compared more favorably with each other than with the semi-continuous data. Semi-continuous sulfate measurements correlated well with the 24-hour measurements, but were biased low by about 0.15 μg/m3. Semi-continuous carbon concentrations did not compare favorably with 24-hour measurements. Fine mass closure calculations suggested that the factor for estimating organic mass from measurements of carbon was approximately 1.8. Furthermore, fine scattering closure calculations showed that the use of 4.0 m2/g for the fine organic mass scattering coefficient was an underestimate by at least 30% for periods with high organic mass concentrations.

[1] Direct observations and size-distribution-based estimates of light-scattering coefficients (bsp) are presented for the Yosemite Aerosol Characterization Study (YACS), which took place during the summer of 2002, an active fire year. Uncertainty in the estimates of bsp were found to be sensitive to the choice of the aerosol refractive indices, which were retrieved by aligning optical particle counter measurements to differential mobility analyzer observations and, in addition, were calculated from composition measurements. Aerosol composition during the study was dominated by organic carbon, with highest levels observed during periods impacted by biomass burning smoke influenced hazes. As a result, estimates of the aerosol refractive index from composition measurements were highly sensitive to the assumed properties of organic carbon. Retrieved and calculated refractive indices were in agreement for reasonable assumptions for properties of organic carbon. Excellent agreement (within 6%) was found between measured bsp and size-distribution derived bsp if the imaginary component of the refractive index, determined from composition measurements and assuming all species were internally mixed, was included in the Mie calculation. Under the internally mixed assumption, aerosols sampled during the study were modeled as weakly absorbing, with computed dry single scattering albedo ranging from 0.86 to 0.94. Calculated light absorption coefficients yielded a study-averaged dry aerosol mass absorption efficiency of 0.37 ± 0.05 m2g−1.

[1] The Yosemite Aerosol Characterization Study (YACS) took place in Yosemite National Park from 15 July to 5 September 2002, during which time air masses arriving at the site were believed to have been influenced by smoke from numerous wildfires active in the western United States. Physical, optical, and chemical aerosol measurements were made to characterize visibility and to help define aerosol sources contributing to haze in the park, with a particular emphasis on the role of prescribed and wild fires. Measurements of dry aerosol size distributions were made with a differential mobility analyzer (DMA) and an optical particle counter (OPC). An iterative alignment method assuming a range of refractive indices was applied to OPC size distributions to match them to DMA size distributions, returning the real refractive index that yielded the best fit and generating a complete size distribution for 0.04 < Dp < 2 μm. Retrieved dry aerosol real refractive indices generally ranged from 1.56 to 1.59 and were comparable to values estimated from composition measurements. Organic carbon was the dominant aerosol species during the study, particularly during periods identified as smoke impacted. Mie theory was used to determine mass scattering efficiencies (λ = 530 nm) from measured dry size distributions using retrieved refractive indices. These ranged from 3 to 6 m2 g−1, with the highest values occurring during smoke-impacted episodes.

[1] The Grand Teton Reactive Nitrogen Deposition Study (GrandTReNDS) was conducted to provide a more complete look at atmospheric concentrations and deposition fluxes of various reactive nitrogen species in and around Grand Teton National Park (GTNP). Daily measurements of wet deposition, PM2.5 composition, and gaseous ammonia and nitric acid concentrations were made at three locations. Weekly measurements of gaseous ammonia were made at eight additional sites. Ammonia concentrations were higher at the western sites; the study average ammonia concentration west of GTNP was 35 nmol m−3 and on the east side of the park it was 18 nmol m−3. Concentrations of other measured reactive nitrogen species were lower than NH3 and fairly similar at all sites, with averages of approximately 9, 1, and 3 nmol m−3 for ammonium, nitrate, and nitric acid, respectively. Wet deposition of ammonium and dry deposition of ammonia were the largest reactive nitrogen deposition pathways, together accounting for 56% and 62% of the nitrogen deposition on the east and west sides of GTNP, respectively. Nitrogen deposition of measured species totaled 2.38 kg N ha−1 west of GTNP at Driggs, ID (6 April to 21 September 2011), 0.85 kg N ha−1 west of GTNP at a high-elevation site (28 July to 21 September 2011) and 1.23 kg N · ha−1 at a location on the east side of GTNP (15 May to 21 September 2011). These measurements highlight the significant inputs of reactive nitrogen to regional ecosystems over the few months studied, the importance of including NH3 dry deposition in nitrogen deposition budgets, and the need to conduct further research to capture sources and the annual cycle of deposition.

[1] Atmospheric organic aerosol concentrations depend in part on the gas-particle partitioning of primary organic aerosol (POA) emissions. Consequently, heating and dilution were used to investigate the volatility of biomass-burning smoke particles from combustion of common North American trees/shrubs/grasses during the third Fire Lab at Missoula Experiment. Fifty to eighty percent of the mass of biomass-burning POA evaporated when isothermally diluted from plume- (~1000 µg m−3) to ambient-like concentrations (~10 µg m−3), while roughly 80% of the POA evaporated upon heating to 100°C in a thermodenuder with a residence time of ~14 sec. Therefore, the majority of the POA emissions were semivolatile. Thermodenuder measurements performed at three different residence times indicated that there were not substantial mass transfer limitations to evaporation (i.e., the mass accommodation coefficient appears to be between 0.1 and 1). An evaporation kinetics model was used to derive volatility distributions and enthalpies of vaporization from the thermodenuder data. A single volatility distribution can be used to represent the measured gas-particle partitioning from the entire set of experiments, including different fuels, organic aerosol concentrations, and thermodenuder residence times. This distribution, derived from the thermodenuder measurements, also predicts the dilution-driven changes in gas-particle partitioning. This volatility distribution and associated emission factors for each fuel studied can be used to update emission inventories and to simulate the gas-particle partitioning of biomass-burning POA emissions in chemical transport models.

The ionic compositions of particulate matter with aerodynamic diameter < or = 2.5 microm (PM2.5) and size-resolved aerosol particles were measured in Big Bend National Park, Texas, during the 1999 Big Bend Regional Aerosol and Visibility Observational study. The ionic composition of PM2.5 aerosol was dominated by sulfate (SO4(2-)) and ammonium (NH4+). Daily average SO4(2-) and NH4+ concentrations were strongly correlated (R2 = 0.94). The molar ratio of NH4+ to SO4(2-) averaged 1.54, consistent with concurrent measurements of aerosol acidity. The aerosol was observed to be comprised of a submicron fine mode consisting primarily of ammoniated SO4(2-) and a coarse particle mode containing nitrate (NO3-). The NO3- appears to be primarily associated with sea salt particles where chloride has been replaced by NO3-, although formation of calcium nitrate (Ca(NO3)2) is important, too, on several days. Size-resolved aerosol composition results reveal that a size cut in particulate matter with aerodynamic diameter < or = 1 microm would have provided a much better separation of fine and coarse aerosol modes than the standard PM2.5 size cut utilized for the study. Although considerable nitric acid exists in the gas phase at Big Bend, the aerosol is sufficiently acidic and temperatures sufficiently high that even significant future reductions in PM2.5 SO4(2-) are unlikely to be offset by formation of particulate ammonium nitrate in summer or fall.

The hygroscopic properties of the organic fraction of aerosols are poorly understood. The ability of organic aerosols to absorb water as a function of relative humidity (RH) was examined using data collected during the 1999 Big Bend Regional Aerosol and Visibility Observational Study (BRAVO). (On average, organics accounted for 22% of fine particulate matter with an aerodynamic diameter less than 2.5 microm (PM2.5) mass). Hourly RH exceeded 80% only 3.5% of the time and averaged 44%. BRAVO aerosol chemical composition and dry particle size distributions were used to estimate PM2.5 light scattering (Bsp) at low and high ambient RH. Liquid water growth associated with inorganic species was sufficient to account for measured Bsp for RH between 70 and 95%.

[1] The Environmental Protection Agency (EPA) and National Park Service (NPS) initiated a comprehensive field experiment called The Big Bend Regional Aerosol and Visibility Observational Study (BRAVO) to investigate the source of visibility-reducing aerosols at Big Bend National Park, Texas. The study was carried out over a period of 4 months starting in the first week of July 1999. One objective of the study was to gain insight into the atmospheric light-scattering properties of ambient aerosols, especially as they relate to their hygroscopicity. This paper will report on comparisons between measured and modeled estimations of dry and ambient scattering and comparisons between measured and modeled ratios of wet and dry scattering, f(RH), as a function of relative humidity (RH). Two equilibrium models, exercised in combination with Mie scattering theory, were used to predict atmospheric aerosol water content and associated increase in atmospheric scattering. Modeled and measured deliquescence and crystallization points were also compared. Measured and modeled deliquescence were always within 10% RH while crystallization RHs were always within a few percentage points. The analysis suggests that on most days some water is retained by the aerosol at low RHs (20–30%) and in most cases the hygroscopic growth of only inorganic salts accounted for all the observed increase in scattering as a function of RH.

[1] Ambient measurements are reported of aerosol light absorption from photoacoustic and filter-based instruments (aethalometer and a particle soot absorption photometer (PSAP)) to provide insight on the measurement science. Measurements were obtained during the Big Bend Regional Aerosol and Visibility Observational Study at the Big Bend National Park in South Texas. The aethalometer measurements of black carbon concentration at this site correlate reasonably well with photoacoustic measurements of aerosol light absorption, with a slope of 8.1 m2/g and a small offset. Light absorption at this site never exceeded 2.1 Mm−1 during the month of collocated measurements. Measurements were also obtained, as a function of controlled relative humidity between 40% and 90%, during the Photoacoustic IOP in 2000 at the Department of Energy Southern Great Plains Cloud and Radiation Testbed site (SGP). PSAP measurements of aerosol light absorption correlated very well with photoacoustic measurements, but the slope of the correlation indicated the PSAP values were larger by a factor of 1.61. The photoacoustic measurements of light absorption exhibited a systematic decrease when the RH increased beyond 70%. This apparent decrease in light absorption with RH may be due to the contribution of mass transfer to the photoacoustic signal. Model results for the limiting case of full water saturation are used to evaluate this hypothesis. A second PSAP measured the light absorption for the same humidified samples, and indicated very erratic response as the RH changed, suggesting caution when interpreting PSAP data under conditions of rapid relative humidity change.

Organic matter was studied in radiation fogs in the San Joaquin Valley of California during the California Regional Particulate Air Quality Study (CRPAQS). Total organic carbon (TOC) concentrations ranged from 2 to 40 ppm of C. While most organic carbon was found in solution as dissolved organic carbon (DOC), 23% on average was not dissolved inside the fog drops. We observe a clear variation of organic matter concentration with droplet size. TOC concentrations in small fog drops (17 microm) were a factor of 3, on average, higher than TOC concentrations in larger drops. As much as half of the dissolved organic matter was determined to have a molecular weight higher than 500 Da. Deposition fluxes of organic matter in fog drops were high (0.5-4.3 microg of C m(-2) min(-1)), indicating the importance of fog processing as a vector for removal of organic matter from the atmosphere. Deposition velocities of organic matter, however, were usually found to be lower than deposition velocities for fogwater, consistent with the enrichment of the organic matter in smaller fog drops with lower terminal settling velocities.

Formaldehyde, formate, and acetate have been determined in fog and cloudwater from several areas of southern California. Up to 190 μM formate and acetate was seen in high pH fog samples from the San Joaquin Valley. Formaldehyde concentrations as high as 500 μM were observed there. Organic acid concentrations were much lower at an acidic site along the margin of the valley. Fog from Riverside, CA had maximum concentrations of 1500 and 500 μM formate and acetate. The highest concentrations were observed in the samples with highest pH. Formaldehyde at Riverside was as high as 380 μM. Average formate and acetate concentrations in intercepted stratus clouds from the Santa Barbara Channel coast and San Pedro Hill were 20–60 μM and 10–30 μM, respectively. Formaldehyde concentrations in stratus clouds were 10–20 μM. Because the lifetime of a cloud or fog droplet is usually much longer than the characteristic time for interfacial mass transport, hydration or dissociation, droplets can be assumed to be in equilibrium with their surroundings. The product of aqueous-phase concentration and liquid water content in the cloud (fog) is nearly always much less than the predicted equilibrium partial pressure. Only the high pH fogs from the San Joaquin Valley have the potential to deplete the gas-phase reservoir of organic acid.DOI: 10.1111/j.1600-0889.1989.tb00303.x

A multidisciplinary field experiment to study wet deposition processes by means of detailed case studies is presented with the help of a video . The field experiment, taking place along the slope of Mount Rigi, Central Switzerland, since 1984 (see Fig. 1 below), combines measurements of radar meteorology, cloud physics, aerosol physics, as well as of inorganic and organic chemistry. It unites researchers of both experimental and theoretical background. It is demonstrated that with the multifaceted data sets obtained many puzzling phenomena of the interaction between air pollutants and precipitation can be observed. State-of-the-art theories are found to be unable to explain many of the observations. Numerical models are so far insufficient to formulate adequate three-dimensional temporal evolutions of precipitating systems and its scavenging effects. Field experiments of this kind are therefore indispensable for a better understanding of the relative importance of the many known interactions, and they are a trigger for the search of still unknown scavenging pathways.

The aerosol at elevated sites in the South Coast Air Basin in California is a mixture of sea salt and pollution-derived secondary aerosol. The influence of sea salt declines with increasing distance from the coast. Nitric acid appears to react with the NaCl in sea salt aerosol to release HCl_(g) and form NaNO_3 in the aerosol. At inland sites, aerosol concentrations differ during periods of onshore and offshore flow. The highest concentrations were observed during the day when the onshore flow transported pollutants to the sites, while lower concentrations were observed at night when drainage flows from nearby mountains influenced the sites. Variations, in liquid water content are a major influence on cloudwater ion concentrations. Comparisons of the ionic concentrations in two size-segregated fractions of cloudwater collected during several sampling intervals suggest that there is a large difference between the average composition of the smaller droplets and that of the larger droplets. The concentrations of Na^+, Ca^(2+) and Mg^(2+) in the large-droplet fraction were observed to be higher than in the small-droplet fraction, while the concentrations of SO_4^(2−), NO_3^−, NH_4^+ and H^+ were higher in the small-droplet fraction. Chloride concentrations were nearly equal in both fractions. Differences in the composition of size-fractionated cloudwater samples suggest that large droplets are formed on sea salt and soil dust, which are large aerosol, and small droplets are formed on small secondary aerosol composed primarily of (NH_4)_2SO_4 and NH_4NO_3. The concentrations of several components that exist partly in the gas phase (e.g. Cl^−, HCOOH and CH_3COOH) appear to be independent of droplet size.

As part of the Rain In Cumulus over the Ocean Experiment (RICO) and the Puerto Rico Aerosol and Cloud Study (PRACS), cloud water was collected at East Peak (EP) in Puerto Rico. The main objective of this study was to determine the concentrations of water-soluble species (Cl−, NO3 −, SO4 2−, NH4 +, Ca2+, H+, Mg2+, K+, and Na+) in water samples taken from clouds influenced by tropical trade winds. The most abundant inorganic species were Na+ (average 465 µeq l−1) and Cl− (434 µeq l−1), followed by Mg2+ (105 µeq l−1), SO4 2− (61 µeq l−1), and NO3 − (25 µeq l−1). High concentrations of nss–SO4 2 (28 µeq l−1), NO3 − (86 µeq l−1), and H+ (14.5 µeq l−1) were measured with a shift in air masses origin from the North Atlantic to North American continent, which reflected a strong anthropogenic influence on cloud chemistry at EP. Long-range transport of particles and acid gases seems to be the factor responsible for fluctuations in concentrations and pH of cloud water at East Peak. When under trade wind influences the liquid phase concentrations of all inorganic substances were similar to those found in clouds in other clean maritime environments.

Interception of cloudwater by forests in the Sierra Nevada Mountains may contribute significantly to acidic deposition in the region. Cloudwater sampled in Sequoia National Park had pH values ranging from 4.4 to 5.7. The advance of cold fronts into the Park appears to lead to higher aerosol and gas phase concentrations than are seen under normal mountain-valley circulations, producing higher cloud-water concentrations than might otherwise be expected. Estimates of annual deposition rates of NO_3^−, SO_4^(2−), NH_4^+ and H^+ due to cloudwater impaction are comparable to those measured in precipitation.

Nylon filters are a popular medium to collect atmospheric fine particles in different aerosol monitoring networks, including those operated by the U.S. Environmental Protection Agency and the Interagency Monitoring of Protected Visual Environments (IMPROVE) program. Extraction of the filters by deionized water or by a basic aqueous solution (typically a mixture of sodium carbonate and sodium bicarbonate) is often performed to permit measurement of the inorganic ion content of the collected particles. Whereas previous studies have demonstrated the importance of using a basic solution to efficiently extract gaseous nitric acid collected using nylon filters, there has been a recent movement to the use of deionized water for extraction of particles collected on nylon filters to eliminate interference from sodium ion (Na+) during ion chromatographic analysis of inorganic aerosol cations. Results are reported here from a study designed to investigate the efficiency of deionized water extraction of aerosol nitrate (NO3-) and sulfate from nylon filters. Data were obtained through the conduct of five field experiments at selected IMPROVE sites. Results indicate that the nylon filters provide superior retention of collected fine particle NO3-, relative to Teflon filters, and that deionized water extraction (with ultrasonication) of collected NO3- and sulfate is as efficient, for the situations studied, as extraction using a basic solution of 1.7 mM sodium bicarbonate and 1.8 mM sodium carbonate.

The ACE-2 HILLCLOUD experiment was carried out on the island of Tenerife in June-July 1997 to investigate the interaction of the boundary layer aerosol with a hill cap cloud forming over a ridge to the north-east of the island. The cloud was used as a natural flow through reactor to investigate the dependence of the cloud microphysics and chemistry on the characteristics of the aerosols and trace gases entering cloud, and to simultaneously study the influence of the physical and chemical processes occurring within the cloud on the size distribution, chemical and hygroscopic properties of the aerosol exiting cloud. 5 major ground base sites were used, measuring trace gases and aerosols upwind and downwind of the cloud, and cloud microphysics and chemistry and interstitial aerosol and gases within the cloud on the hill. 8 intensive measurement periods or runs were undertaken during cloud events, (nocturnally for seven of the eight runs) and were carried out in a wide range of airmass conditions from clean maritime to polluted continental. Polluted air was characterised by higher than average concentrations of ozone (> 50 ppbv), fine and accumulation mode aerosols (> 3000 and > 1500 cm-3, respectively) and higher aerosol mass loadings. Cloud droplet number concentrations N, increased from 50 cm-3 in background maritime air to > 2500 cm-3 in aged polluted continental air, a concentration much higher than had previously been detected. Surprisingly, N was seen to vary almost linearly with aerosol number across this range. The droplet aerosol analyser (DAA) measured higher droplet numbers than the corrected forward scattering spectrometer probe (FSSP) in the most polluted air, but at other times there was good agreement (FSSP = 0.95 DAA with an r2 = 0.89 for N

Ammonia (NH3) plays a critical role in atmospheric chemistry and can exacerbate haze formation. Agricultural emissions have been known as a primary source of global atmospheric NH3. However, with accelerating urbanization and optimized agricultural production, the dominance of agricultural emissions has become less clear. We investigated the contributions of nonagricultural and agricultural sources to atmospheric NH3 based on measurements of NH3 isotopes at nine sites in Quzhou County, a typical agricultural county in the North China Plain. We found that Quzhou had extremely high NH3 concentrations (annual average across all sites of 40.3 +/- 3.3 mu g m-3). We compared the sources of seasonal NH3 contributions in rural and urban areas through 15N-stable isotope analyses, which provides new insights into NH3 sources compared with the traditional emission inventories. In rural areas, agricultural sources (fertilizer application and livestock production) make significant contributions (56 +/- 3%) to NH3 emissions in the winter, whereas there were larger contributions of nonagricultural sources [fossil fuel, waste, and biomass burning (56 +/- 2%)] relative to agricultural sources in urban areas. More effective strategies are still needed for better manure management and vegetable/fruit production in the winter and for controlling nonagricultural sources, even in counties dominated by agriculture.

Short Communication - no abstract