Processes aerosol scavenging

The key features of soot are its chemical inertness, its physical and chemical adsorption properties, and its light absorption. The large surface area coupled with the presence of various organic functional groups allow the adsorption of many different materials onto the surfaces of the particles. This type of sorption occurs both in the aerosol phase and in the aqueous phase once particles are captured by cloud droplets. As a result, complex chemical processes occur on the surface of soot particles, and otherwise volatile species may be scavenged by the soot particles. 

In the atmosphere DMS is oxidised mainly in the gas phase. Oxidation in cloud-water droplets is insignificant as the low solubility of DMS mitigates the effects of its rapid aqueous oxidation by ozone ( , McElroy, W.J., Central Electricity Research Laboratories, personal communication). Gas-phase oxidation is initiated principally by reaction with OH radicals ( ) and methanesulphonic acid (MSA) is one of the products (2). MSA has a very low vapour pressure and will be rapidly scavenged by aqueous aerosols and cloud droplets wherein further oxidation to sulphate by OH may occur. Although the kinetics and mechanism of this process have yet to be unambiguously determined, it is possible that emissions of DMS could be both a significant source of "background" sulphur and, upon oxidation, of non sea-salt sulphate. 

Atmospheric aerosols are multicomponent particles ranging from 0.001 to 10 pm in diameter. Particles are introduced into the atmosphere by combustion processes and a variety of other anthropogenic and natural sources. They evolve by gas-to-particle conversion and coagulation, are augmented due to the formation of fresh particles by nucleation, and are removed by deposition at the earth s surface and scavenging by airborne droplets. Atmospheric aerosols are the main cause of the visibility degradation accom-... 

The current version of GEM-AQ has five size-resolved aerosols types, viz. sea salt, sulphate, black carbon, organic carbon, and dust. The microphysical processes which describe formation and transformation of aerosols are calculated by a sectional aerosol module (Gong et al. 2003). The particle mass is distributed into 12 logarithmically spaced bins from 0.005 to 10.24 pm radius. This size distribution leads to an additional 60 advected tracers. The following aerosol processes are accounted for in the aerosol module nucleation, condensation, coagulation, sedimentation and dry deposition, in-cloud oxidation of SO2, in-cloud scavenging, and below-cloud scavenging by rain and snow. 

Because of similarity of the behavior of Be and Pb in the atmosphere, Be/ Pb is little affected by processes other than production and transport. Both Pb and Be are formed in the atmosphere as energetic single atoms. Since neither is volatile, each of them attaches to the first particles they encounter. The most abundant aerosols in terms of surface area are typically those with a diameter of 0.1 -0.5 p.m, the so-called accumulation mode. This size class carries many of the chemical species in the atmosphere that have low volatility and also have gaseous precursors, such as sulfate as discussed above. Accumulation-mode aerosols are most subject to long-distance transport. Scavenging by precipitation is the principal mechanism of removal of these aerosols from the atmosphere. 

Sulphur dioxide takes part in chemical reactions with substances naturally present in the atmosphere and with other pollutants, some of them driven by sunlight and others by the presence of cloud droplets. The end product of the oxidation of sulphur dioxide is sulphuric acid, together with ammonium sulphate, in the form of suspended particles. These sulphur particles, known collectively as sulphate aerosol, tend not to be removed particularly efficiently by dry deposition and have timescales limited only by the scavenging during rain events. Sulphate aerosols may have lifetimes up to 10 days and may travel hundreds and thousands of kilometres before encountering rain. The capture of sulphate aerosol by rain leads to the process of wet deposition and this process accounts for the remaining one third of the total removal of sulphur species. 

Evidence was found for the major processes responsible for the acidification of fogwater (i) the scavenging of acidic precursor aerosol, (ii) the scavenging of gaseous nitric acid, and (iii) oxidation of reduced sulfur components to sulfate. Conversion of S02(g) to sulfate in fogwater does not appear to proceed faster than 10% hour" and therefore cannot account for the high acidities observed at the beginning of fog events however, sulfate production in the precursor air parcel can lead to sulfuric acid fog condensation nuclei. 

The possibility that the depletion of chloride in the marine aerosol is due to fractionation during the formation of sea-salt particles by bursting bubbles can be discounted. Laboratory studies of Chesselet et al (1972b) and Wilkness and Bressan (1972) showed no deviation of the Cl /Na+ mass ratio from seawater in the bubble-produced sea-salt particles. It may be mentioned in passing that bromide in marine aerosols shows a deficit similar to chloride, whereas iodide is present in excess. The latter observation is attributed to both chemical enrichment at the sea s surface and scavenging of iodine from the gas phase. A portion of iodine is released from the ocean as methyl iodide, which in the atmosphere is subject to photodecomposition and thereby provides a source of scavengable iodine. The process has been reviewed by Duce and Hoffman (1976). In continental aerosols, chloride and bromide are partly remnants of sea salt, but there exists also a contribution from the gas phase. 

This value has the same magnitude as the residence time for the bulk aerosol derived in Section 7.6 in a different manner. The congruence of results shows that in-cloud scavenging and precipitation is the dominant process for the removal of particulate matter from the troposphere. 

Nucleation scavenging of aerosols in clouds refers to activation and subsequent growth of a fraction of the aerosol population to cloud droplets. This process is described by (17.70) and has been discussed in Section 17.5. 

Species can be incorporated into cloud and raindrops inside the raining cloud. These processes determine the initial concentration Caq° of the raindrops, before they start falling below the cloud base, and have been discussed previously. Let us summarize the rates of in-cloud scavenging for gases and aerosols. 

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