Atmospheric Environment (v.35, #1)

This review describes databases of small-scale spatial variations and indoor, outdoor and personal measurements of air pollutants with the main focus on suspended particulate matter, and to a lesser extent, nitrogen dioxide and photochemical pollutants. The basic definitions and concepts of an exposure measurement are introduced as well as some study design considerations and implications of imprecise exposure measurements. Suspended particulate matter is complex with respect to particle size distributions, the chemical composition and its sources. With respect to small-scale spatial variations in urban areas, largest variations occur in the ultrafine (<0.1 μm) and the coarse mode (PM10–2.5, resuspended dust). Secondary aerosols which contribute to the accumulation mode (0.1–2 μm) show quite homogenous spatial distribution. In general, small-scale spatial variations of PM2.5 were described to be smaller than the spatial variations of PM10. Recent studies in outdoor air show that ultrafine particle number counts have large spatial variations and that they are not well correlated to mass data. Sources of indoor particles are from outdoors and some specific indoor sources such as smoking and cooking for fine particles or moving of people (resuspension of dust) for coarse particles. The relationships between indoor, outdoor and personal levels are complex. The finer the particle size, the better becomes the correlation between indoor, outdoor and personal levels. Furthermore, correlations between these parameters are better in longitudinal analyses than in cross-sectional analyses. For NO2 and O3, the air chemistry is important. Both have considerable small-scale spatial variations within urban areas. In the absence of indoor sources such as gas appliances, NO2 indoor/outdoor relationships are strong. For ozone, indoor levels are quite small. The study hypothesis largely determines the choice of a specific concept in exposure assessment, i.e. whether personal sampling is needed or if ambient monitoring is sufficient. Careful evaluation of the validity and improvements in precision of an exposure measure reduce error in the measurements and bias in the exposure–effect relationship.
Keywords: Exposure assessment; Particles; PM10; PM2.5; NO2; O3; Bioaerosols;

Discrimination of micrometre-sized ice and super-cooled droplets in mixed-phase cloud by E. Hirst; P.H. Kaye; R.S. Greenaway; P. Field; D.W. Johnson (33-47).
Preliminary experimental results are presented from an aircraft-mounted probe designed to provide in situ data on cloud particle shape, size, and number concentration. In particular, the probe has been designed to facilitate discrimination between super-cooled water droplets and ice crystals of 1–25 μm size within mixed-phase clouds and to provide information on cloud interstitial aerosols. The probe acquires spatial light scattering data from individual particles at throughput rates of several thousand particles per second. These data are logged at 100 ms intervals to allow the distribution and number concentration of each particle type to be determined with 10 m spatial resolution at a typical airspeed of 100 m s−1. Preliminary results from flight data recorded in altocumulus castellanus, showing liquid water phase, mixed phase, and ice phase are presented to illustrate the probe's particle discrimination capabilities.
Keywords: Aircraft measurements; Cloud ice; Cloud droplets; Ice particle growth process; Light scattering;

The concentrations of total gaseous mercury (Hg) were determined at hourly intervals along with relevant environmental parameters that include both meteorological plus criteria pollutant data during two field campaigns (September 1997 and May/June 1998). The mean concentrations of Hg for the two study periods were computed as 3.94 and 3.43 ng m−3, respectively. By separating the data into daytime and nighttime periods, we further analyzed diurnal variation patterns for both seasons. Using our Hg data sets, we were able to recognize two contrasting diurnal variation patterns of Hg between two different seasons that can be characterized as: (1) the occurrences of peak Hg concentration during daytime (fall) and (2) slight reductions in daytime Hg concentration relative to nighttime (summer). To study the systematic differences in diurnal patterns between two different seasons, we analyzed Hg data in terms of different statistical approaches such as correlation (and linear regression) and factor analysis. Results of these analyses consistently indicated that different mechanisms were responsible for controlling the daytime distribution patterns of Hg. When the relationship between Hg and concurrently determined O3 is considered, its reaction with ozone is unlikely to limit Hg levels as the dominant sink mechanism (within the ranges of ozone concentrations found during this study, regardless of season). It is on the other hand suspected that the variation of boundary layer conditions between day/night periods may have been important in introducing the relative reduction in daytime Hg levels during summer. To further provide a general account of short-term variations in Hg distribution data, it is desirable to describe other unknown sink mechanisms.
Keywords: Urban; Diurnal; Nocturnal; Dilution; Ozone; Distribution; Source; Sink;

Chloroform – concentration gradients in soil air and atmospheric air, and emission fluxes from soil by Eddo J Hoekstra; Jan H Duyzer; Ed W.B de Leer; Udo A.Th Brinkman (61-70).
Since we demonstrated the natural formation of chloroform in soil, the question arose to which extent this contributes to the chloroform present in the atmosphere. Concentration gradients in soil air and atmospheric air of different forests were measured. Chloroform concentration gradients indicating emission occur in forest soils and the atmosphere under the canopy, whereas this was not observed for other chlorinated solvents. Above the canopy all concentration gradients observed for chloroform and 1,1,1-trichloroethane indicate deposition. The emission flux was measured using enclosures and calculated from the observed concentration gradients in soil air and atmospheric air. Wood-degrading areas and soils with a humic layer were found to emit up to 1000 ng chloroform m−2  h−1 and seem to be larger chloroform sources than the other areas of study. Rather unexpectedly, some points of one sampling site appeared to emit 1,1,1-trichloroethane, tetrachloromethane and tetrachloroethene. A reasonable agreement was found between the fluxes using enclosures and those derived from the concentration gradients in soil air and atmospheric air.
Keywords: (Soil) air concentration gradient; Terrestrial natural source; Chloroform; Tetrachloromethane; 1,1,1-trichloroethane;

Ground-based, high-resolution measurements of downward atmospheric thermal emission spectra are reported for a northern mid-latitude location for summer and winter conditions. These measurements clearly show the presence of the 11.3-μm thermal emission band of nitric acid situated between 850–920 cm−1. By using the FASCOD3 line-by-line radiation code to simulate the background thermal emission, the measured seasonally averaged surface radiative forcing due to nitric acid is determined to be 0.055 W m−2±15%. The zenith column amounts of nitric acid are found to vary between 7.9×1015 and 1.1×1016  molecules cm−2±15%. An estimation is made of the contribution of nitric acid to the direct radiative forcing of the Earth's surface since pre-industrial times for northern mid-latitudes. This work suggests that nitric acid may play a role that is comparable to that of other greenhouse gases, such as CFC-11, in the forcing of the Earth's climate system. Under polluted conditions, nitric acid may contribute about half of the radiative forcing that is currently associated with tropospheric ozone.
Keywords: Global warming; Greenhouse effect; Nitric acid; Radiative forcing; Climate change;

The degradation of organic matter in peat bogs is complex and not yet well understood. Recent investigations of the trace gases CO2 and CH4 focussed on the impact of these greenhouse gases on global warming. However, there have to be metabolic intermediates between complex organic structures (i.e., humic acids) and gaseous end products (CH4, CO2, N2, NO x and H2S) other than water-soluble substances (i.e., aromatic acids, amino acids, fatty acids). Deoxygenation during microbial decomposition of plant material also produces anoxic conditions that favor the formation of kinetically stable hydrocarbons. In this study, volatile organic substances (VOS) in peat bogs were investigated using two techniques: purge-and-trap and closed-loop stripping. Coupled gas chromatography–mass spectroscopy analysis revealed mainly branched hydrocarbons (C8H18) in concentrations up to 260 nM in peat pore-water. Additionally, alkylated benzenes were found in concentrations of up to 464 nM, in the peat pore-water, and up to 23 pptv in the headspace of peat cores. However, one-third of all the compounds in the complex VOS-fraction extracted from the peat system remain to be identified, especially those substances containing oxygen.
Keywords: Closed-loop stripping; Aromatic hydrocarbons; Alkanes; Production; Emission;

Decamethyl cyclopentasiloxane (D5) and decamethyl tetrasiloxane (MD2M) were injected into a smog chamber containing fine Arizona road dust particles (95% surface area <2.6 μM) and an urban smog atmosphere in the daytime. A photochemical reaction – gas–particle partitioning reaction scheme, was implemented to simulate the formation and gas–particle partitioning of hydroxyl oxidation products of D5 and MD2M. This scheme incorporated the reactions of D5 and MD2M into an existing urban smog chemical mechanism carbon bond IV and partitioned the products between gas and particle phase by treating gas–particle partitioning as a kinetic process and specifying an uptake and off-gassing rate. A photochemical model PKSS was used to simulate this set of reactions. A Langmuirian partitioning model was used to convert the measured and estimated mass-based partitioning coefficients (K P) to a molar or volume-based form. The model simulations indicated that >99% of all product silanol formed in the gas-phase partition immediately to particle phase and the experimental data agreed with model predictions. One product, D4TOH was observed and confirmed for the D5 reaction and this system was modeled successfully. Experimental data was inadequate for MD2M reaction products and it is likely that more than one product formed. The model set up a framework into which more reaction and partitioning steps can be easily added.
Keywords: Gas/particle partitioning; Siloxanes; Partitioning kinetics model; Road dust particles; Photochemistry;

The influence of dissolved NO2 and iron on the oxidation rate of S(IV) species in the presence of dissolved oxygen is presented. To match the conditions in the real environment, the concentration of iron in the reaction solution and trace gases in the gas mixture was typical for a polluted atmosphere. The time dependence of HSO3 , SO4 2−, NO2 and NO3 and the concentration ratio between Fe(II) and total dissolved iron were monitored. Sulphate formation was the most intensive in the presence of an SO2/NO2/air gas mixture and Fe(III) in solution. The highest contribution to the overall oxidation was from Fe-catalysed S(IV) autoxidation. The reaction rate in the presence of both components was equal to the sum of the reaction rates when NO2 and Fe(III) were present separately, indicating that under selected experimental conditions there exist two systems: SO2/NO2/air and SO2/NO2/air/Fe(III), which are unlikely to interact with each other. The radical chain mechanism can be initiated via reactions Fe(III)–HSO3 and NO2–SO3 2−/HSO3 .
Keywords: Acid rain formation; Aqueous S(IV) oxidation; Iron; NO2;

Survival of bacterial and mold spores in air filter media by R Maus; A Goppelsröder; H Umhauer (105-113).
The present study deals with the survival of bacterial and mold spores (B. subtilis, A. niger) in new and used air filter media. In an filtration test unit samples of different filter media were challenged with specific microbial aerosols and the viability or survival of the microorganisms collected in the filter media was studied. No notable decline or increase in the viability of B. subtilis in new or used filter samples was observed within 5 days. No differences were observed when filter media were either continuously exposed to air flow or stored under static conditions. No influence of relative humidity (RH=30–85%) on the viability of B. subtilis spores was detected as well. Under ideal humidity conditions (RH>98%) no bacterial growth occurred within all the investigated filter media which is due to the lack of nutrients. Similar results were obtained when employing A. niger spores at low relative humidities (RH<35%). However, in two new filter media the viability declined notably at high relative humidity (RH>85%). This trend is attributed to the combined effect of spore rehydration and diffusion of fiber substances into the spores which rendered the spores prone to air flow and air toxics. Under static conditions in a climatic chamber (RH>98%) abundant mold growth occurred in two filter media. The results indicate that atmospheric dust deposited in air filters may serve as nutrient for molds if humidity is sufficient and filters are not exposed to an air flow.
Keywords: Air filters; Bioaerosols; Viability; Filter material; Spores; Ventilation effects; Growth on filters;

Biogenic nitric oxide emissions from cropland soils by Paul A. Roelle; Viney P. Aneja; B. Gay; C. Geron; T. Pierce (115-124).
Emissions of nitric oxide (NO) were determined during late spring and summer 1995 and the spring of 1996 from four agricultural soils on which four different crops were grown. These agricultural soils were located at four different sites throughout North Carolina. Emission rates were calculated using a dynamic flow-through chamber system coupled to a mobile laboratory for in-situ analysis. Average NO fluxes during late spring 1995 were: 50.9±47.7 ng N m−2  s−1 from soil planted with corn in the lower coastal plain. Average NO fluxes during summer 1995 were: 6.4±4.6 and 20.2±19.0 ng N m−2  s−1, respectively, from soils planted with corn and soybean in the coastal region; 4.2±1.7 ng N m−2  s−1 from soils planted with tobacco in the piedmont region; and 8.5±4.9 ng N m−2  s−1 from soils planted with corn in the upper piedmont region. Average NO fluxes for spring 1996 were: 66.7±60.7 ng N m−2  s−1 from soils planted with wheat in the lower coastal plain; 9.5±2.9 ng N m−2  s−1 from soils planted with wheat in the coastal plain; 2.7±3.4 ng N m−2  s−1 from soils planted with wheat in the piedmont region; and 56.1±53.7 ng N m−2  s−1 from soils planted with corn in the upper piedmont region. An apparent increase in NO flux with soil temperature was present at all of the locations. The composite data from all the research sites revealed a general positive trend of increasing NO flux with soil water content. In general, increases in total extractable nitrogen (TEN) appeared to be related to increased NO emissions within each site, however a consistent trend was not evident across all sites.
Keywords: Nitric oxide; Biogenic emissions; Dynamic chamber; Agricultural soils;

Aerosols have several important influences on the climate system. Among the more important of these are their roles in absorbing and scattering radiation, and as condensation nuclei in cloud-forming processes. Despite their importance, knowledge of their spatial and temporal variability and, in turn, their influence on climate, is incomplete. Constraints associated with conventional approaches to measuring atmospheric turbidity – including the requirements for clear skies and costly equipment – have contributed to a paucity of turbidity data. This paper presents a methodology for estimating atmospheric turbidity from readily available surface-weather data, regardless of cloud cover. Using a high-resolution spectral radiation model, clear-sky beam irradiance is parameterized as a function of atmospheric attenuation processes, including scattering and absorption by aerosols. The model is integrated over the day to obtain an expression for estimating potential daily clear-sky beam irradiation. Turbidity can then be estimated by forcing the model with monthly averaged climate data. The methodology can be applied at any location where the requisite climate data are available and therefore holds promise for a more complete, and possibly global, climatology of aerosols.
Keywords: Aerosols; Climate; Radiation; Irradiance; Modeling;

Monoaromatic compounds in ambient air of various cities: a focus on correlations between the xylenes and ethylbenzene by Anne Monod; Barkley C Sive; Pasquale Avino; Tai Chen; Donald R Blake; F Sherwood Rowland (135-149).
Speciation of o-xylene, m-xylene, p-xylene and ethylbenzene was performed by gas chromatography from ambient air and liquid fuel samples collected at various locations in 19 cities in Europe, Asia and South America. The xylene's mixing ratios were compared to each other from the various locations, which included urban air, traffic air and liquid fuel. For all samples, the xylenes exhibited robust correlations, and the slopes remained constant. The m-xylene/p-xylene ratio was found to be 2.33±0.30, and the m-xylene/o-xylene ratio was found to be 1.84±0.25. These ratios remain persistent even in biomass combustion experiments (in South America and South Africa). Comparing the xylenes to toluene and benzene indicate that combustion, but not fuel evaporation, is the major common source of the xylenes in areas dominated by automotive emissions. Although a wide range of combustion types and combustion efficiencies were encountered throughout all the locations investigated, xylenes and ethylbenzene ratios remained persistent. We discuss the implications of the constancies in the xylenes and ethylbenzene ratios on atmospheric chemistry.
Keywords: Gas chromatography; m-xylene and p-xylene speciation; Alkylbenzenes; Tropospheric air; Urban plume;

A closure study of extinction apportionment by multiple regression by L.A. de P. Vasconcelos; E.S. Macias; P.H. McMurry; B.J. Turpin; W.H. White (151-158).
Multiple regression has been widely used to apportion particle light scattering among distinct chemical species. The resulting scattering budgets are shown here to be unbiased estimates under certain theoretical conditions. The theory allows species’ particle size distributions and water uptakes to vary from sample to sample, as they are known to do in reality. The sole constraint is that variations in each species’ characteristics be statistically independent of all species’ concentrations. Individual violations of this condition cause identifiable biases, and multiple violations can offset each other to yield regression estimates that are accurate by accident. Detailed and summary accountings of statistical errors are illustrated using a mechanistic model derived from measurements in southern California.
Keywords: Aerosols; Estimation; Scattering efficiency; Radiative forcing; Visibility; Southern California Air Quality Study (SCAQS);

Evaluation of the DMS flux and its conversion to SO2 over the southern ocean by Zang-Ho Shon; D. Davis; G. Chen; G. Grodzinsky; A. Bandy; D. Thornton; S. Sandholm; J. Bradshaw; R. Stickel; W. Chameides; G. Kok; L. Russell; L. Mauldin; D. Tanner; F. Eisele (159-172).
A total of 16 boundary layer (BL) DMS flux values were derived from flights over the Southern Ocean. DMS flux values were derived from airborne observations recorded during the Aerosol Characterization Experiment (ACE 1). The latitude range covered was 55°S–40°S. The method of evaluation was based on the mass-balance photochemical-modeling (MBPCM) approach. The estimated flux for the above latitude range was 0.4–7.0 μmol m−2  d−1. The average value from all data analyzed was 2.6±1.8 μmol m−2  d−1. A comparison of the MBPCM methodology with several other DMS flux methods (e.g., ship and airborne based) revealed reasonably good agreement in some cases and significant disagreement in other cases. Considering the limited number of cases compared and the fact that conditions for the comparisons were far from ideal, it is not possible to conclude that major agreement or differences have been established between these methods. A major result from this study was the finding that DMS oxidation is a major source of BL SO2 over the Southern Ocean. Model simulations suggest that, on average, the conversion efficiency is 0.7 or higher, given a lifetime for SO2 of ∼1 d. A comparison of two sulfur case studies, one based on DMS–SO2 data generated on the NCAR C-130 aircraft, the other based on data recorded on the NOAA ship Discoverer, revealed qualitative agreement in finding that DMS was a major source of Southern Ocean SO2. On the other hand, significant disagreement was found regarding the DMS/SO2 conversion efficiency (e.g., 0.3–0.5 versus 0.7–0.9). Although yet unknown factors, such as vertical mixing, may be involved in reducing the level of disagreement, it does appear at this time that some significant portion of this difference may be related to systematic differences in the two different techniques employed to measure SO2. It would seem prudent, therefore, that further instrument intercomparison SO2 studies be considered. It also would be desirable to stage new intercomparison activity between the MBPCM flux approach and the air-to-sea gradient as well as other flux methods, but under far more favorable conditions.
Keywords: Marine; Sulfur; Chemistry; DMS; SO2; ACE 1;

Box models are widely used in air pollution modeling. They allow the use of simple computational tools instead of the simulation of 3D Eulerian grid models, given by a large set of partial differential equations. We investigate here the theoretical justification of such box models. The key point is the comparison with the underlying Eulerian model describing the dispersion of pollutants in the atmosphere. We restrict the study to a vertical monodimensional case for more clarity. The main result is that the nonlinearity of the chemical kinetics, which is a characteristic feature of chemistry, induces the loss of accuracy.
Keywords: Box models; Reaction–diffusion; Chemical kinetics; Air pollution modelling;

NO y removal from the Cumberland Power Plant Plume by R.E. Imhoff; M. Luria; R.J. Valente; R.L. Tanner (179-183).
Airborne measurements were performed in the plume of the Cumberland Power Plant during August 1998 using a highly sensitive SO2 instrument. The measurements confirmed previous suggestions that NO y species are removed from the plume at a faster rate than SO2. The differential removal rate (the difference between loss rate of NO y and that of SO2) was estimated to be 0.06 h−1. This value implies that the NO y loss rate is in the range of 0.09–0.14 h−1. The application of a mathematical argument, based on the convolution integral, enabled improved synchronization of the data from the SO2 and NO y instruments. Examination of the synchronized data revealed that the concentration ratio of SO2 and NO y varies across the plume. Near the source it is higher at the wings of the plume, while in the core of the plume it is similar to the ratio at the release point. Two possible explanations of the observations are discussed: conversion to non-measurable NO y species, and in-plume loss of NO y (as HNO3) via dry deposition.
Keywords: Power plant plume; Oxides of nitrogen (NO y ); Nitric acid (HNO3); Sulfur dioxide (SO2); Differential loss rate;

New Directions: VOCs and biosphere–atmosphere feedbacks by J.D Fuentes; B.P Hayden; M Garstang; M Lerdau; D Fitzjarrald; D.D Baldocchi; R Monson; B Lamb; C Geron (189-191).