Cloud Interstitial Air Research Articles

Vertical distribution of gaseous elemental mercury in Canada

Measurements of gaseous elemental mercury (GEM) were made in three locations in Canada at altitudes from 0.1 to 7 km. In the summer in southeastern Canada, northwesterly winds bring air with a constant mixing ratio of GEM at altitudes up to 7 km, with a concentration near 1.5 nanograms per standard cubic meter of air (ng sm−3). In the winter in southern and central Ontario the mixing ratio is still approximately constant with altitude, but the concentration is 1.7 ng sm−3. In the spring in the Arctic the concentration of gaseous elemental mercury at altitudes above 1 km is near 1.7 ng sm−3; however, there is evidence of episodic depletion of elemental mercury near the surface with mixing of depleted air to altitudes of 1 km. Measurements of GEM in cloud interstitial air and of mercury in cloud water indicate that the influence of a single cycling of air through cloud has little effect on the concentration of GEM. The GEM in air masses transported over the relatively unpopulated terrain of northern Canada during the summer indicates a lower limit of 5000 ng m−2 for an atmospheric column from the surface to 5 km. This gives a global burden of at least 2500 t for that altitude range. These data demonstrate the existence of a vast pool of mercury aloft, provide evidence for a long atmospheric lifetime, and illustrate the potential for long‐range atmospheric transport of this metal at altitudes up to at least 7 km.

Oxidation of SO2 in Clouds at Whiteface Mountain

In-cloud oxidation of SO2 byH2O2 was investigated using a tracertechnique based on SO2− 4/Se ratios atWhiteface Mountain, New York during summer months from1990 to 1998. Cloud water samples collected at themountain's summit (1.5 km above mean sea level) andaerosols at a below cloud site (Lodge) located at 0.6km amsl and in cloud interstitial air at the summitwere analyzed for SO2− 4 and selectedtrace elements. Gaseous SO2 andH2O2were measured in realtime. Cloud water pH wasgenerally below 5.0 with a mean value of 3.6. Theresults show that significant in cloud oxidation occursin clouds during summer months varying from belowdetection to 62% with on average approximately 24% ofthe cloud water SO2− 4 produced from in-situ SO2 oxidation. During summer the clouds wereoxidant limited for approximately one third of thetime.

Case studies of the SO2 + H2O2 reaction in clouds

The in‐cloud reaction between SO2 and H2O2has been investigated using data from field campaigns of July 1995 and July–August 1997 at Whiteface Mountain, New York. Cloud water samples were collected at the mountain's summit (1.5 km, above mean sea level (amsl)), and aerosols in clear air at a site (Lodge) located at 0.6 km amsl and in cloud interstitial air at the summit. Cloud water and aerosol samples were analyzed for SO42− and selected trace elements. Gaseous SO2 and H2O2 were measured at both sampling sites. Criteria based on total sulfur and O3 concentrations were developed to decide when the two sites were coupled (i.e., air masses at the two sites had the same composition for the chemical species studied). Four case studies are presented that exhibit varying concentration levels and different meteorological conditions. In two cases the sites were coupled, and in two others they were decoupled. The clouds showed high pollutant concentrations with a mean pH of 3.5. A tracer technique was used to quantitatively determine the SO42− produced from in situ SO2 oxidation. In general, good agreement between the SO2 consumed and SO42− produced was observed. Similarly, H2O2 concentrations in precloud air showed stoichiometric mass balance with the sum of H2O2 observed in cloud interstitial air, in cloud water, and equimolar amounts needed to produce SO4in2− as determined by the tracer technique. Data from all 24 clouds showed that (1) ∼28% of the cloud water SO42− was due to in situ oxidation; (2) ∼75% of the SO2 in the precloud air was oxidized in situ at Whiteface Mountain; (3) the mean SO2 concentration in air masses associated with cloudy periods is approximately 1.4 ppb, significantly higher than the summer average of 0.5 ppb; (4) the mean H2O2 concentration in cloud‐free periods during summer is 1.1 ppb sufficient to oxidize SO2 present; (5) the frequent presence of H2O2 in cloud water indicated that the reaction had not often reached completion.

Phase partitioning of aerosol constituents in cloud based on single-particle and bulk analysis

Single-particle analysis, performed by laser microprobe mass spectrometry and bulk analytical techniques were used to study aerosol-cloud interactions within the third field campaign of the EURO-TRAC subproject “ground-based cloud experiments” at the Great Dun Fell, Cumbria, U.K. in spring 1993. The shape of the ridge made it possible for ground-based instrumentation to sample similar parcels of air before, during and after their transit through the cloud. A single jet five-stage minicascade impactor was used for sampling particles of the interstitial aerosol. A second impactor worked in tandem with a counterflow virtual impactor and collected residues of cloud droplets. Considering marine conditions largest droplets nucleated on sea-salt particles, whereas smaller droplets were formed on sulphate and methane sulphonate containing particles. This clearly indicates chemical inhomogeneities in the droplet phase. Particles, which were disfavoured by droplet formation, often contained the highest amounts of water-insoluble carbonaceous matter. For the submicron size range we found that the carbonaceous matter was always internally mixed with sulphate. The fraction of carbonaceous matter increased with decreasing size. A detectable fraction of particles remained in the cloud interstitial air, although they were in size as well as in composition suitable to form cloud droplets. The findings confirm that nucleation is the most important process affecting phase partitioning in cloud, but that spatial and temporal variations of water vapour supersaturation have also an influence on the observed phase partitioning. Proton induced X-ray emission analysis and light absorption measurements of filter samples showed that the average scavenged fraction was 0.77 for sulphur and 0.57 for soot in clouds formed by continental influenced air and 0.62 and 0.44. respectively, for marine influenced clouds.

Scavenging and solubility of trace element bearing aerosols in frontal clouds at Whiteface Mountain, NY

Scavenging efficiencies for aerosol Se, As, and Sb in cloud water are reported for frontal stratus clouds at Whiteface Mountain, NY. Solubility for these aerosols is also presented from seven clouds associated with air that had passed through highly diverse pollution source regions. Solubilities of Se and As aerosols were determined to be 98 ± 1 % and 96 ± 3%, respectively. Antimony aerosols yielded a slightly lower value, 92 ± 4%. Scavenging efficiencies were determined by comparing the concentrations below the cloud base with that in the cloud interstitial air. Scavenging efficiencies for Se, As, Sb, and SO 4 were determined to be 53%v or less in the moderate to highly polluted air masses studied.

SO2Oxidation in a rainband: Effects of in‐cloud H2O2Production

Abstract Aqueous‐phase H2O2 production in a rainband and its possible effect on sulphate production are studied by means of a two‐dimensional numerical model. In‐cloud peroxide production is incorporated into this chemistry model and its simulation results are compared with those in which aqueous‐phase H2O2 came only from the dissolution of gaseous H2O2 from the cloud interstitial air. Results are presented for two different polluted situations ‐ Case 1 having initial SO2 and sulphate aerosol profiles representative of a moderately polluted air mass, and Case 2 having chemical profiles expected to increase the relative importance of oxidation to nucleation as a means of contributing sulphate to cloud and rain. Sulphate production increased in both cases, although in Case 1 the effect of this increase on the concentration of sulphate in rain is negligible because nucleation and scavenging of aerosol are the major processes by which sulphate enters cloud and rain. In Case 2, sulphate concentrations in rain ...

Efficient scavenging of aerosol sulfate by liquid-water clouds

Continuous aircraft measurements of aerosol sulfate concentration, [SO 4 2−], (flame-photometric detector) and light-scattering coefficient, b scat, (integrating nephelometer) in cloud interstitial air and in associated clear air are reported for several locations in the eastern United States. Measurements were made in situations where, on the basis of other physical and chemical measurements, it can be argued that the cloud air and the clear air have similar history and composition. It is found in cloud interstitial air that [SO 4 2−] approaches zero and b scat approaches the Rayleigh scattering coefficient of air. Sulfate concentration and b scat in successive subsaturated regions along a given flight path at a given altitude were frequently nearly constant or slowly varying, the pattern presented appearing to be that of a fairly uniform concentration of sulfatecontaining, light-scattering aerosol from which segments had been removed in regions corresponding to the presence of cloud. The strong decrease in both [SO 4 2−] and b scat within cloud interstitial air relative to clear air is attributed to nearly complete uptake of sulfate-containing and light-scattering aerosol particles into cloud droplets. From comparison of the in-cloud signal to the non-cloud signal it is possible to set an upper bound to the fraction of unscavenged sulfate aerosol (frequently as low as 0.25) and to b scat (frequently as low as 0.1), although for both quantities the measurements would be consistent with essentially complete scavenging. Additionally, the observation in these situations that no cloud-free regions exhibited the low values of [SO 4 2−] and b scat characteristic of the in-cloud regions is indicative of the return of the dissolved material to the air as clear-air aerosol upon cloud evaporation.

Sulfate and nitrate chemistry in cumuliform clouds

Continuous aircraft measurements of aerosol sulfate concentration, [SO42−], (flame-photometric detector) and light-scattering coefficient, bscat, (integrating nephelometer) in cloud interstitial air and in associated clear air are reported for several locations in the eastern United States. Measurements were made in situations where, on the basis of other physical and chemical measurements, it can be argued that the cloud air and the clear air have similar history and composition. It is found in cloud interstitial air that [SO42−] approaches zero and bscat approaches the Rayleigh scattering coefficient of air. Sulfate concentration and bscat in successive subsaturated regions along a given flight path at a given altitude were frequently nearly constant or slowly varying, the pattern presented appearing to be that of a fairly uniform concentration of sulfatecontaining, light-scattering aerosol from which segments had been removed in regions corresponding to the presence of cloud. The strong decrease in both [SO42−] and bscat within cloud interstitial air relative to clear air is attributed to nearly complete uptake of sulfate-containing and light-scattering aerosol particles into cloud droplets. From comparison of the in-cloud signal to the non-cloud signal it is possible to set an upper bound to the fraction of unscavenged sulfate aerosol (frequently as low as 0.25) and to bscat (frequently as low as 0.1), although for both quantities the measurements would be consistent with essentially complete scavenging. Additionally, the observation in these situations that no cloud-free regions exhibited the low values of [SO42−] and bscat characteristic of the in-cloud regions is indicative of the return of the dissolved material to the air as clear-air aerosol upon cloud evaporation.

Measurements of the chemical composition of stratiform clouds

Measurements are reported of the chemical composition of the liquid water and interstitial air in warm (> 0°C), non-precipitating stratus and strato-cumulus clouds at various locations in the eastern United States. Inorganic ionic composition of the cloud water was generally dominated by H +, NH 4 +, NO 3 − and SO 4 2−, similar to the composition of precipitation in this region of the U.S. Concentrations of the corresponding interstitial aerosol species and gaseous HNO 3 were invariably low in comparison to concentrations of the respective ionic species in cloudwater. In contrast, the concentration of NO x (i.e. NO + NO 2 + organic nitrates) was invariably comparable to or in excess of that of cloudwater nitrate. Sulfur dioxide was found at varying concentrations relative to cloudwater sulfate. In many cases, the SO 2 concentration was quite low (< 0.2 ppb) even in the presence of substantial quantities of cloudwater SO 4 2− (> 1 ppb equivalent gas-phase concentration), suggesting large fractional conversion and incorporation into cloudwater. In other cases, in which dilute SO 2 plumes (pso, > 5 ppb) were observed in the cloud interstitial air, the gaseous SO 2 concentration substantially exceeded the cloudwater sulfate concentration. Concentrations of H 2O 2 in cloudwater were found to exhibit strong inverse correlation with interstitial SO 2. Appreciable concentrations of SO 2 in cloud interstitial air and H 2O 2 in cloudwater were only rarely observed to coexist, for the most part only one or the other being present above the limit of detection. These observations are consistent with aqueous-phase oxidation of SO 2 by H 2O 2, as has been inferred previously on the basis of laboratory kinetic studies, and with the hypothesis that depending on relative concentrations, either of these species can be a limiting reagent for in-cloud SO 2 oxidation. The uptake of NO x as cloudwater nitrate is less complete than the uptake of SO 2 as sulfate, and evidence for the occurrence of similar in-cloud processes causing the conversion of NO or NO 2 to cloudwater nitrate has not been found.