Gases, atmospheric removal mechanisms

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. 

Once PAHs enter the atmosphere, they are distributed between gas and particle phases and subject to removal mechanisms, such as oxidative and... 

The importance of OH as a removal mechanism for atmospheric trace gases is illustrated in Table 2, which shows the global emissions of each gas and the approximate percentage of each of these emitted gases which is destroyed by reaction with OH (Ehhalt, 1999). 

Nitrogen fixation. The two aspects of the nitrogen cycle having the greatest impact on the biological pump are nitrogen fixation and denitrification. The first provides a mechanism for drawing on the extensive atmospheric pool of N2 gas in support of primary production. The second provides a pathway for DIN to be converted back to N2 gas and removed from the ocean system. 

Bowman, C.T. Gas phase reaction mechanisms for nitrogen oxide formation and removal in combustion. In Pollutants From Combustion Formation and Impact on Atmospheric Chemistry Vovelle, C., Ed. NATO Science Series C Kluwer Dordrecht, The Netherlands, 2000. 

Figure 12 shows the optical drift due to the guiding laser. The input laser beam was again chopped and the output intensity was measured by the lock-in amplifier with and without an N2 gas environment after the optical and electrical packaging. The stability of the device related to the 1.3-/u,m wavelength has been measured with time. Initially, the device was in an Nj environment to prevent photo-oxidation [17] for 2 h, and then the Nj gas was removed, as shown in Fig. 12a. For the first 2 h the output is stable. After the N2 gas is removed and the device is exposed to the ambient atmosphere, the output intensity decreases to 0.7 of the initial value after 11 h. After that, however, the output intensity increases to about twice the initial value. This phenomenon has not been observed before. The decay rates R depend linearly on the input intensities. For an input intensity (/q) of 10 W/cm, the is 0.2 dB/h, and for 5/q the R is 1.06 dB/h as shown in Fig. 12. This linear optical power loss mechanism may be explained by a single-photon process [23]. On the other hand, the increase rates R do not depend linearly on the input intensities. For input intensities of/ and S/q, the rates R are 1 and 0.12 dB/h, respectively. Since depends on (intensity), the recovery mechanism could be related to a two-photon process [24]. Table 3 summarizes the R and...

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