Atmosphere removal processes

Deposition is the atmospheric removal process by which gaseous and particulate contaminants are transferred from the atmosphere to surface receptors - soil, vegetation, and surface waters (22,27,28, 32). This process has been conveniently separated into two categories dry and wet deposition. Dry deposition is a direct transfer process that removes contaminants from the atmosphere without the intervention of precipitation, and therefore may occur continuously. Wet deposition involves the removal of contaminants from the atmosphere in an aqueous form and is therefore dependent on the precipitation events of rain, snow, or fog. 

Bidleman T.F., Christinsen E.J. (1979) Atmospheric removal process for high molecular weight organochlorines. J. Geophysical Res. 84(cl2), 7857-7862. 

Methane is found throughout the troposphere in concentrations now exceeding 1.6 parts per million by volume (1 ppmv = 10" ), and is the most abundant source of C-H bonds in the atmosphere. Its primary atmospheric removal process is also reaction with HO radicals, as in (6). The atmospheric lifetimes for CHa and CILCCh can be connected through the relative rates of reactions (5) and (6), and the value observed in the laboratory... 

Given its high vapor pressure of over 2 atm at 25°C, methylamine will remain in the vapor phase, if released to the atmosphere where it will react with photochemically produced hydroxyl radicals (Tyx of 18h). Dissolution into rain droplets is also an important removal process. Other atmospheric removal processes, for example, photolysis and hydrolysis are not significant. 

Public Comment on Atmospheric Removal Processes. A public comment questioned whether photolysis is the dominant removal pathway for airborne PCBs, as the profile currently suggests (page 306). One panelist thought precipitation, not photolysis, is the dominant removal pathway. 

In the atmosphere, ammonia can be removed by rain or snow washout. Reactions with acidic substances, such as H2SO4, HCl, or HNO3 (all produced in high concentrations from anthropogenic activities) produce ammonium aerosols, which can undergo dry or wet deposition. The gas phase reaction of ammonia with photochemically produced hydroxyl radicals is thought to contribute about 10% to the overall atmospheric removal process. The best estimate of the half-life of atmospheric ammonia is a few days. 

PAN acts as a reservoir species for both CH3C(0)02 radicals and NO. Because of this, the atmospheric lifetime of PAN is important if its lifetime is relatively long, PAN can act as an effective reservoir for NO. Potential atmospheric removal processes for PAN include thermal decomposition (reaction 4 above), UV photolysis, and OH reaction. PAN is not highly water-soluble it is more soluble than NO or N02 but considerably less soluble than HNO3. Thus, wet deposition is a minor removal process. Dry deposition is also unimportant. The PAN-OH rate constant is <3 x 10 14 cm3 molecule-1 s 1, and OH reaction is not an effective removal process. PAN absorbs UV radiation up to 350 nm (Libuda and Zabel 1995 Talukdar et al. 1995). Thus, thermal decomposition and photolysis are the principal removal processes for PAN. 

The primary atmospheric removal processes for halocarbons are photolysis and reaction with tropospheric hydroxyl radicals (OH). For the fully halogenated CFCs and halons, photolysis is the only important sink and their atmospheric lifetimes are dependent on their absorption cross-sections, the solar flux, and the surface to stratosphere transport time. As a general rule, the greater the number of Cl, Br, or I atoms on any one carbon atom, the larger the cross-section and the shorter the lifetime. For example, the lifetimes of CCIF3 (CFC-13), CCI2F2... 

PFCs and SFg are highly resistant to normal atmospheric removal processes and will persist in... 

The gas-phase reactions of OH and NO3 radicals with dimethylphenols are rapid as described in this section and are expected to be the major atmospheric removal processes. The estimated atmospheric lifetimes with respect to reactions with OH and NO3 are approximately 1 h. 

Removal Processes. Pollutant removal processes, particularly dry deposition and scavenging by rain and clouds, are a primary factor in determining the dynamics and ultimate fate of pollutants in the atmosphere. 

The major purpose of ambient particulate sampling is to obtain mass concentration and chemical composition data, preferably as a function of particle diameter. This information is valuable for a variety of problems effects on human health, identification of particulate matter sources, understanding of atmospheric haze, and particle removal processes. 

To assess the relative importance of the volatilisation removal process of APs from estuarine water, Van Ry et al. constructed a box model to estimate the input and removal fluxes for the Hudson estuary. Inputs of NPs to the bay are advection by the Hudson river and air-water exchange (atmospheric deposition, absorption). Removal processes are advection out, volatilisation, sedimentation and biodegradation. Most of these processes could be estimated only the biodegradation rate was obtained indirectly by closing the mass balance. The calculations reveal that volatilisation is the most important removal process from the estuary, accounting for 37% of the removal. Degradation and advection out of the estuary account for 24 and 29% of the total removal. However, the actual importance of degradation is quite uncertain, as no real environmental data were used to quantify this process. The residence time of NP in the Hudson estuary, as calculated from the box model, is 9 days, while the residence time of the water in the estuary is 35 days [16]. 

Figure 10. Cumulative distribution curves of dust in card room atmosphere while processing cotton with an electrostatic precipitator (ESP) in filtration line. Key curve for ESP energized O, curve for ESP not energized and A, size distribution removed by ESP.

The atmospheric and chemical processes controlling the spatial and temporal variability of psychoactive substances in urban atmospheres are largely uncertain, mostly due to the fact that the atmospheric residence time of these compounds is so far unclear. The transport, transformation and deposition/atmospheric removal... 

Dichlorobenzene will exist predominantly in the vapor-phase in the atmosphere, and its detection in rainwater suggests that atmospheric removal via washout is possible (Ligocki et al. 1985). Depending on soil type, the compound is expected to be moderately mobile in soil and to volatilize from surface water and soil surfaces to the atmosphere. Volatilization, sorption, biodegradation, and bioaccumulation are likely to be competing processes, with the dominant fate being determined by local environmental conditions. 

Carbon monoxide, which is predominantly produced during combustion processes, may exhibit an 0 excess of up to 7.5%c in summer at high northern latitudes (Rockmann et al. 1998). The major source of this fractionation is its atmospheric removal reaction CO + OH = CO2 + H2, in which the remaining CO gains excess 0. 

Gierczak et al. (1998) have also measured the temperature dependence for the absorption cross sections in addition to the quantum yields as a function of pressure and temperature. They have used these data, combined with the kinetics of the OH-acetone reaction, which is the other major removal process, to calculate the contributions of the OH reactions and of photolysis to the loss of acetone in the atmosphere as a function of altitude. Figure 4.31 shows that photolysis is a significant, but not the major, contributor at the... 

In short, particles in the atmosphere are now frequently treated in terms of the four modes summarized in Fig. 9.7, which also shows the major sources and removal processes for each one. Although the vertical axis is not shown, it could in theory be any of the distributions discussed, that is, number, mass, surface, or volume. 

However, about this time, a variety of research indicated that even with full implementation of the Montreal Protocol, the atmospheric abundance of chlorine could reach as much as 6-9 ppb between the years 2050 and 2075. This delay is due to the relatively long time between emission of these compounds into the troposphere and when they reach the stratosphere and photolyze to produce an active chlorine atom. Figure 13.1, for example, compares the estimated equivalent effective stratospheric chlorine from 1960 until the year 2100 with no controls and a 3% increase per year in CFC and methylchloroform emissions to those with the controls agreed to in the Montreal Protocol. Equivalent effective stratospheric chlorine loading depends on emissions as well as removal processes, which determine what fraction of the CFCs emitted at the earth s... 

---