In tropospheric clouds

The aerosols of sulfuric acid so formed increase the reflectivity (albedo) of the Earth s atmosphere, cutting down the solar radiation that reaches the Earth s surface and so counteracting to some extent the greenhouse warming due to CO2 emissions that accompany the SO2, as mentioned earlier. Airborne sulfuric acid may be neutralized by traces of ammonia in the air, giving particulate NH4HSO4 and (NH4)2S04 hazes, but in the absence of such neutralization the aqueous sulfuric acid droplets in tropospheric clouds may reach pH 1.5 or lower. 

The use of the sun or moon as the light source allows one to measure the total column abundance, i.e., the concentration integrated through a column in the atmosphere. This approach has been used for a number of years (e.g., see Noxon (1975) for NOz measurements) and provided the first measurements of the nitrate radical in the atmosphere (Noxon et al., 1978). As discussed later in this chapter, such measurements made as a function of solar zenith angle also provide information on the vertical distributions of absorbing species. Cloud-free conditions are usually used for such measurements as discussed by Erie et al. (1995), the presence of tropospheric clouds can dramatically increase the effective path length (by an order of... 

Estimates of radiative forcing due to tropospheric ozone are listed in Table 2. CTM calculations of changes in tropospheric ozone and the radiative forcing are based on a variety of assumptions. Most models, but not all, include temperature adjustment in the stratosphere and effects of clouds. Neglect of either of these effects has been estimated to lead to an overestimate of radiative forcing by approximately 10-25% (Berntsen et al., 1997). 

In addition clouds and haze level (Bais et al., 1993, Estupinan, et al., 1996, Seckmeyer et al. 1996), as well as other tropospheric minor constituents such as S02 (Zerefos et al. 1995a) and surface albedo (WMO, 1994) are also very important in determining UV-B levels at ground, and are among the controlling factors of UV-B transfer through the atmosphere (Zerefos, 1997). 

ABLE (AirBome Lidar Experiment) is a Nd-Yag high energy lidar for the measurement of aerosols and PSC. It will operate in either a dual wavelength configuration 532 nm or 355 nm emissions or at 532 nm with dual polarisation detection capacity. It will detect aerosols, PSC and tropospheric clouds. 

Analysis of aerosol samples obtained at several locations in Western Europe has shown that about 60% of the content of organic carbon in tropospheric aerosol is the share of water-soluble organic compounds. According to observational data, at a rural location in Austria, mono- and dicarboxylic acids constitute about 11 % (with respect to the total content of organic carbon in cloud water). While insoluble organic compounds hamper the assimilation of water by aerosol, soluble organic matter, as a rule, favors water assimilation. 

The final step in removal of any species from the atmosphere involves heterogeneous deposition to the Earth s surface. Removal processes include wet deposition via rain-out (following uptake into tropospheric clouds) and dry deposition to the Earth s surface, principally to the oceans. The rates of these processes are largely determined by the species chemistries in aqueous solution. Heterogeneous lifetimes of the parent HFCs, HCFCs and HFEs are of the order of hundreds of years because of their low aqueous solubility and reactivity. 

Extrapolation to the K/T boundary requires consideration of the time scales of acid deposition. Nitric acid formation occurs rapidly by aqueous phase reaction of NO and NO2 with liquid water produced by tlie incident K/T bolide on both impact and infall of ejecta. For tlie quantities of NO produced by the K/T impact ( 10 5 moles), conversion to HNO3 occurred wiUiin days, assuming sufficient liquid water was available in the posl-K/T atmosphere. The nitric acid will form an acid rain of pH 0 for a liquid water content of 1 g/m (typical of tropospheric clouds) but will contain enough protons to weather only 3 x 10 moles of Sr, for Sr/(Ta -0.003 in soil and bedrock minerals. Sulfuric acid formation occurred on a time scale of years [7] due to the slow rate of gas phase SO2 oxidation. Spread evenly over 10 years, 10 moles of SO2 produced a global acid rain of pH —4, and released —3 x 10 moles of Sr. 

Clouds occur frequently in the troposphere, and from an airborne vantage point it is evident that clouds are not randomly scattered in the vertical direction. Usually, clouds have bases at relatively well-defined altitudes, because rising air containing water vapor (moisture) systematically becomes cooler with height, eventually reaching its dew point, the temperature at which water vapor condenses (Section 4.2.2). Water in both liquid and solid phases in the clouds facilitates a host of chemical reactions. 

Sion In Nonurban Tropospheric Clouds," Atmos. Environ. 1983, 17 341-345. — ... 

The multiple scattering of photons in the cloud and between the cloud and the ground enlarges the average photon path length. Therefore any absorption by tropospheric ozone or aerosols is amplified and thus the spectral shape of the spectrum is significantly modified by clouds. 

Maahs, H. G. (1983), Measurements of the Oxidation Rate of Sulfur (IV) by Ozone in Aqueous Solution and Their Relevance to S02 Conversion in Nonurban Tropospheric Clouds, Atmos. Environ. 17, 341-345. 

Dimethyl sulfide — and possibly also methyl sulfide — is oxidized in the troposphere to sulfuric and methanesulfonic acids, and it has been suggested that these compounds may play a critical role in promoting cloud formation... 

Because the mass of ammonium sulfate and sulfuric acid particles is mainly in the size range of active condensation nuclei, it is believed that this process provides a very effective removal mechanism for the tropospheric background aerosol. However, we have to emphasize that other processes are also operating in the cloud to remove small aerosol particles, of which the most important process is the coagulation of particles and cloud drops. As we have seen (Subsection 4.1.1), thermal coagulation is particularly effective in the range of very small particles inactive in condensation. To estimate this process, let us consider a cloud in which the number concentration of drops with radius rc is Nc. If the number concentration... 

Petrenchuk and Selezneva (1970) made model calculations to evaluate the removal both in the cloud and beneath the cloud. They found that, in a clean tropospheric environment, 55 % of the trace constituents found in precipitation collected at the ground level is due the rain-out processes, in agreement with Georgii s results discussed in Subsection 5.3.4. 

Atmospheric chemistry is dominated by trace species, ranging in mixing ratios (mole fractions) from a few parts per million, for methane in the troposphere and ozone in the stratosphere, to hundredths of parts per trillion, or less, for highly reactive species such as the hydroxyl radical. It is also surprising that atmospheric condensed-phase material plays very important roles in atmospheric chemistry, since there is relatively so little of it. Atmospheric condensed-phase volume to gas-phase volume ratios range from about 3 x KT7 for tropospheric clouds to 3 x ICE14 for background stratospheric sulfate aerosol. 

Sprenger U, Bachmann K. 1987. Determination of ammonium in aerosols, cloud and rain water, and of gaseous ammonia in the troposphere. Fresenius Z Anal Chem 327 16. 

Maahs, H. G Kinetics and mechanism of the oxidation of S(IV) by ozone in aqueous solution with particular reference to S02 conversion in nonurban tropospheric clouds. /. Geophys. Res. 88, 10721 (1983). 

The column density of water in the troposphere is determined largely by the vertical distribution of the vapor. Liquid droplets and ice crystals represent only a minor fraction of the total abundance of H20. Even in dense clouds the mass of water in the vapor phase predominates over that... 

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