Removal processes

So far, we have examined the sources of trace gases and pollutants in the atmosphere and the way in which they are chemically transformed. Now we need to look at the removal process to complete the source-reservoir-sink model of trace gases that we have adopted.

Our discussions have emphasized the importance of the OH radical as a key entity in initiating reactions in the atmosphere. Attack often occurs through hydrogen abstraction, and subsequent reactions with oxygen and nitrogen oxides (as illustrated in Box 3.6). This serves to remind us that the basic transformation that takes place in the atmosphere is oxidation (see also Box 4.3). This is hardly unexpected in an atmosphere dominated by oxygen, so we can argue that reactions within the atmosphere generally oxidize trace gases.

Oxidation of non-metallic elements yields acidic compounds, and it is this that explains the great ease with which acidification occurs in the atmosphere. Carbon compounds can be oxidized to organic compounds, such as formic acid (HCOOH) or acetic acid (CH3COOH) or, more completely, to carbonic acid (H2CO3, i.e. dissolved CO2). Sulphur compounds can form H2SO4 and, in the case of some organosulphur compounds, methane sulphonic acid (CH3SO3H). Nitrogen compounds can ultimately be oxidized to HNO3. The solubility of many of these compounds in water makes rainfall an effective mechanism for their removal from the atmosphere. The process is known as 'wet removal'.

It is important to note that, even in the absence of SO2, atmospheric droplets will be acidic through the dissolution of CO2 (Box 3.7). This has implications for the geochemistry of weathering (see Section 4.4). The SO2, however, does make a substantial contribution to the acidity of droplets in the atmosphere. It can, so to speak, acidify rain (Box 3.7). However, let us consider the possibility of subsequent reactions that can cause even more severe acidification:

H2O2(aq) + HSO-(aq) ^ SO^q) + H+aq) + H2O(l) eqn. 3.30

O3(aq) + HSO-(aq) ^ SO4-aq) + H+aq) + O^) eqn. 3.31

Hydrogen peroxide (H2O2) and O3 are the natural strong oxidants present in rainwater. These oxidants can potentially oxidize nearly all the SO2 in a parcel of air. Box 3.8 shows that under such conditions rainfall may well have pH values lower than 3. This illustrates the high acid concentrations possible in the atmosphere as trace pollutants are transferred from the gas phase to droplets. Liquid water in the atmosphere has a volume about a million times smaller than the gas phase; thus a substantial increase in concentration results from dissolution.

After the water falls to the Earth, further concentration enhancement can take place if it freezes as snow. When snow melts the dissolved ions are lost preferentially, as they tend to accumulate on the outside of ice grains which make up snowpacks. This means that at the earliest stages of melting it is the dissolved H2SO4 that comes out. Concentration factors of as much as 20-fold are possible. This has serious consequences for aquatic organisms, and especially their young, in the spring as the first snows thaw. It is not just acid rain, but acid rain amplified.

It is also possible for gaseous or particulate pollutants to be removed directly from the atmosphere to the surface of the Earth under a process known as dry deposition. This removal process may take place over land or the sea, but it is

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