Atmospheric Environment (v.34, #15)

A tracer technique has been developed in this laboratory which allows the measurement of aqueous-phase oxidation of SO2 in clouds. We are now extending this technique to the chemistry of nitrogen species in clouds. Using a conservative tracer such as selenium we can determine the fraction of cloudwater nitrate that is the result of aerosol scavenging. At remote sites this allows the calculation of the nitric acid mixing ratio in the precloud air from measurements made during a cloud event. We have used this technique to determine the pre-cloud nitric acid partial pressure at Whiteface Mountain (WFM) for three cloud events. The calculated concentrations for an event on 2 August 1997 agree with measurements made at a second site below cloud level using filterpacks. Nitric acid concentrations for cloud events during July 1995, yielded values of <0.1–10.0 ppbv. At highly polluted sites closer to NO x sources the technique may provide a method of determining if there is any aqueous-phase oxidation of N(III) occurring.
Keywords: Nitric acid; Cloud chemistry; Nitrogen oxides; Acid precipitation;

An approximate solution to the advection–diffusion equation was used to back-calculate the source strength from downwind concentration measurements of single and multipoint (4 and 16 points) tracer gas (SF6 and CH4) release experiments conducted over microplots over an open field. These microplots were 2 m×2 m and 4 m×8 m in size so that maximum downwind sampling distances were ∼4 m. Best estimates (within ±20%) of the source strength were obtained when downwind concentrations measured at or near the mean plume height and along the centreline of the plume were used in the model. Predictions of the source strength degraded when concentration observations made progressively further away (in the y and z sense) from the centreline of the diffusing tracer gas plume were used. For the small scale used in this study, the performance of the model was not sensitive to the number of point sources, source configuration or downwind sampling distance.
Keywords: Atmospheric dispersion; SF6; Sulphur hexafluoride;

Contributions to light extinction during project MOHAVE by Douglas H Lowenthal; John G Watson; Pradeep Saxena (2351-2359).
The contribution of aerosols to light extinction at Meadview, AZ, during summer 1992 was estimated using Mie theory and size-resolved aerosol chemical measurements. Sulfate particle size increased as a function of relative humidity. Twelve-hour average light scattering was estimated to within 15%. Sulfate was the most abundant chemical component in the fine aerosol fraction. On average, Rayleigh scattering, coarse particles, and fine sulfates contributed 39, 21, and 19% to total light extinction. Average estimated light scattering was largely insensitive to assumptions about mixing state, degree of sulfate neutralization and organic carbon water uptake properties. It was estimated that a reduction of Mohave Power Plant (MPP) SO2 emissions corresponding to a contribution of 19% to ambient sulfate would have resulted in a decrease in total light extinction of between 3.3 and 5.3%.
Keywords: Light scattering; Extinction efficiency; Particle growth mechanisms; Water uptake; Aerosol acidity;

In Europe ammonia (NH3), volatilization from animal manure is the major source of NH3 in the atmosphere. From March to July 1997, NH3 volatilization from trail hose applied slurry was measured for seven days after application in six experiments. A statistical analysis of data showed that NH3 volatilization rate during the first 4–5 h after slurry application increased significantly (P<5%) with wind speed and soil slurry surface water content. NH3 volatilization in the six measuring periods during the experiments increased significantly (P<5%) with relative water content of the soil slurry surface, global radiation, pH, and decreased with increasing rainfall during each measuring period and rainfall accumulated from onset of each experiment. A mechanistic model of NH3 volatilization was developed. Model inputs are climate variables, soil characteristics and total ammoniacal nitrogen (TAN=ammonium+ammonia) in the soil surface layer. A pH submodel for predicting pH at the surface of the soil slurry liquid was developed. The measured NH3 volatilization was compared with model simulations. The simulated results explained 27% of the variation in measured NH3 volatilization rates during all seven days, but 48% of measured volatilization rates during the first 24 h. Calculations with the model showed that applying slurry in the morning or in the afternoon reduced volatilization by 50% compared with a noon application. Spreading the slurry with trail hoses to a 60 cm high crop reduced losses by 75% compared with a spreading onto bare soil. Ammonia volatilization was 50% lower when the soil had dried out after slurry application compared with a wet slurry surface.
Keywords: Emission; Ammonia; Livestock; pH model; Soil; Slurry;

Behaviour of radon, radon progenies and particle levels during room depressurisation by Pirjo Korhonen; Helmi Kokotti; Pentti Kalliokoski (2373-2378).
The aim of the study was to investigate the effect of room depressurisation (up to −28 Pa) on indoor radon, radon progenies and particle concentrations in a wooden school building (A) and in a wooden double-family house (B). The airborne concentrations of particles, radon, and radon progenies decreased with increased depressurisation. At the same time, the air exchange rates increased by a factor of 9.5 (house A) and 13 (house B). The calculated radon entry rates thus rose almost in the same ratio, 6.3 and 12, respectively. The equilibrium factors between radon progenies and radon were the same before and after the depressurisation being 0.04 in house A and 0.16 in house B. When the pressure was the lowest, radon concentrations decreased by a factor of 0.7 in house A and 0.9 in house B, and the radon progeny levels fell by a factor of 0.6 in both houses. The particle levels (particles exceeding 0.01 μm) decreased by a factor of 10 (house A) and of 1.2 (house B). The dilution by outdoor air was more effective in house A, which was less tight.
Keywords: Depressurisation; Radon; Radon progeny; Particle; Radon entry rate; Air exchange rate;

Generation of hydroxyl radicals by urban suspended particulate air matter. The role of iron ions by Athanasios Valavanidis; Anastasia Salika; Anna Theodoropoulou (2379-2386).
Recent epidemiologic studies showed statistical associations between particulate air pollution in urban areas and increased morbidity and mortality, even at levels well within current national air quality standards. Inhalable particulate matter (PM10) can penetrate into the lower airways where they can cause acute and chronic lung injury by generating toxic oxygen free radicals. We tested inhalable total suspended particulates (TSP) from the Athens area, diesel and gasoline exhaust particles (DEP and GED), and urban street dusts, by Electron Paramagnetic Resonance (EPR). All particulates can generate hydroxyl radicals (HO), in aqueous buffered solutions, in the presence of hydrogen peroxide. Results showed that oxidant generating activity is related with soluble iron ions. Leaching studies showed that urban particulate matter can release large amounts of Fe3+ and lesser amounts of Fe2+, as it was shown from other studies. Direct evidence of HO was confirmed by spin trapping with DMPO and measurement of DMPO-OH adduct by EPR. Evidence was supported with the use of chelator (EDTA), which increases the EPR signal, and the inhibition of the radical generating activity by desferrioxamine or/and antioxidants (d-mannitol, sodium benzoate).
Keywords: Urban air pollution; Air particulate matter; Oxygen free radicals; Electron paramagnetic resonance; Respiratory diseases;

Dry deposition of large, airborne particles onto a surrogate surface by Eugene Kim; David Kalman; Timothy Larson (2387-2397).
Simultaneous measurements of particle dry deposition flux and airborne number concentration in the open atmosphere were made using three different types of artificially generated particles in the size range 10–100 μm – perlite, diatomaceous earth and glass beads. A combination of gravimetric analysis, automated microscopy and sonic anemometry provided size-resolved estimates of both the inertial and gravitational components of the quasi-laminar layer particle deposition velocity, (V d)b, as a function of size. Eddy inertial deposition efficiency (η dI) was determined as a function of dimensionless eddy Stokes number (Stke). In the range 3<Stke<100, the observed η dI agree with the model of . In addition, the observed (V d)b values agree well with the predictions of Pleim et al. (1984, ADOM/TADAP Model Development Program. Volume 4. The Dry Deposition Module. Ontario Ministry of Environment, Rexdale, Ontario). However the dry deposition model of Sehmel and Hodgson (1978, a model for predicting dry deposition of particles and gases to environmental surfaces. DOE Report PNL-SA-6721, Pacific Northwest Laboratories, Richland, WA), used in several regulatory models, significantly under-predicted (up to seven times) (V d)b for large particles (d a>10 μm).
Keywords: Dry deposition velocity; Aerodynamic layer; Quasi-laminar layer; Eddy inertial deposition efficiency; Eddy Stokes number;

Impingement of rain drops on a tall building by V Etyemezian; C.I Davidson; M Zufalla ; W Daia ; S Finger; M Striegel (2399-2412).
Soiling on the walls of limestone buildings can be washed off when the surface erodes due to rain impingement. In this study, the delivery of rain to the 42-story Cathedral of Learning in Pittsburgh, Pennsylvania, represented by a 30 m×30 m×160 m rectangular block, was modeled using the RNG Kε model for turbulence and Lagrangian trajectory calculations for individual rain drops. Local Effect Factors (LEF) for the rectangular block compared well with earlier work in the literature. LEFs increased with wind speed, raindrop size, and height along the block. Wind speed, direction, and rain intensity were measured continuously over a seven-week period and provided input parameters for modeling rain fluxes to the Cathedral of Learning. Model results suggested that sections of the building receiving larger amounts of rain corresponded to white areas, indicating that rain fluxes have a significant effect on the soiling patterns. Intermediate wind speeds (2.5 and 5 m s−1) resulted in high rain fluxes. Although less frequent, high wind speeds also resulted in high rain fluxes. Much of the rain was delivered to the block as 1.25 and 2.5 mm drops with 5 mm drops having a smaller effect. Consideration of wind incidence angles other than 0° was shown to be important for future modeling efforts.
Keywords: Soiling; Buildings; Driving rain; Acid rain; CFD;

The output from the regional oxidant model (ROM) is compared to observed ozone over northern Illinois for June, July and August 1987. The 8-h daily average ozone at the ozone monitoring stations is interpolated to the ROM grid cells using a spatial statistical method. Differences between the model output and spatial predictions are compared at three levels of spatial averaging (with approximate scales of 19, 100, 400 km) and three levels of temporal averaging (daily, weekly, 3 months). In addition two ozone monitoring stations are paired with weather stations and with ROM cells in order to investigate the performance of ROM as a function of meteorological conditions. For daily values the root mean squared error (RMSE) between the ROM values and those predicted from the monitoring network varies between 14 and 25 ppb with the largest discrepancies occurring near Lake Michigan. Weekly averages reduce the RMSE by approximately 30% but spatial aggregation is not helpful in improving the agreement. The difference between ROM ozone predictions and the observed ozone at two paired sites depends most strongly on temperature and to a lesser extent on dew point temperature. The R 2 from linear regressions is approximately 35%. An examination of the synoptic-scale and meso-scale weather patterns during this period indicates that ROM is sensitive to dynamic situations such as a frontal passage.
Keywords: Ozone; Model evaluation;

The chemical component of a reactive pollution dispersion model often consumes much of the total computational effort involved. If savings can be made in the calculation of the chemical sub-model without significant loss of accuracy then higher resolution can be afforded in the spatial domain leading to better overall solution accuracy. The usual approach to reducing chemical models is by combining species with similar reactivities into single variables. Compact representations of atmospheric chemical mechanisms can be found of the order of 30–100 species. Dynamical systems analysis however shows that the long-term behaviour of chemical systems is usually restricted to much lower-dimensional manifolds in the total species space, due to many of the fast time-scales quickly reaching local equilibrium. This suggests that if appropriate representations can be found, further reductions can be made in the number of variables required to represent tropospheric chemistry.This paper will demonstrate using time-scale analysis that the intrinsic dimension of a typical tropospheric chemical model is low (varying between 2 and 9) and therefore by using a lower-dimensional representation of the chemistry, savings can be made in terms of the number of equations which need to be solved in the chemical sub-model of a dispersion code. An alternative method for chemical modelling will be described which uses simple difference equations rather than the solution of differential rate equations; a technique called repro-modelling. This technique defines difference equations representing species concentrations as functions of concentrations at previous time-points and important parameters, by fitting orthonormal polynomial functions to large data sets. The use of such fitted algebraic representations makes the repeated chemical kinetic simulations used in reactive dispersion codes more efficient. The paper will present a dimensional analysis of a reduced version of the Carbon-Bond scheme and will show that the scheme can be accurately represented over a wide range of concentration conditions using a nine-dimensional repro-model rather than the 90 variables in the original scheme.
Keywords: Repro-model; Tropospheric chemistry; Low-dimensional manifold; Chemical modelling; Time-scales;

Comparison of measured and calculated motorway emission data by B. Vogel; U. Corsmeier; H. Vogel; F. Fiedler; J. Kühlwein; R. Friedrich; A. Obermeier; J. Weppner; N. Kalthoff; D. Bäumer; A. Bitzer; K. Jay (2437-2450).
A comprehensive field campaign was carried out to check the quality of modelled on-road emissions of NO x and CO by comparing them with real-world emissions on a motorway in the southwestern part of Germany. Three different sets of emission data were determined for a section of the motorway A 656 that connects Mannheim and Heidelberg. First, emission data were pre-calculated, that means before the field campaign took place, using literature data for traffic volume, types of vehicles and driving speeds. Second, detailed traffic measurements during two intensive measurement phases were used to calculate improved emission data based on the actual traffic situation. Third, real-world emission data were determined by meteorological and chemical measurements. It shows that the differences of the pre-calculated emission data and the emission data improved by the traffic measurements differ by less than 15% in case of CO and approx. by 35% in case of NO x . A comparison of the observed emissions with the calculated ones gives an agreement for NO x within the error bars. For CO, however, a discrepancy by a factor of two was found. The real-world emissions were much higher than calculated.
Keywords: Realistic motorway emissions; Emission model; Field campaign; Traffic measurements; Emission factors;

The impact of updated rate constants for several key reactions of tropospheric gas-phase chemistry, namely the reactions of OH with NO2 (reduced by 30%) and the reaction of HO2 with NO (increased by 20%) are studied using the gas-phase chemical scheme RADM2. Furthermore, the earlier value of the rate constant of CH4 with OH used in RADM2 is reduced by 30% due to recent literature. The updated rate constants influence in particular the calculated budget of HO x and O3. For trace gas mixes in a rural environment the average calculated OH concentration increases by 20%, while HO2 exhibits only a small decrease of 2%. The net formation of ozone is about 12% larger. The long-term impact on the globally averaged calculated OH concentration is estimated in a simple box model to be 48%, i.e. much larger than the short-term effect due to the feedback on the mixing ratios of long-lived precursors and reaction partners. The predicted global methane and ozone mixing ratios are unaffected because of compensating effects of the simultaneous change of all three rate parameters.

Measurements of nitro-PAH in the atmospheres of two cities by Marwan Dimashki; Stuart Harrad; Roy M Harrison (2459-2469).
Analytical techniques have been developed based upon both HPLC with fluorescence detection of amino derivatives and upon GC/NICIMS for analysis of nitro-PAH in the atmosphere. Samples taken within a busy road tunnel, which should be representative of the emission source profile, indicated the expected presence of 9-nitroanthracene and 1-nitropyrene. Additionally, vapour-phase 1-nitronaphthalene and 2-nitronaphthalene were detected, and it is unclear whether these are directly emitted or formed from low activity free radical chemistry within the tunnel atmosphere, or a combination of the two. A comprehensive suite of nitro-PAH (1-nitronaphthalene, 2-nitronaphthalene, 9-nitroanthracene, 1-nitropyrene, 2-nitrofluoranthene and 7-nitrobenz[a]anthracene) have been determined in the city atmospheres of Birmingham, UK and Damascus, Syria. Whilst the nitronaphthalenes were detectable only in the vapour phase, 9-nitroanthracene was detectable in both particulate and vapour forms, and 1-nitropyrene, 2-nitrofluoranthene and 7-nitrobenz[a]anthracene were present solely in association with airborne particles. Concentrations measured in the winter months were highly comparable in the two cities despite differences in the motor vehicle parc and reflect the presence of both directly emitted compounds and compounds likely to have been formed within the atmosphere. Concentrations were broadly comparable with those measured at other urban sites across the world, although in most other studies a more limited suite of compounds was determined.
Keywords: Nitro-PAH; Nitration reactions; Filter artefact;

Sources to formic acid studied by carbon isotopic analysis and air mass characterization by M. Glasius; S. Wessel; C.S. Christensen; J.K. Jacobsen; H.E. Jørgensen; K.C. Klitgaard; L. Petersen; J.K. Rasmussen; T. Stroyer Hansen; C. Lohse; E. Boaretto; J. Heinemeier (2471-2479).
The carbon isotopic composition of formic acid samples from a rural and a semi-remote area in Denmark has been investigated by collection of 5–6 day integrated samples during winter and spring. The results show that 80–100% of formic acid stems from biogenic VOC emitted from terrestrial sources. Even during winter there is a dominating biogenic contribution, which is surprising since biogenic emissions are very low at these latitudes in that period.Quality assurance samples have been used to check the reliability of the collection and pre-treatment procedures for atmospheric formic acid samples. The carbon isotopic investigation was supplemented by measurements of benzene, toluene, formaldehyde, acetone, acetaldehyde and nitrogen dioxide in order to characterise the air masses of one formic acid sample. The general air mass analysis showed that the sampling site, a semi-remote area, was without significant local air pollution sources. During the formic acid sampling period, the air masses were influenced by both direct anthropogenic emissions (benzene, toluene, nitrogen dioxide and acetone) and compounds formed during long-range transport of anthropogenic hydrocarbons (formaldehyde and acetaldehyde). Nevertheless, formic acid still had a predominantly (89±5%) biogenic origin.
Keywords: Formic acid; Radiocarbon analysis; Volatile Organic Compounds;

The paper contains the description of numerical model of the airborne pollution transport developed by a scientific team of M. Galperin. During several years the model was used for the calculations in Meteorological Synthesizing Centre – East, within the scope of Co-operational Programme for Monitoring and Evaluation of the Long-Range Transmission of air Pollutants in Europe (EMEP). The paper presents the extensive description of the current version of the model and outlines the results of its quality assurance. The model is based on an original Eulerian advection scheme with variable time step, which can be classified as a “pseudo-Lagrangian” scheme. Vertical concentration profiles are parameterized by truncated Gaussian function with dynamic parameters calculated at every model time step. The model includes two types of chemistry and deposition units: acid compounds and ammonia, and heavy metals carried with aerosol particles. One of the model advantages is the ability to make the evaluation of source–receptor pollution exchange for substances with non-linear atmospheric chemistry and deposition. The model evaluation was carried out by the comparison of multi-annual calculated concentrations with measured values for several acidifying substances.
Keywords: Atmospheric pollution; Numerical modelling; Acidification;

Erratum (2497).