Troposphere over continents

The first aircraft flights to measure NH3 concentration in the troposphere over continents were carried out by Georgii and Muller (1974), while Gravenhorst (1975)... 

The values 4 X 10" g/m (the lower troposphere over the continents) and 0.1 X 10" g/m (the middle and upper troposphere) are proposed as an averaged characteristic of the mass concentration of sulfate aerosols... 

In the absence of atmospheric motion and removal by precipitation, Be and Pb would remain where they originated—the upper troposphere and stratosphere and the lowermost meter of the atmosphere over continents and islands, respectively. In the real atmosphere, Be is mixed downward and Pb is mixed upwards and both are removed by precipitation. They are distributed through the atmosphere by eddy mixing. The residence time of aerosols is short compared to... 

Ammonia is the primary basic gas in the atmosphere and, after N2 and N20, is the most abundant nitrogen-containing compound in the atmosphere. The significant sources of NH3 are animal waste, ammonification of humus followed by emission from soils, losses of NH3-based fertilizers from soils, and industrial emissions (Table 2.8). The ammonium (NH ) ion is an important component of the continental tropospheric aerosol. Because NH3 is readily absorbed by surfaces such as water and soil, its residence time in the lower atmosphere is estimated to be quite short, about 10 days. Wet and dry deposition of NH3 are the main atmospheric removal mechanisms for NH3. In fact, deposition of atmospheric NH3 and NH4" may represent an important nutrient to the biosphere in some areas. Atmospheric concentrations of NH3 are quite variable, depending on proximity to a source-rich region. NH3 mixing ratios over continents range typically between 0.1 and lOppb. 

These isolines derived from out balloon data have been combined with those computed from aircraft data of project AIRSTREAM measured, up to about 20 km, over the American continent, between 10°S and 80°N [26], These aircraft data covering the time span 1973-1983 were averaged in relative units with respect to tropospheric abundances, in the same fashion as described for the balloon data. No seasonal effects were considered. As figs. 14-16 demonstrate, both data sets match fairly well, at least for middle and higher latitudes, yielding complete "hemispheric" distributions of CFC-12, CFC-11 and CH3CCI3. It should be mentioned that no intercalibration of the balloon and aircraft data sets has been carried out so far. 

In summer, surface inversions over the ice cap are weaker, which stimulates aerosol mixing in the troposphere and its motion to the surface. A relative spreading of the tropopause favours the air transport from the lower stratosphere to the upper one. The transport takes place, apparently, near the boundaries of the continent, but it may also take place over its interiors. As a result, rather a homogeneous distribution of aerosol concentrations and composition over the Antarctic continent is observed [36]. 

Although the vertical distribution of radon over the continents is a direct consequence of supply from soils, convection upward (treated as turbulent diffusion), and radioactive decay, the pattern is different over the oceans. This difference is due to the fact that no significant source of radon exists over the oceans and the pattern is set by longdistance transport from continents. Off the northwest coast of the United States, for example, Andreae et al. (1988) show vertical patterns up to 4 km ranging from constancy with elevation to increases with elevation. The concentration range from 6 pCi (STP) to 10 pCi m (STP) is more typical of upper troposphere air over the continents and not like the —400 pCi m (STP) in the continental boundary layer (Figure 3). 

The seasonal pattern of Be/ Pb at island sites is six months out of phase with continental sites (Figure 15). During the summer, Be/ Pb is nearly the same over the continent and the ocean, indicating vigorous mixing of the troposphere. In the winter, however, Be/ Pb is higher at oceanic sites than continental sites by a factor of 4. 

An atmospheric sulfur inventory for the whole European continent has been recently constructed by E. Meszaros et al. (1978). These authors show on the basis of the comparison of anthropogenic sulfur emission (Semb, 1978) and sulfur advection from the Atlantic that the sulfur gained by advection is small. 70-85 % of the sulfur emitted and imported is removed over the continent equally by dry (mostly in form of S02) and wet deposition. Meszaros and his associates have estimated the dry deposition of S02 by using an average European S02—S concentration calculated from data in Table 13 (3.2 /tg m-3) and a dry deposition velocity of 1cm s (Garland, 1978). The value of wet deposition was based on precipitation chemistry measurements. It follows from this quantitative calculation that Europe contributes 15-30 % of its sulfur emission to the tropospheric sulfur cycle of other areas. 

The application of Eq. (1-10) makes it convenient to estimate the air masses contained in the atmospheric domains of principal interest here, namely, the troposphere and the stratosphere. The total mass of the atmosphere is obtained as follows. The air mass over the oceans is (p0/ g) Asea, where the zero subscript refers to sea level and Asea = 3.61 x 1014 m2 is the total ocean surface area. The average elevation of the continents was estimated by Kossina (1933) as 874 m above sea level (a.s.l.). From Eq. (1-9) one estimates with T0 = 284 K that the average column density of air over the land areas is reduced to 0.91 of that over the oceans. The surface area of the continents is /4conl = 1.49 x 1014 m2. The total mass of the atmosphere thus sums to... 

The reactions with ozone, H02, CH302, and other R02 radicals cause NO to be rapidly reverted back to N02 so that steady-state conditions are set up for NO. The lifetimes of H02 and CH302 are quite sensitive to this stationary NO concentration. The tropospheric background level of N02 is of the order of 30 pptv in marine air, and higher over the continents. Measurements of NO in the free troposphere indicate daytime mixing ratios of about 10 pptv or less (see Table 9-12). In rural continental air, values of... 

Fig. 5-7. Ozone mixing ratios in the upper troposphere as a function of latitude, from aircraft observations. Top Data from one flight on July 19, 1971 (Fabian and Pruchniewicz, 1977), flight altitude 11-12 km. Center. Seasonally averaged data from about 40 individual flights between Norway and South Africa (Fabian and Pruchniewicz, 1977) at altitudes of 11-12 km. Bottom Average ozone mixing ratios at altitudes of 5-6.5 km over the North American continent and the Pacific Ocean (Routhier et al, 1980).

Hoffman et al. (1974) found the same procedure applicable to data obtained from measurements on board of ships in the central Atlantic Ocean. Table 7-15 includes mean (X)/(Na) ratios from their work. Shown in parentheses are the values derived from the slopes of regression lines. They are distinctly lower than the averaged data. Hoffman et al. (1974) measured also the abundance of iron in the aerosols. Since the samples were taken in a region partly affected by fallout from the Saharan dust plume, iron serves as a convenient indicator for the contribution of material from continental sources. Not surprisingly, the enrichment of the elements Mg, Ca, K, and Sr was well correlated with the iron content. The (X)/(Na) ratios approached those of sea water only when the Fe concentrations were very low. These results demonstrate that materials from both marine and crustal sources are present over the open ocean. In addition, they provide some verification for the existence of a tropospheric background aerosol having the continents as a source, and they confirm the absence of a significant fractionation of alkali and alkaline earth elements in the production of sea salt. 

Fig. 7-26. Model for the vertical distribution of particulate matter in the troposphere. The model assumes a superposition of the tropospheric background aerosol with boundary layer aerosol over the continents, and with sea-salt aerosol over the oceans.

Evidence for the second viewpoint comes from measurements of longer-lived radionucleides within the radium decay sequence, specifically bismuth-210 and lead-210. The major routes for nuclei conversion within the radium decay scheme are shown in Fig. 7-27. The direct decay product of radium-226, an alpha-emitter, is radon-222, which escapes the Earth surface. Only the continents are a source the contribution from the oceans is negligible. Since the half-life time of radon-222 is only 3.8 days, its distribution in the troposphere is rather uneven. Over the continents the mixing ratio declines with increasing altitude (see Fig. 1-9). Over the oceans, the vertical gradient is reversed, as the oceans act as a sink and the zonal circulation keeps supplying material from the middle and upper troposphere. The immediate... 

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