Troposphere aerosol lifetimes

Junge (128) estimated that tropospheric aerosols had a lifetime of approximately 10 days, although the rate of removal decreased rapidly with increasing altitude. On the basis of the distribution of fission products,... 

Particles are eventually removed from the atmosphere by two mechanisms deposition at the Earth s surface (dry deposition) and incorporation into cloud droplets during the formation of precipitation (wet deposition). Because wet and dry deposition lead to relatively short residence times in the troposphere, and because the geographical distribution of particle sources is highly nonuniform, tropospheric aerosols vary widely in concentration and composition over the Earth. Whereas atmospheric trace gases have lifetimes ranging from less than a second to a century or more, residence times of particles in the troposphere vary only from a few days to a few weeks. 

The resultant O3 layer is critically important to life on Earth as a shield against LTV radiation. It also is responsible for the thermal structure of the upper atmosphere and controls the lifetime of materials in the stratosphere. Many substances that are short-lived in the troposphere (e.g. aerosol particles) have lifetimes of a year or more in the stratosphere due to the near-zero removal by precipitation and the presence of the permanent thermal inversion and lack of vertical mixing that it causes. 

S02 has a short lifetime in the troposphere where it is oxidised in both the gas and liquid phases to form H2S04. Its gas phase oxidation by OH leads to the formation of cloud condensation nuclei. In rain and aerosols it is oxidised by H202 to sulphuric acid. In the dry lower stratosphere the lifetime of S02 is expected to be longer compared to that in the troposphere. 

It is thought that rainout and washout may also be important removal mechanisms for soluble tropospheric gases with photochemical lifetimes longer than a few days. From the data for aerosols and fission debris, we can estimate a lifetime of at least one week, and possibly longer. It would also appear that this process is significant in only the first 5 km of the troposphere [Junge (128)]. 

In summary, we find that vertical mixing and intra-hemispherical mixing, both longitudinal and latitudinal, have a time scale of approximately one month, while inter-hemispherical mixing and mixing through the tropopause have a time scale of one year or more. Rainout and washout provide lifetimes of one week or more for aerosols and soluble gases in the lower troposphere. 

On the average, the free ion lifetime in the troposphere is on the order of 10 -10 seconds. Taking typical Q-values of 1-10 cm a , a steady-state free ion concentration, , on the order of 10 -10 cm is obtained. Due to temporal and spatial changes of the aerosol content, tropospheric free ion concentrations may undergo marked changes. 

Formation of ammonium nitrate aerosols also affects the global troposphere by transporting NO from polluted regions to remote locations (Horowitz et al., 1998). Gas-phase organic nitrates such as PAN, formed in polluted regions and exported to the remote troposphere, are often a significant source of in remote locations. Because ammonium nitrate is relatively long-lived (with a lifetime of days to weeks, similar to other fine particulates) it can also transport NO to the remote troposphere. 

The impact of deposition on global distribution has been noted for the CFC replacements hydro-chlorofluorocarbons (HCFCs), the chlorinated solvents tetrachloroethene (PCE), and trichloro-ethene (TCE), as these compounds undergo gas phase oxidation and photochemical degradation, resulting in the formation of carbonyl halides (e.g., CCI2O) and haloacetyl halides (e.g., bromo-, chloro-, and fluoroacetates). As these compounds are polar and water soluble, they are transported via aerosols, rain, and fog, which impacts their tropospheric lifetime and depositional fluxes (Rompp et al., 2001 de Bmyn et al., 1995). It is not clear whether and to what extent there is evidence of latitudinal fractionation of these compounds. 

A further study of the transport of aerosol pollution in die troposphere in relation to die actual optical-microphysical properties of aerosols, determined by the mediod of multiwave lidar sensing of atmosphere, allows the identification with high confidence of the various sources of aerosols. The effectiveness of the pollutants transport under equal conditions is determined by the lifetimes of pollutants in the atmosphere, which in turn depend on the character of the surface over which the air masses flow. 

Table VI summarizes aerosol mass concentrations and composition in different regions of the troposphere. It is interesting to note that average total fine particle mass (that associated with particles of diameter less than about 2 /im) in non-urban continental, i.e., regional, aerosols is only a factor of two lower than urban values. This reflects the relatively long residence time of particles (recall the estimate of a lifetime of fine particles by dry deposition of 10 days). Correspondingly, the average composition of non-urban continental and urban aerosols is roughly the same. The average mass concentration of remote aerosols is a factor of three lower than that of non-urban continental aerosols. The elemental carbon component, a direct indicator of anthropogenic combustion sources, drops to 0.3% in the remote aerosols, but sulfate is still a major compo-...

Spatial scales characteristic of various atmospheric chemical phenomena are given in Table 1.1. Many of the phenomena in Table 1.1 overlap for example, there is more or less of a continuum between (1) urban and regional air pollution, (2) the aerosol haze associated with regional air pollution and aerosol-climate interactions, (3) greenhouse gas increases and stratospheric ozone depletion, and (4) tropospheric oxidative capacity and stratospheric ozone depletion. The lifetime of a species is the average time that a molecule of that species resides in the atmosphere before removal (chemical transformation to another species counts as removal). Atmospheric lifetimes vary from less than a second for... 

On the basis of atmospheric measurements, Chin and Davis (1995) estimated the total quantity of OCS in the atmosphere to be 5.2 Tg, of which 4.63 Tg is in the troposphere and 0.57 Tg in the stratosphere. Based on the estimated global OCS source strength of 0.86 Tg yr 1, the global atmospheric lifetime of OCS is estimated to be about 6 years. We will return to the global cycle and chemistry of OCS in Chapter 5 in connection with the stratospheric aerosol layer. 

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