Atmospheric Environment (v.41, #S)

Organic Matter of the Troposphere—II. by Bernd R.T. Simoneit; Monica A. Mazurek (4-24).
Higher plant waxes are the predominant natural components in the lipid fractions (> C15) of aerosols sampled over rural and oceanic regions. Hydrocarbon, fatty acid, ketone and fatty alcohol fractions of the lipids were characterized in terms of their contents of homologous compound series and specific biogenic molecular markers. Particulate samples from the rural western United States have been analyzed and compared with samples from urban Los Angeles and remote areas over the Atlantic Ocean. The samples from rural sites contained predominantly vascular plant wax and lesser amounts of higher plant sterols and resin residues. Urban samples and, to varying degrees, some rural samples contained primarily higher weight residues of petroleum products. The loadings of hydrocarbons derived from higher plant waxes ranged approximately from 10 to 160 ng m−3 of air (for fatty acids, 10–100 ng m−3 and for fatty alcohols, 10–200 ng m−3). Higher molecular weight lipids (i.e. plant epicuticular wax, terpenes, etc.) from flora comprise a significant component of the organic carbon in rural aerosols. Primary biogenic residues are major components of aerosols in all areas and they are important components in the global cycling of organic carbon.

The physical characteristics of sulfur aerosols by Kenneth T. Whitey (25-49).
A review of the physical characteristics of sulfur-containing aerosols, with respect to size distribution of the physical distributions, sulfur distributions, distribution modal characteristics, nuclei formation rates, aerosol growth characteristics, and in situ measurement, has been made.Physical size distributions can be characterized well by a trimodal model consisting of three additive lognormal distributions.When atmospheric physical aerosol size distributions are characterized by the trimodal model, the following typical modal parameters are observed:1. Nuclei mode – geometric mean size by volume, DGVn, from 0.015 to 0.04  μm. σ gn=1.6, nucler mode volumes from 0.0005 over the remote oceans to 9  μm3  cm−3 on an urban freeway.2. Accumulation mode – geometric mean size by volume, DGVa, from 0.15 to 0.5  μm, σ ga=1.6–2.2 and mode volume concentrations from 1 for very clean marine or continental backgrounds to as high as 300  μm3  cm−3 under very polluted conditions in urban areas.3. Coarse particle mode – geometric mean size by volume, DGVc, from 5 to 30  μm, σ gn=2–3, and mode volume concentrations from 2 to 1000  μm3  cm−3.It has also been concluded that the fine particles (Dp <2  μm) are essentially independent in formation, transformation and removal from the coarse particles (Dp >2  μm).Modal characterization of impactor-measured sulfate size distributions from the literature shows that the sulfate is nearly all in the accumulation mode and has the same size distribution as the physical accumulation mode distribution.Average sulfate aerodynamic geometric mean dia. was found to be 0.48±0.1  μm (0.37±0.1  μm vol. dia.) and σ g=2.00±0.29. Concentrations range from a low of about 0.04 μg m−3 over the remote oceans to over 8 μg m−3 under polluted conditions over the continents.Review of the data on nucleation in smog chambers and in the atmosphere suggests that when SO2, is present, SO2-to-aerosol conversion dominates the Aitken nuclei count and, indirectly, through coagulation and condensation, the accumulation mode size and concentration. There are indications that nucleation is ubiquitous in the atmosphere, ranging from values as low as 2 cm−3  h−1 over the clean remote oceans to a high of 6×106  cm−3  h−1 in a power plant plume under sunny conditions.There is considerable theoretical and experimental evidence that even if most of the mass for the condensational growth of the accumulation mode comes from hydrocarbon conversion, sulfur conversion provides most of the nuclei.

Methods for estimating the dry deposition velocities of atmospheric gases in the U.S. and surrounding areas have been improved and incorporated into a revised computer code module for use in numerical models of atmospheric transport and deposition of pollutants over regional scales. The key improvement is the computation of bulk surface resistances along three distinct pathways of mass transfer to sites of deposition at the upper portions of vegetative canopies or structures, the lower portions, and the ground (or water surface). This approach replaces the previous technique of providing simple look-up tables of bulk surface resistances. With the surface resistances divided explicitly into distinct pathways, the bulk surface resistances for a large number of gases in addition to those usually addressed in acid deposition models (SO2, O3 NO x , and HNO3) can be computed, if estimates of the effective Henry's Law constants and appropriate measures of the chemical reactivity of the various substances are known. This has been accomplished successfully for H2O2, HCHO3 CH3CHO (to represent other aldehvdes), CH3O2H (to represent organic peroxides), CH3C(O)O2H, HCOOH (to represent organic acids), NH3, CH3C(O)O2NO2 and HNO2. Other factors considered include surface temperature, stomata1 response to environmental parameters, the wetting of surfaces by dew and rain, and the covering of surfaces by snow. Surface emission of gases and variations of uptake characteristics by individual plant species within the landuse types are not considered explicitly.
Keywords: Dry deposition; Surface resistance; SO2; Acid deposition; Numerical modeling.;

Recent data collected in the Arctic have demonstrated the transport of atmospheric aerosol of anthropogenic origin into that region. Concern over the radiative effect of the highly-absorbing soot component of this aerosol has resulted in a variety of atmospheric sampling efforts aimed at assessing the climatic impact of this component. However, little attention has been given to the measurement of soot deposited on the Arctic snowpack and the resulting perturbation of snow albedo, snowmelt rates and radiative transfer. Here we report measurements of light-absorbing material in the Arctic snowpack for longitudes from 25 E to 160 W. The contributions to light absorption due to natural crustal and soot aerosol are identified by their wavelength dependence. Reductions in Arctic snow albedo of one to several percent appear probable for the soot/ice mass fractions obtained to date. Estimates of the impact of this reduced albedo on the Arctic radiation budget over a season are shown to approximately equal that of the Arctic haze itself. The absorption of shortwave radiation by the springtime snowpack is estimated to be 5–10% higher than that of soot-free snow for this data.
Keywords: Arctic haze; soot aerosol; snow albedo; radiation budget; light absorption;

A partitioning model is developed to allow the modeling of the dynamics of secondary organic aerosol (SOA) formation. The gas/aerosol partitioning is assumed to be governed by equilibrium partitioning into an absorptive, well-mixed liquid (or at least amorphous) organic matter (om) phase. The model is represented using a set of coupled linear equations. It may be especially applicable when photochemical smog is being formed in the summer. The model permits (indeed, it requires) partitioning of a given compound i to occur even when i is present at a level below its saturation vapor pressure. During early periods of SOA formation, to determine the partitioning for each compound of interest, the model must be solved iteratively for each time and location of interest. Iteration is required because the partitioning is assumed to be governed by mole fraction concentrations in the om phase, and because prior to solving the problem, one does not know the total number of mols of condensed compounds in the om phase. During later stages of SOA formation, if the amount and general composition of the SOA begin to become constant, the partitioning coefficient of each of the compounds will also stabilize, and an iterative solution will be less needed.
Keywords: Secondary organic aerosol formation; Smog; Gas-particle partitioning; Gas-particle distribution; Absorption; Adsorption; Sorption; Organic compounds; Particulate material; Urban particulate material; Total suspended particulate material; TSP;

A gas-phase reaction mechanism for the atmospheric photooxidations of over 100 alkanes, alkenes, aromatic hydrocarbons, alcohols, ethers and other compounds representative of the range of reactive organics emitted into polluted atmospheres is described. Most of these organic species are represented using generalized reactions with variable rate constants and product yield coefficients for which individual assignments were made or estimated. This mechanism employs 19 species to represent the reactive oxygenated and organic nitrate products, and includes the gas-phase reactions of SO2, but does not include heterogeneous or liquid-phase reactions. The evaluation of this mechanism, by comparison of its predictions against the results of over 500 environmental chamber experiments, is described in a separate paper. This detailed mechanism can be used in assessments of relative atmospheric reactivities of organic compounds, and can provide the basis for the derivation of more condensed mechanisms for use in air quality simulation models.
Keywords: Atmospheric chemistry; Photochemical smog; Air pollution; Computer modeling; Air quality simulation models; Kinetic mechanisms; Gas-phase reactions; Organic compounds; Alkanes; Alkenes; Aromatic hydrocarbons; Oxygenated organic compounds; Organic nitrates; Ozone; Atmospheric reactivity;

2004 Introductions (118-119).

Pollution and the Planetary Albedo by S. Twomey (120-125).
Addition of cloud nuclei by pollution can lead to an increase in the solar radiation reflected by clouds. The reflection of solar energy by clouds already may have been increased by the addition of man-made cloud nuclei. The albedo of a cloud is proportional to optical thickness for thin clouds, but changes more slowly with increasing thickness. The optical thickness is increased when the number of cloud nuclei is increased. Although the changes are small, the long-term effect on climate can be profound.

Expressions for predicting the temperature and relative humidity dependence of the NH4NO3 dissociation constant are derived from fundamental thermodynamic principles. The general trends predicted by the theory agree with the atmospheric data of Appel et al. (1979,1980), Pitts (1978,1979) and Tuazon et al. (1980).

2003 Introductions (136-137).

Aqueous-phase oxidation of SO2 occurs via a sequence of steps consisting of gas-phase diffusion, mass transfer at the gas–water interface, hydrolysis and ionization of the dissolved sulfur-IV, aqueous-phase diffusion, and oxidation reaction. Expressions are given for the characteristic times of these several processes for reaction in aqueous droplets. Readily applicable criteria are developed in terms of these characteristic times to delimit the conditions, in the laboratory or in the ambient atmosphere, under which the rate of reaction in an aqueous droplet is equal to the intrinsic oxidation rate or is restricted by the 6nite rates of the several other processes. Under most conditions of concern in the ambient atmosphere, or in laboratory simulation of these conditions, the characteristic times of hydrolysis and ionization are sufilciently short compared to that of aqueous-phase reaction that the several dissolved sulfur-IV species may be considered to be a single pool of equilibrated reactant species. Similarly, the S(IV) solubility equilibria at the gas–water interface may also be considered to be achieved on a time scale that is short compared to that of aqueousphase reaction,except perhaps at high pH (pH>7) where the characteristic time of this process becomes long (∼10 sec at 25 °C) because of the high solubility of S(IV).A more detailed treatment of the problem of simultaneous diffusion and reaction establishes the domain of applicability of the steady-state assumption for reaction in aqueous dropkts. Within the. steady state approximation, we examine the magnitude of limitation to the overall rate of reaction resulting from tbe finite rate of mass transport in the gas and aqueous phases and from the finite rate of achieving the solubility equilibrium at the interface. Expressions are presented that permit this treatment to be readily applied to laboratory kinetic data.The foregoing treatment also permits examination of the conditions under which limitation to the overall rate of reaction is controlled by one or another of the above mechanisms. For gas- and aqueous-phase mass transport by molecular diffusion it is found (again for 25 °C) that gas-phase mass transport is more controlling than aqueous-phase mass transport for pH>3.3, and that the onset of departure from the solubility equilibrium at the phase interface is more controlling than gas-phase diffusion only for very small droplets (radius<0.16μm). For SO2 oxidation by atmospheric O2 aqueous-phase diffusion of O2 is more controlling than gas-phase diffusion of SO2 only for quite high SO2 partial pressure ( p SO 2 > 10 ppm ).

The process by which sulphur dioxide is oxidised in atmospheric droplets has been studied in laboratory experiments designed to collect a large amount of chemical data pertinent to the atmospheric situation. Thus the oxidation of sodium sulphite solutions by oxygen, ozone and hydrogen peroxide has been studied at different pH's and temperatures. In all cases the reaction is first order with respect to sulphite ion but the order with respect to oxidant differs. For oxygen the order is zero whereas the order for ozone and hydrogen peroxide is one. Varying the hydrogen ion concentration has little effect on the oxygen reaction rate between pH 6 and 9; the ozone reaction rate is inversely proportional to the square root of the hydrogen ion concentration and the hydrogen peroxide rate is almost directly proportional to the hydrogen ion concentration. These last two observations are very important since in the case of ozone it indicates that the reaction proceeds via a free radical mechanism involving hydroxyl radicals and in the case of hydrogen peroxide it is the only oxidation process of sodium sulphite so far investigated that shows a positive response to the presence of hydrogen ions.The experimental data was used to calculate the rate of sulphate formation in water droplets under atmospheric conditions for each of the three oxidants. If it is assumed that the ozone and hydrogen peroxide gas phase concentrations are initially 50 parts in 109 and 1 part in 109 by volume respectively, then the rates of sulphate formation are equal in cloud water at pH 5.8. Above this pH the ozone reaction is faster and below it the hydrogen peroxide reaction is faster due to the positive catalysis by hydrogen ions; the oxygen rate is unimportant by comparison at all pH's below 7. The rate of hydrogen peroxide reaction is such that substantial amounts of sulphate can still be formed rapidly in water droplets at pH values from 3 to 5, and thus this process will be very important in creating acidity in rainwater.

2002 Introductions (169-170).

The anthropogenic emissions of SO2 and NOx for 25 Asian countries east of Afghanistan and Pakistan have been calculated for 1975, 1980, 1985, 1986 and 1987 based on fuel consumption, sulfur content in fuels and emission factors for used fuels in each emission category. The provincial- and regional-based calculations have also been made for China and India. The total SO2 emissions in these parts of Asia have been calculated to be 18.3 and 29.1 Tg in 1975 and 1987, respectively. The calculated total NOx emissions were 9.4 and 15.5 Tg in 1975 and 1987, respectively. The SO2 and NOx emissions in East Asia (China, Japan, South Korea, North Korea and Taiwan) were 23.4 and 10.7 Tg in 1975 and 1987, respectively. Keyword: Emission inventories, sulfur dioxide emissions, nitrogen oxide emissions, Asian emissions, anthropogenic emissions.

Chamber experiments were conducted to study the aerosol products from the ozonolysis of the major atmospheric monoterpenes; α-pinene, β-pinene and limonene. It was found that the α-pinend–O3 reaction produced mainly 2′. 2′-dimethyl-3′-acetyl cyclobutyl ethanal (pinonaldehyde), the β-pinene–O3 reaction, mainly 6,6-dimethyl-bicyclo [3.1.1] heptan-2-one and the limonene–O3 reaction, several unidentified products. These products were sought in forest aerosols and pinonaldehyde was detected in the atmosphere.
Keywords: Terpenes; Ozonolysis; Natural aerosols; Pinonaldehyde.;

2001 Introductions (198-199).

The current knowledge of the gas-phase reactions occurring in the troposphere for alkanes, alkenes, alkynes, oxygenates and aromatic hydrocarbons and their photooxidation products is reviewed,and areas of uncertainty identified.
Keywords: Tropospheric chemistry; Organic compounds; Gas-phase reactions.;

Source apportionment of airborne particulate matter using organic compounds as tracers by James J. Schauer; Wolfgang F. Rogge; Lynn M. Hildemann; Monica A. Mazurek; Glen R. Cass; Bernd R.T. Simoneit (241-259).
A chemical mass balance receptor model based on organic compounds has been developed that relates source contributions to airborne fine particle mass concentrations. Source contributions to the concentrations of specific organic compounds are revealed as well. The model is applied to four air quality monitoring sites in southern California using atmospheric organic compound concentration data and source test data collected specifically for the purpose of testing this model. The contributions of up to nine primary particle source types can be separately identified in ambient samples based on this method, and approximately 85% of the organic fine aerosol is assigned to primary sources on an annual average basis. The model provides information on source contributions to fine mass concentrations, fine organic aerosol concentrations and individual organic compound concentrations. The largest primary source contributors to fine particle mass concentrations in Los Angeles are found to include diesel engine exhaust, paved road dust, gasoline-powered vehicle exhaust, plus emissions from food cooking and wood smoke, with smaller contribution from tire dust, plant fragments, natural gas combustion aerosol, and cigarette smoke. Once these primary aerosol source contributions are added to the secondary sulfates, nitrates and organics present, virtually all of the annual average fine particle mass at Los Angeles area monitoring sites can be assigned to its source.
Keywords: Receptor models; Organic aerosol; Fine particles; Source contributions; Emissions;