Instrument to collect fogwater for chemical analysis

An instrument is presented which collects large samples of ambient fogwater by impaction of droplets on a screen. The collection efficiency of the instrument is determined as a function of droplet size, and it is shown that fog droplets in the range 3–100-µm diameter are efficiently collected. No significant evaporation or condensation occurs at any stage of the collection process. Field testing indicates that samples collected are representative of the ambient fogwater. The instrument may easily be automated, and is suitable for use in routine air quality monitoring programs

The H_2SO_4-HNO_3-NH_3 System at High Humidities and in Fogs: 1. Spatial and Temporal Patterns in the San Joaquin Valley of California

A systematic characterization of the atmospheric H_2SO_4-HNO_3-NH_3 system was conducted in the fog water, the aerosol, and the gas phase at a network of sites in the San Joaquin Valley of California. Spatial patterns of concentrations were established that reflect the distribution of SO_2, NO_x, and NH_3 emissions within the valley. The concept of atmospheric alkalinity was introduced to interpret these concentrations in terms of the buffering capacity of the atmosphere with respect to inputs of strong acids. Regions of predominantly acidic and alkaline fog water were identified. Fog water was found to be alkaline in most of the valley, but small changes in emission budgets could lead to widespread acid fog. An extended stagnation episode was studied in detail: progressive accumulation of H_2SO_4-HNO_3-NH_3 species was documented over the course of the episode and interpreted in terms of production and removal mechanisms. Secondary production of strong acids H_2SO_4 and HNO_3 under stagnant conditions resulted in a complete titration of available alkalinity at the sites farthest from NH_3 sources. A steady SO_2 conversion rate of 0.4–1.1% h^(−1) was estimated in the stagnant mixed layer under overcast conditions and was attributed to nonphotochemical heterogeneous processes. Removal of SO_2 was enhanced in fog, compared to nonfoggy conditions. Conversion of NO_x to HNO_3 slowed down during the stagnation episode because of reduced photochemical activity; fog did not appear to enhance conversion of NO_x. Decreases in total HNO_3 concentrations were observed upon acidification of the atmosphere and were attributed to displacement of NO_3− by H_2SO_4 in the aerosol, followed by rapid deposition of HNO_3(g). The occurrence of fog was associated with general decreases of aerosol concentrations due to enhanced removal by deposition

Fogwater chemistry in an urban atmosphere

Analyses of fogwater collected by inertial impaction in the Los Angeles basin and the San Joaquin Valley indicated unusually high concentrations of major and minor ions. The dominant ions measured were NO_3^−, SO_4^(2−), NH_4^+, and H^+. Nitrate exceeded sulfate on an equivalent basis by a factor of 2.5 in the central and coastal regions of the Los Angeles basin but was approximately equal in the eastern Los Angeles basin and the San Joaquin Valley. Maximum observed values for NH_4^+, NO_3^−, and SO_4^(2−) were 10.0, 12.0, and 5.0, meq 1^(−1), while the lowest p;H observed was 2.2. Iron and lead concentrations of over 0.1 mM and 0.01 mM, respectively, were observed. High concentrations of chemical components in fog appeared to correlate well with the occurrence of smog events. Concentrations in fogwater were also affected by the physical processes of condensation and evaporation. Light, dissipating fogs routinely showed the highest concentrations

The H_2SO_4-HNO_3-NH_3 System at High Humidities and in Fogs: 2. Comparison of Field Data With Thermodynamic Calculations

Concentrations of HNO_3(g) and NH_3(g) determined in the field were compared to predictions from aerosol equilibrium models. The products of HNO_3(g) and NH_3(g) concentrations measured under cool and humid nonfoggy conditions agreed in magnitude with predictions from a comprehensive thermodynamic model for the atmospheric H_2SO_4-HNO_3-NH_3-H_2O system. Observed concentrations of NH_3(g) in fogs were generally consistent with those predicted at equilibrium with fog water, but important discrepancies were noted in some cases. These discrepancies may be due to fluctuations in fog water composition over the course of sample collection or to the sampling of nonfoggy pockets of air present within the fog. Detectable concentrations of HNO_3(g) (up to 23 neq m^(−3)) were often found in fogs with pH 5 were below the detection limit of 4–8 neq m^(−3)

Chemical characterization of stratus cloudwater and its role as a vector for pollutant deposition in a Los Angeles pine forest

Highly concentrated, acidic stratus cloudwater was monitored as it intercepted a pine forest (Henninger Flats) 25 km northeast of Los Angeles. Observed pH values ranged from 2.06 to 3.87 for over 100 samples collected in 1982 and 1983 with a median value below pH 3. The ratio of nitrate/sulfate in cloudwater samples was between 1.5 and 2: rainwater at the same site had a ratio of approximately 1. The solute deposition accompanying several light. spring rains (summing to ~1 % of annual rainfall) was a disproportionate fraction of the annual total: H^+. NO_3 and SO _4^2 were -20% or more. Based on a reasonable estimate of fog precipitation, deposition of sulfate, nitrate and free acidity due to intercepting stratus clouds may be of comparable magnitude as that due to the incident rainfall at Henninger Flats. Cloudwater that had deposited on local pine needles was collected. It was in general more concentrated than ambient cloudwater but with comparable acidity. Enrichment of K^+ and Ca^(2+) in those samples and in throughfall is believed to be due to leaching from foliar surfaces. Injury to sensitive plant tissue has been noted in the literature when prolonged exposure to this severe kind of micro-environment has been imposed

Nutrient leaching from pine needles impacted by acidic cloudwater

In coastal and mountainous environments, fog and cloud droplets are frequently deposited onto terrestrial surfaces; this deposition pathway may account for a large fraction of both moisture and chemical loading. Stratus cloudwater was collected for chemical analyses at Henninger Flats (870 m MSL), a site in the foothills of the San Gabriel Mountains, 25 km northeast of downtown Los Angeles. Concurrent samples of intercepted cloudwater were manually removed, drop-by-drop, from needles on various species of pine trees upon which it had deposited. Samples were analyzed for pH, major cations, and anions. Solute concentrations were substantially higher in samples removed from pine needles compared to the suspended cloudwater. For example, cloudwater concentrations for nitrate and sulfate were measured between 0.25 and 4.5 meq L^(−1) while deposited samples were 0.4 to 90 meq L^(−1). This solute enhancement was due to evaporation following droplet deposition and to nutrient leaching. Nutrient leaching was indicated by (a) a disproportionate increase in the concentrations of cations such as K and Mg (a factor of 2 to 15 enrichment relative to sulfate), and (b) reductions in the leachate acidity and ammonium relative to the incident droplets. A relationship was observed between the enhancement of Na, Ca, and nitrate in pine needle leachate. This suggests that reactions at the foliar surfaces are occurring which involve gaseous HNO_3 and accumulated soil dust and sea-salt particles

Depositional Aspects of Pollutant Behavior in Fog and Intercepted Clouds

Droplet deposition during fog is shown to play an important role in the removal of anthropogenic pollutants from the atmosphere. Relevant theoretical principles are reviewed. The in-cloud scavenging of aerosols and soluble gases coupled with the small size of fog droplets results in higher chemical concentrations in fog water than in rainwater. In the urban regions of southern California and the southern San Joaquin Valley, fog water chemistry is dominated by sulfate, nitrate, and ammonium ions, which are measured at millimolar levels. The formation of fog is shown to accelerate deposition rates for water-scavenged atmospheric constituents. During stagnation episodes, pollutant removal by ventilation of valley air requires at least 5 days, while the enhancement of deposition by fog formation leads to pollutant lifetimes on the order of 6-12 h. Thus, in an environment characterized by flat, open landscape and low wind speed, droplet sedimentation can be the dominant removal mechanism of pollutants during prolonged stagnation episodes with fog

The H 2 SO 4 -HNO 3 -NH 3 system at high humidities and in fogs: 1. Spatial and temporal patterns in the San Joaquin Valley of California

A systematic characterization of the atmospheric H2SO4-HNO3-NH3 system was conducted in the fog water, the aerosol, and the gas phase at a network of sites in the San Joaquin Valley of California. Spatial patterns of concentrations were established that reflect the distribution of SO2, NOx, and NH3 emissions within the valley. The concept of atmospheric alkalinity was introduced to interpret these concentrations in terms of the buffering capacity of the atmosphere with respect to inputs of strong acids. Regions of predominantly acidic and alkaline fog water were identified. Fog water was found to be alkaline in most of the valley, but small changes in emission budgets could lead to widespread acid fog. An extended stagnation episode was studied in detail: progressive accumulation of H2SO4-HNO3-NH3 species was documented over the course of the episode and interpreted in terms of production and removal mechanisms. Secondary production of strong acids H2SO4 and HNO3 under stagnant conditions resulted in a complete titration of available alkalinity at the sites farthest from NH3 sources. A steady SO2 conversion rate of 0.4–1.1% h−1 was estimated in the stagnant mixed layer under overcast conditions and was attributed to nonphotochemical heterogeneous processes. Removal of SO2 was enhanced in fog, compared to nonfoggy conditions. Conversion of NOx to HNO3 slowed down during the stagnation episode because of reduced photochemical activity; fog did not appear to enhance conversion of NOx. Decreases in total HNO3 concentrations were observed upon acidification of the atmosphere and were attributed to displacement of NO3− by H2SO4 in the aerosol, followed by rapid deposition of HNO3(g). The occurrence of fog was associated with general decreases of aerosol concentrations due to enhanced removal by deposition.Engineering and Applied Science