Particulate nitrate conversions

Figure 11.27 summarizes the ratio of the sum of the individual components of NOy to the total NOy measured using conversion to NO (Parrish et al., 1993). These data summarize measurements of NO, N02, PAN, HN03, and particulate nitrate as well as total NOy at Whitetop Mountain (Tennessee), Bondville (Illinois), Scotia (Pennsylvania), and Egbert (Ontario, Canada). The median value (which is less influenced by extremes) of the percentage NOy that can be accounted for ranges from 75 to 94%, with all but the Bondville site being within experimental error of 100%. 

However, as expected from the chemistry discussed in Chapter 7.E, simultaneous control of NH3 has a significant effect on particle nitrate formation, since the formation of ammonium nitrate is a major mechanism for conversion of gaseous HN03 to particulate nitrate. Thus, Meng et al. (1997) predict that a 50% reduction in both NO. and NH3 would give about the same reduction in particulate nitrate. 

It is well documented that N02 can be transformed in the air to particulate nitrate. For this reason the nitrate content of aerosol particles in clear air is around 0.1 fig m 3 (Soderlund and Svensson, 1976). This gas-to-particle conversion process is initiated either by reaction [3.24] or by the gas phase hydrolysis of nitrogen dioxide ... 

The last anion to be discussed is nitrate. In the marine aerosol, nitrate is associated mainly with coarse particles. This fact is apparent in Fig. 7-21, but it has also been observed by Savoie and Prospero (1982). Sea water contains insignificant amounts of nitrate, so that the particulate nitrate must derive from the gas phase, that is, from gaseous nitric acid. As a gas-to-particle conversion process, one would expect the condensation of nitric acid to take place in the accumulation mode. The volatility of HN03 is much greater than that of H2S04, however. It appears that the condensation of sulfuric acid prevents the simultaneous condensation of nitric acid in the same size range. In this connection one should remember that with... 

According to a number of studies are the particles emitted from motor vehicles made up of mineral oxides, soot, numerous organic compounds, a variety of metals and metal compounds and water (Puxbaum et al. 2000). The inorganic ions sulfate, nitrate and ammonium, which were normally major constituents of airborne particulate matter, were also found in particulate vehicle emissions, but only to a minor degree since these compounds were primary formed by gas-to-particle conversion processes of gaseous precursor substances. 

The essential elements of urban aerosol chemistry are shown in Figure 2, in which we have represented the chemistry in terms of the conversion of SO2, NO and hydrocarbons to particulate sulfate, nitrate, and organics, respectively. Table VII summarizes the key unknown aspects of the processes depicted in Figure 2. There are many feat ires of atmospheric aerosol chemistry that must be elucidated before we understand fully the formation and growth of atmospheric particles. 

It has been established that the principal growth mechanism for urban atmospheric aerosols in the 0.1-1.0 ym diameter size range (the so-called Accxamulation Mode) is gas-to-particle conversion (68,71) The major secondary components in atmospheric aerosols have been identified as sulfates, nitrates and particulate organic species (68,71,72,73). The qualitative picture of polluted... 

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