Nitrate secondary aerosol

FIGURE 3-5 Hourly variatioiis of secondary aerosol organics, nitrates, sulfates, and ammonium as percent of total aerosol. Pasadena, Califixnia, July 25, 1973. Reprinted with permis from Grosjean and Friedlandn. ... 

Chemical radicals—such as hydroxyl, peroxyhydroxyl, and various alkyl and aryl species—have either been observed in laboratory studies or have been postulated as photochemical reaction intermediates. Atmospheric photochemical reactions also result in the formation of finely divided suspended particles (secondary aerosols), which create atmospheric haze. Their chemical content is enriched with sulfates (from sulfur dioxide), nitrates (from nitrogen dioxide, nitric oxide, and peroxyacylnitrates), ammonium (from ammonia), chloride (from sea salt), water, and oxygenated, sulfiirated, and nitrated organic compounds (from chemical combination of ozone and oxygen with hydrocarbon, sulfur oxide, and nitrogen oxide fragments). ... 

The source contributions of aerosol formed from gaseous emissions, such as sulfate, nitrate and certain organic species, cannot be quantified by chemical mass balance methods, Watson (9>) proposes a unique source type which will put an upper limit on the contributions of secondary aerosol sources, but it cannot attribute those contributions to specific emitters. 

A receptor model analysis in western Germany separated nitrate-rich from sulphate-rich secondary aerosols, with the latter being accompanied with vanadium and nickel [7]. Such factor composition pinpoints to heavy oil combustion sources which can be found, e.g. in oil refineries, off-shore platforms and overseas ships. In addition, trans-boundary pollution from eastern European countries is a significant source. 

Nitrate-rich secondary aerosol, mainly composed of ammonium nitrate, on the other hand, is predominantly formed on medium spatial scale and hence may have its major origin within Germany. Large sources for NH3, one of the aerosol precursor compounds, are located in the agricultural areas of north-western Germany ( Swine belt ). The other precursors, oxides of nitrogen, stem mainly from industrial and traffic-related combustion sources. 

The reason why SIA is higher in urban areas is less obvious as these are secondary aerosols. The observed increment is predominantly caused by more nitrate and sulphate. The reaction of nitric acid and sulphuric acid with the sea-salt aerosol in a marine urbanised environment follows an irreversible reaction scheme. In essence, the chloride depletion stabilises part of the nitrate and sulphate in the coarse mode and may partly explain part of the observed increment. However, it also raises the question how to assign the coarse mode nitrate in the mass closure. The sea salt and nitrate contributions cannot simply be added any more as nitrate replaces chloride. Reduction of NOx emissions may cause a reduction of coarse mode nitrate, which is partly compensated by the fact that chloride is not lost anymore. A reduction would yield a net result of ((N03-C1)/N03 = (62-35)/62=) 27/62 times the nitrate reduction (where the numbers are molar weights of the respective components), and this factor could be used to scale back the coarse nitrate fraction in the chemical mass balance. A similar reasoning may be valid for the anthropogenic sulphate in the coarse fraction. Corrections like these are uncommon in current mass closure studies, and consequences will have to be explored in more detail. 

Water is a major component of the accumulation mode aerosol in amounts that depend on the relative humidity. The uptake of water is driven by the strongly hygroscopic nature of the secondary aerosol components, especially the ainmoniuin sulfates and nitrate. The water content depends in a complex way on both the inorganic and organic components. The resulting aerosol phase solutions are likely to be highly concentrated compared with fog droplets, for example. 

The sources and chemical compositions of the fine and coarse urban particles are different. Coarse particles are generated by mechanical processes and consist of soil dust, seasalt, fly ash, tire wear particles, and so on. Aitken and accumulation mode particles contain primary particles from combustion sources and secondary aerosol material (sulfate, nitrate, ammonium, secondary organics) formed by chemical reactions resulting in gas-to-particle conversion (see Chapters 10 and 14). 

CMB Application to Central California PM Chow et al. (1992) apportioned source contributions to aerosol concentrations in the San Joaquin Valley of California. The source profiles used for CMB application are shown in Table 26.1. The standard deviations oa.. of the profiles (three or more samples were taken) are also included. To account for secondary aerosol components in the CMB calculations, ammonium sulfate, ammonium nitrate, sodium nitrate, and organic carbon were expressed as secondary source profiles using the stoichiometry of each compound. The average elemental concentrations observed at one of the receptors—Fresno, California, in 1988-1989— are shown in Table 26.2. The ambient concentrations of some species (c.g., Ga, As, Y, Mo, Ag) included in the source profiles were below the detection limits. These species... 

As aerosols in the atmosphere originate from multiple sources, the composition of nutrients changes considerably over the four seasons. Nitrate, NOJ, and NHJ are defined as secondary aerosol-associated species and are not associated with primary aerosols. The combustion of fossil fuels is a significant source of NO, whereas NHj may originate from anthropogenic emissions such as animal waste and the application of chemical fertilizers. 

Soil-derived partieles sueh as aluminosilicates, CaCOs, and Si02 are the seeond most abundant partiele type in indoor environment of subway stations. The relative abundanee of the sod-derived particles in subway stations are the lowest in the tunnel and the highest at the waiting room. Also secondary particles sueh as nitrates and sulfates are more abundantly encountered in the waiting room than in the platform area. The soil-derived and secondary aerosol particles are likely from the outdoor atmosphere. Therefore, the contents of those particles are higher for the samples eoUeeted at the loeations closer to the outdoor. 

The presence of hydrocarbons, therefore, can accelerate the oxidation of nitric oxide to NO2, which in turn reacts in the light to produce more ozone. Atmospheric oxygen is the source of oxidation capacity for both this reaction and the transformation of propene to more oxidized compounds. While the ultimate fate of much of the hydrocarbon is oxidation to carbon dioxide, it is equally important to note that this complex web of reactions involves many volatile intermediates, such as formaldehyde (HCHO), acetaldehyde (CH3CHO), and PAN (peroxyacetyl nitrate, not shown in Fig. 4.40), all of which can cause human health effects and/or damage to materials. The oxidation process also leads to the production of secondary aerosols, which are responsible for the decreased visibility caused by smog. 

Review of the literature provides ample evidence that aerosol formation is an important part of the atmospheric chemistry linked with photochemical-oxidant production. The important chemical constituents of concern include sulfate, nitrate, and secondary organic material. 

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

Sulphates (SO42 ), ammonium (NH4+) and nitrates (NO3 ) are the main secondary inorganic aerosol ions as they account for about two thirds of the total ionic mass in PMj and for about 50% in PM10 in Athens [21], These ions represent two different major source categories fuel combustion and vehicular circulation. Theodosi et al. [21] studied the spatial variability of these ions in 2 sites within GAA Lykovrisi (LYK) and Goudi (GOU). LYK is a moderately populated municipality, in the northern part of the GAA, 10 km from the city centre GOU is located... 

Also special care should be taken to reduce uncertainties on emission data and measurements. The validation of an aerosol model requires the analysis of the aerosol chemical composition for the main particulate species (ammonium, sulphate, nitrate and secondary organic aerosol). To find data to perform this kind of more complete evaluation is not always easy. The same applies to emissions data. The lack of detailed information regarding the chemical composition of aerosols obliges modellers to use previously defined aerosols components distributions, which are found in the literature. Present knowledge in emission processes is yet lacunal, especially concerning suspension and resuspension of deposited particles [37]. 

Oxidation/hydroxylation of aromatic compounds by OH and HOONO is expected to enhance their degradation rate and hence decrease their lifetime on particulate matter, which in the case of pollutants is beneficial from the point of view of human health. Oxidation of PAHs could also lead to the production of photosensitizers such as quinones and aromatic carbonyls [10, 40, 41]. These compounds, if present in the gas phase, are also able to form aggregates and are therefore involved in the formation of secondary organic aerosol [42]. In contrast, nitration induced by OH + N02 or HOONO could lead to highly mutagenic nitro-PAHs [43] or phytotoxic nitrophenols [44, 45], in which case the health and environmental impact of the reaction intermediates is not negligible and is sometimes higher than that of the parent molecules. 

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