Sulfate, atmospheric aerosols

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]. 

Considerable time elapsed before there was general acceptance of Haagen-Smit s important discovery, in part because of its subtle nature. For the first time, a major air pollution problem was demonstrated to be caused by a pollutant generated in the atmosphere. Its effect often did not become apparent until many miles downwind from the source. (The same suspicion has been attached to sulfate-containing aerosols for many years, but the proof that the sulfate is damaging is not as well established.) In addition, a new pollution source, automobile exhaust, had been shown to be of prime importance. 

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-... 

Oatis, S., D. Imre, R. McGraw, and J. Xu, Heterogeneous Nucleation of a Common Atmospheric Aerosol Ammonium Sulfate, Geophys. Res. Lett., 25, 4469-4472 (1998). 

The amount of atmospheric aerosol sulfate, expressed as the column burden Bso2-. This term can... 

Toon, O. B., J. B. Pollack, and B. N. Khare, 1976. The optical constants of several atmospheric aerosol species ammonium sulfate, aluminum oxide and sodium chloride, J. Geophys. Res.. 81, 5733-5748. 

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... 

Descriptions of analytical methods for strong acid and acidic sulfate content of atmospheric aerosols have been reviewed (6-10). Methods for acidic aerosol determination are reviewed in this chapter according to the measurement principle either filter collection and post-collection extraction, deriv-atization or thermal treatment, and analysis or in situ collection (real-time or stepwise) and analysis. 

One other in situ technique can be used to determine fractional acidity in atmospheric aerosols by means of Fourier transform infrared (FTIR) spectroscopy (46). Originally, impactor samples were collected and were pressed into a KBr matrix, and then the IR spectrum was taken by attenuated total reflectance (ATR) FTIR spectroscopy to determine relative acidity, based on differences in absorption bands for sulfate and bisulfate species. Aerosols with [H+]/[S042 ] ratios greater than 1 could also be qualitatively identified. More recent innovations in the FTIR technique (47, 48) have made possible... 

Continuous Sampling and Determination. There are no truly continuous techniques for the direct determination of sulfuric acid or other strong acid species in atmospheric aerosols. The closest candidate method is a further modification of the sensitivity-enhanced, flame photometric detector, in which two detectors are used, one with a room-temperature de-nuder and one with a denuder tube heated to about 120 °C. Sulfuric acid is potentially determined as the difference between the two channels. In fact, a device based on this approach did not perform well in ambient air sampling (Tanner and Springston, unpublished data, 1990). Even with the SF6-doped H.2 fuel gas for enhanced sensitivity, the limit of detection is unsuitably high (5 xg/m3 or greater) because of the difficulty in calibrating the two separate FPD channels with aerosol sulfates. 

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.

Addition of EGA to the analysis of atmospheric aerosol particles has permitted an independent speciation and determination of the nitrogenous component for samples which have not had chemical or physical pretreatment. The discovery from ESCA analyses that a substantial fraction of the particulate nitrogen exists chemically bound to the carbonaceous fraction has been confirmed by EGA. The indication from ESCA and EGA that inorganic sulfate... 

Accomplishment of the complex observational experiment LACE-98 made it possible to obtain extensive information about atmospheric aerosol (aircraft measurements of the size distribution and number density of fine aerosols, coefficients of aerosol absorption, backscattering and depolarization, chemical composition of aerosol, as well as surface observations of the spectral optical thickness of the atmosphere, coefficients of extinction and backscattering). Fiebig et al. (2002) compared the observational data on optical parameters obtained from the results of numerical modeling for total H2S04 aerosol near the tropopause as well as for the ammonium sulfate/soot mixture in the remainder of the air column (Osborne et al., 2004). 

Atmospheric Aerosol Non-Seasalt Sulfate Non-Remote Marine -10 to +13 San Francisco Bay -10 to +12 N.W. Atlantic + 7 to +13 HBEF, non-urban US +1 to +4 Miami, Florida +1 to +2 Mauna Loa Observatory +4 to +6 Ludwig (22) Gravenhorst (22) Saltzman et al. (24) Saltzman (pers. commun.) Zoller Kelly (pers. commun.)... 

Atmospheric Aerosol Sulfate. Isotope measurements of non-seasalt sulfate in marine aerosols (24.52.631 require that sulfate from sea spray be either physically or mathematically removed from the sample medium. Mathematically, mass balance relationships are used to correct the value for the presence of seasalt sulfate in the sample. Physical means employ impactors or cyclone separators to segregate particles based on size so that value for non-seasalt sulfate can be directly measured. 

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 is now well known that mass-independent isotopic compositions are observed in atmospheric aerosol sulfates (Lee and Thiemens, 1997, 2001 Lee et al., 1998, 2001a,b, 2002 Savarino et al., 2000 Johnson et al., 2001). The details of these papers are reviewed in Thiemens et al. (2001) and Thiemens (2002). The measurements of the 5 0 isotopic composition of aerosol sulfate have revealed the following ... 

NH3 and to a lesser extent mono-, di-, and trimethylamines are the only significant gaseous bases in the atmosphere, and there has been considerable interest in whether the oceans are a source or sink of these gases. Early attempt to assess the air-sea flux from concentration measurements are probably suspect because of the ease with which sample contamination can occur during laboratory processing and analysis. It should be noted here that due to its high solubihty (low value of Henry s law constant), the air-water transfer of NH3 (and the methylamines for the same reason) is under gas phase control (see Section 6.03.2.1.1). The first reliable measurements were probably from the North and South Pacific and indicated that the flux of NH3 from sea to air is of a size similar to that for emission of DMS (Quinn et al., 1990, 1988). Indeed, the authors showed that this similarity was mirrored in the molar ratio of (non-sea-salt) sulfate to ammonium (1.3 0.7) in atmospheric aerosol particles collected on the cruise, indicating that for clean marine air remote from terrestrial sources, the emission of DMS and NH3 from the sea appears to control the composition of the aerosol. 

An important example related lo the atmospheric aerosol is the droplet containing dissolved sulfates that form as a result of the oxidation of SO2 in solution. The sulfates may be present a.s sulfuric acid or in a partially neutralized form as ammonium salts or metallic salts from sources such as flyash. The droplet size distribution and chemical composition are determined by a combination of thermodynamic and rate processes. In this section, we consider only equilibrium thermodynamics as it affects the vapor pressure of the drop. 

Sulfate ion i.s the chemical component usually present in highest concentration in the submieron atmospheric aerosol. Almo.st all of the sulfate results from the atmospheric oxidation of SO either by homogeneous gas-phase reactions or by aerosol- or droplet-phase reactions. Reaction with the hydroxyl radical OH is thought to be the major ga.s-phase mechanism. Many solution-phase processes are possible, including reaction with dissolved HiO and reactions with 0 catalyzed by dissolved metals such as Fe and Mn (Seinfeld and Pandis, 1998). 

---