Nitrates atmospheric aerosols

All of these species are very soluble in a rain or cloud drop and are an important source of atmospheric aerosols. For ammonia and ammonium, the condensed phases (I and s) represent approximately two-thirds of the total atmospheric burden, whereas for nitric acid and nitrates, about two-thirds is in the gas phase (Soderlund and Svensson, 1976). 

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

Atmospheric aerosols are hygroscopic, taking up and releasing water as the RH changes (see also Section C.l) because some of the chemical components are themselves deliquescent in pure form. For example, sodium chloride, the major component of sea salt, deliquesces at 298 K at an RH of 75%, whereas ammonium sulfate, (NH4)2S04, and ammonium nitrate, NH4N03, deliquesce at 80 and 62% RH, respectively. (See Table 9.16 for the deliquescence points of some common constituents of atmospheric particles.) De-... 

Atmospheric aerosols are complex mixtures of particles derived from diverse sources. Soot from diesel engines, fly ash from coal combustion, and sulfates, nitrates, and organic compounds produced by atmospheric reactions of gaseous pollutants all contribute to the aerosol. Particle size and composition depend upon the conditions of aerosol formation and growth and determine the effects of atmospheric aerosols on human health, ecosystems, materials degradation, and visibility. Much of the research on environmental aerosols has focused on fine particles ranging from a few micrometers in... 

Fig. 5. An analysis of a coarse atmospheric aerosol extract by CE and IC [49]. CE conditions a 57 cmX75 xm I.D. capillary, distance to detector, 50 cm. Electrolyte 2.25 mM PMA (pyromel-litic acid), 0.75 mM HMOH (hexamethonium hydroxide), 6.50 mM NaOH and 1.60 mM TEA (triethanolamine), pH 7.7 or 2.0 mM NDC (2,6-naphthalenedicarboxylic acid), 0.5 mM TTAB (tetradecyltrimethylammonium bromide) and 5.0 mM NaOH, pH 10.9 30 kV (PMA) or 20 kV (NDC) pressure injection for 10 s indirect UV detection at 254 nm (PMA) or 280 nm (NDC). IC conditions an IonPac-ASlO column with an IonPac-AGlO guard precolumn conductivity detection using an anion self-regenerating suppressor (ASRS-I) in the recycle mode. Analytes 2, chloride 3, sulfate 5, nitrate 6, oxalate 7, formate 10, hydrocarbonate or carbonate 11, acetate 12, propionate 14, benzoate.

The study described here demonstrates that ESCA provides information regarding the chemical nature of the surface of an unperturbed sample which would be difficult to acquire by other methods. A major weakness of ESCA, the necessity of exposing the sample to vacuum, together with its attendant problem of sample volatilization, can also be one of its strengths. The volatility of some nitrogenous species in atmospheric aerosol particles can be used to provide strong evidence for chemical identity of ionic compounds (e.g., ammonium nitrate) rather than simply ionic identities as provided by wet chemical methods. This volatility is accelerated by x-ray irradiation, so that similar results could be achieved only by extended vacuum exposure alone if another analytical technique were used. Also, with ESCA, volatile losses can be conveniently monitored since the sample remains in the spectrometer throughout the process. 

An increase of C02 concentration in the atmosphere does not determine substantial fertilization of marine bioproductivity—but does lead to pH decrease. As temperature grows, C02 assimilation by the ocean decreases, but C02 emissions due to upwellings are reduced and the transport of excess carbon to deep layers of the ocean diminishes. The anthropogenically induced input of nutrients to the oceans through river run-off and deposition of atmospheric aerosols (especially nitrate and iron as elements of atmospheric aerosols) can affect bioproductivity. 

Another debatable approach to pollution control involves the methods currently used to reduce hydrocarbons and CO in automotive exhausts. The need to control CO is based on its direct health effects while the need to control the hydrocarbons is based on their interactions with the N02 photolytic cycle which leads to elevated concentrations of N02, 03, peroxyacyl nitrates, and aerosols. The solution adopted was to increase the efficiency of the combustion process, thereby reducing hydrocarbon and CO emissions. Unfortunately, the method adopted also leads to dramatic increases in NO emissions. When this increase in NO was objected to, the answer came back that increased NO in the atmosphere is beneficial since it rapidly reacts with and destroys ozone, one of the very health-related substances requiring control. This is another example of failure to view the total air pollution system. Of course NO destroys 03, but one product of this reaction is N02 which is also detrimental to health. Furthermore, this N02 is the beginning point of sunlight absorption which leads to all the products of photochemical interactions. In a certain location excess NO will tend to reduce 03 levels. However, downstream of these locations excess N02 will promote more photochemical reactions and perhaps even higher ozone levels. In part this nonsolution to automotive pollution may be a major cause of the substantial increases in ozone in many areas during the past few years. This automotive example clearly illustrates the need for in-depth analysis when plans are made to change any part of the system of air pollution. Decisions based on such an analysis are all the more important because the tradeoffs involve human health and welfare. 

Figure 1 shows a schematic of a typical atmospheric aerosol particle (if such an entity can be assumed to exist). The particle consists of sulfates, nitrates, water, ammonium, elemental and organic carbon, metals, and dust. After a primary particle is emitted, gas-phase reactions occur, converting oxides of nitrogen to nitric acid, sulfur dioxide to sulfuric acid, and hydrocarbons to oxidized, low-vapor-pressure condensable organics. 

It has been shown that atmospheric aerosol nitrate possesses one of the largest mass-independent oxygen isotopic compositions observed in nature... 

Finally, in the case of present-day aerosol nitrate isotopic measurements, samples collected as a function of particle size provide another level of detection in the resolution of sources and atmospheric transformation mechanisms. Large particles (1-10 pm) typically are crustal or oceanic sea spray, depending upon where the particular samples are collected. Small particles (less than 0.1 pm) generally are gas-to-particle conversion process products. Using combined multi-isotope ratio measurements and size-fractionated collection processes, it is possible to provide sophisticated details of atmospheric aerosol fates. 

SANIQMI RD 4600430.21 Method guidelines. Method of measurement of mass concentration of fluorine and chlorine anions, hydro-phosphates, nitrates and sulphates in atmospheric aerosols, using the technique of ionic chromatography. SANIGMI (1993a) (in Russian). 

In addition to the above gases, ammonia (NH3) is also an important atmospheric trace substance. An essential characteristic of ammonia and NOx is that these trace gases transform in the air into ammonium and nitrate-containing aerosol particles. These particles are of importance for the control of many atmospheric processes (see Chapter 4). 

Wet-chemical analyses of aqueous extracts of aerosol samples have established the presence of anions such as sulfate, nitrate, and the halides, and of cations such as ammonium and the ions of the alkali and alkaline earth elements. Table 7-13 shows selected data to illustrate the abundances of important inorganic components in the urban, continental, arctic, and marine aerosols. Included for comparison are the concentrations of silicon, aluminum, and iron, which are the major elements of crustal origin. They occur in oxidized form, such as in aluminosilicates, which are practically insoluble. Taken together, the elements listed in Table 7-13 account for 90% of all inorganic constituents of the atmospheric aerosol. 

The concentration of atmospheric aerosols varies considerably in space and time. This variability of the aerosol concentration field is determined by meteorology and the emissions of aerosols and their precursors. For example, the annual average concentration of PM2.5 in North America varies by more than an order of magnitude as one moves from the clean remote to the polluted urban areas of Mexico City and southern California (Figure 8.24). Sulfate dominates the fine aerosol composition in the eastern United States, while organics are major contributors to the aerosol mass everywhere. Nitrates are major components of the PM2.5 in the western United States. The EC makes a relatively small contribution to the particle mass in many areas, but because of its ability to absorb light and its toxicity, it is an important component of atmospheric particulate matter. 

Atmospheric aerosols at high relative humidities are aqueous solutions of species such as ammonium, nitrate, sulfate, chloride, and sodium. Cloud droplets, rain, and so on are also aqueous solutions of a variety of chemical compounds. 

Stelson and Seinfeld (1981) have shown that solution concentrations of 8-26 M can be expected in wetted atmospheric aerosol. At such concentrations the solutions are strongly nonideal, and appropriate thermodynamic activity coefficients are necessary for thermodynamic calculations. Tang (1980), Stelson and Seinfeld (1982a-c), and Stelson et al. (1984) have developed activity coefficient expressions for aqueous systems of nitrate, sulfate, ammonium, nitric acid, and sulfuric acid at concentrations exceeding 1M. 

Solution of (10.96) for a given temperature requires calculation of the corresponding molalities. These concentrations depend not only on the aerosol nitrate and ammonium but also on the amount of water in the aerosol phase. Therefore calculation of the aerosol solution composition requires estimation of the aerosol water content. As we have seen in Section 10.2.1, the water activity will be equal to the relative humidity (expressed in the 0-1 scale). While this is very useful information, it is not sufficient for the water calculation. One needs to relate the tendency of the aerosol components to absorb moisture with their availability and the availability of water given by the relative humidity. In atmospheric aerosol models (Hanel and Zankl 1979 Cohen et al. 1987 Pilinis and Seinfeld 1987 Wexler and Seinfeld 1991) the water content of aerosols is usually predicted using the ZSR relationship (Zdanovskii 1948 Stokes and Robinson 1966)... 

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