Particulate nitrate

Acid deposition and the associated particulate nitrates and sulfates are implicated in the deterioration of certain sensitive ecosystems, decreased visibility, negative human health effects, and increased degradation of certain stone building materials and cultural resources, especially those made of limestone and marble. Fine particulate nitrate and sulfate particles... 

Secondary pollutants H2S04, sulfate aerosols, etc. 03, PAN, HNO, aldehydes, particulate nitrate and sulfate, etc. 

Evidence for the uptake of NO, by aqueous solutions has been sought in both laboratory and field studies. A lower limit for the mass accommodation coefficient for NO, on liquid water of > 2.5 X 10 3 was reported by Thomas et al. (1989) and Mihelcic et al. (1993). Li et al. (1993) followed the formation of particulate nitrate in a rural area and, by comparing their measurements to model predictions, suggested that the mass accommodation coefficient for NO, on aqueous (NH4)2S04-NH4HS04-H2S04 aerosols is approximately unity, i.e. NO, is taken up into the particle on every collision. 

Air is drawn through a filter that consists of, or is coated with, a substance that takes up the species of interest. The filter is then extracted and the ions of interest are measured. For most species of interest, care must be taken to avoid interference from particles. For example, as discussed later, filter collection of HN03 will have a contribution from particulate nitrate if particles are not removed first. Figure 11.22 shows a typical filter pack used to measure gaseous HNO-, and NH-, (Anlauf et al., 1988). 

Denuders have been used in several different ways. One is to extract the walls of the denuder and measure the adsorbed gas directly by ion chromatography. Denuders have also been used as difference denuders. For example, in nitric acid measurements, the combination of gas-phase HNO-, and particle nitrate has been measured using a nylon filter or Teflon-nylon filter combination in one sampling train. In a parallel sampling train, particulate nitrate alone is measured by first passing the airstream through a denuder to re-... 

Table 11.7, for example, summarizes measurements of the components of NOy made at Niwot Ridge, Colorado, in mid-1987 (Ridley et al., 1990a). The sum of NO, (NO + N02), HN03, particulate nitrate, PAN,... 

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%. 

FIGURE 11.27 Ratio of the sum of (NO + N02 + PAN + HNO, + particulate nitrate) measured individually to total measured NO, at three sites in the United States (Whitetop Mountain, Tennessee Bondville, Illinois and Scotia, Pennsylvania) and one in Canada (Egbert, Ontario). The bars represent the range of results and the mid-range lines the median values (adapted from Parrish et al., 1993). 

Figure 11.28 shows similar data for measurements made at Idaho Hill, Colorado, in the fall of 1993 (Williams et al., 1997). Measurements were made of NO, N02, PAN, PPN, HN03, and particulate nitrate, as well as total NO. Two sets of meteorological conditions were encountered, one where the wind was downslope and from the west where there were few sources nearby, and one where the wind was upslope, carrying pollutants from urban areas to the east. Figure 11.28a shows that for upslope air masses from the east with relatively fresh emissions, the sum of the measured compounds accounts, within experimental error, for the total NOy. The average ratio of NOy/total NOy was 1.06 + 0.15. On the other hand, during periods with cleaner, downslope air from the west (which has also had more time to react), the sum of the individual compounds frequently does not add up to the total measured NOy (Fig. 11.28b). The deficit ranges...

FIGURE 11.28 Ratio of sum of individual compounds (NO + N02 + PAN + PPN + HN03 + particulate nitrate) to total measured NO, under two types of overall meteorological conditions (a) episodes with winds from the south and east with fresh emissions and (b) winds primarily from the west with cleaner but more aged air (adapted from Williams et al., 1997). 

Buhr, S. M., M. P. Buhr, F. C. Fehsenfeld, J. S. Flolloway, U. Karst, R. B. Norton, D. D. Parrish, and R. E. Sievers, Development of a Semi-Continuous Method for the Measurement of Nitric Acid Vapor and Particulate Nitrate and Suflate, Atmos. Environ., 29, 2609-2624 (1995). 

Trends in particulate matter with diameters less than 10 nm, PM10, in the Los Angeles air basin from 1985 to 1994 are shown in Fig. 16.43 (Prasad, personal communication, 1997). A downward trend in both the mean and the maximum levels is evident, occurring along with similar trends in 03, NOx, and particulate nitrate and sulfate in this region (see Figs. 16.36 and 16.37). This is qualitatively consistent with expectations... 

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. 

The need to determine accurately the phase-specific concentrations of these pollutants reflects several concerns Compared to gaseous materials, particle-phase materials may penetrate more deeply into the human respiratory tract particle-phase pollutants scatter light much more effectively than gaseous materials, and they thus have a greater contribution to visibility reduction gaseous nitric acid has a much higher deposition velocity than particulate nitrates and can be a substantial contributor to the acidification of lakes, streams, forests, and vegetation. 

Common Pathways for Loss of Nitric Acid (Without Artifact Particulate Nitrate Formation). Introduction. Loss of HN03 can result from sorption on the surfaces of sampler inlets and inlet lines this situation is common to all collection procedures for atmospheric HN03. Such inlet losses are rarely determined, so it is difficult to assess the accuracy of atmospheric HN03 measurements or to pinpoint the source of bias between methods in intermethod comparisons (e.g., see reference 26). In addition to sampler-associated losses, N03 can be lost from samples during storage. 

Table II. Summary of Manual Techniques and Sources of Error in Sampling Particulate Nitrate and Nitric Acid ...

Commercial Solvents Corp Safe Aqueous Slurries of Particulate Nitrated Explosives and Polyacrylamides... 

Nitrate (N03 ) behaves differently at FKL and the GAA. At FKL on average, about 94% of particulate nitrate (N03 ) is associated with coarse particles, strongly... 

Practical methods also have been reported for semicon-tinuous measurement of nitrate and carbon in particles from ambient air. For example, an instrument for nitrate monitoring uses collection of particles on an impactor surface, followed by flash volatilization and determination of the nitrate present using a chemiluminescence technique. Ion chromatographs also have been adopted for semicontinuous determination of gaseous and particulate nitrate. Real-time carbon analyzers also are available, one of which uses differential thermal analysis of impactor-collected material. 

Inorganic particulate nitrate products would be added if it were necessary to complete the material balance to account for nitrogen (36). 

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