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Atmospheric Environment (v.40, #5)

Editorial board (pp. i).

List for forthcoming papers (pp. i-ii).

Atmospheric organic and bio-aerosols as cloud condensation nuclei (CCN): A review by Jiming Sun; Parisa A. Ariya (pp. 795-820).

Organic substances have been recognized as active cloud condensation and ice formation nuclei for several decades. In some regions of the world, these organic compounds (OC) consist predominantly of suspended matter mass, which can have local (e.g. toxicity, health hazards) and global (e.g. climate change) impacts. However, due to the complexity of their chemical nature, the significance of organic molecules in driving physical and chemical atmospheric processes is still very uncertain and poorly understood. The aim of this review paper is to assess the current state of knowledge regarding the role of organic aerosols (including bioaerosols) as cloud condensation nuclei (CCN), as well as to compare the existing theoretical and experimental data. It seems that classical Kohler theory does not adequately describe the hygroscopic behaviour of predominantly identified organic CCN such as pure dicarboxylic acid particles. Factors such as surface tension, impurities, volatility, morphology, contact angle, deliquescence, and the oxidation process should be considered in the theoretical prediction of the CCN ability of OC and the interpretation of experimental results. Major identified constituents of organic CCN, their main sources and their CCN properties will be herein reviewed. We will also discuss areas of uncertainty and expose key issues deserving of future research.

Keywords: Organic cloud condensation nuclei; Sources of organic CCN; CCN ability of pure OC; (bio) chemical reaction

Using multiple continuous fine particle monitors to characterize tobacco, incense, candle, cooking, wood burning, and vehicular sources in indoor, outdoor, and in-transit settings by Wayne R. Ott; Hans C. Siegmann (pp. 821-843).

This study employed two continuous particle monitors operating on different measurement principles to measure concentrations simultaneously from common combustion sources in indoor, outdoor, and in-transit settings. The pair of instruments use (a) photo-charging (PC) operating on the principle ionization of fine particles that responds to surface particulate polycyclic aromatic hydrocarbons (PPAHs), and (b) diffusion charging (DC) calibrated to measure the active surface area of fine particles. The sources studied included: (1) secondhand smoke (cigarettes, cigars, and pipes), (2) incense (stick and cone), (3) candles used as food warmers, (4) cooking (toasting bread and frying meat), (5) fireplaces and ambient wood smoke, and (6) in-vehicle exposures traveling on California arterials and interstate highways. The ratio of the PC to the DC readings, or the PC/DC ratio, was found to be different for major categories of sources. Cooking, burning toast, and using a “canned heat�? food warmer gave PC/DC ratios close to zero. Controlled experiments with 10 cigarettes averaged 0.15ngmm⁻² (ranging from 0.11 to 0.19ngmm⁻²), which was similar to the PC/DC ratio for a cigar, although a pipe was slightly lower (0.09ngmm⁻²). Large incense sticks had PC/DC ratios similar to those of cigarettes and cigars. The PC/DC ratios for ambient wood smoke averaged 0.29ngmm⁻² on 6 dates, or about twice those of cigarettes and cigars, reflecting a higher ratio of PAH to active surface area. The smoke from two artificial logs in a residential fireplace had a PC/DC ratio of 0.33–0.35ngmm⁻². The emissions from candles were found to vary, depending on how the candles were burned. If the candle flickered and generated soot, a higher PC/DC ratio resulted than if the candle burned uniformly in still air. Inserting piece of metal into the candle's flame caused high PPAH emissions with a record PC/DC reading of 1.8ngmm⁻². In-vehicle exposures measured on 43- and 50-min drives on a California arterial highway gave PC/DC ratios of 0.42 and 0.58ngmm⁻², with one-min average PC/DC ratios varying along the roadway due to the different types of vehicles. Interstate highways had PC/DC ratios of approximately 0.5ngmm⁻² with ratios above 1ngmm⁻² when driving behind diesel trucks. These PC/DC ratios were higher than the “signature�? value of the cigarette (0.11–0.19ngmm⁻²) measured in a large Indian gaming casino with smoking. Simultaneous continuous monitors operating together can provide useful information to help differentiate source categories. The PC/DC ratio reflects the mass of PAHs per unit of active surface area of the particles, and therefore we expect it to be relevant to the toxicity of fine particles.

Keywords: Photoelectric aerosol sensor; Diffusing charging monitor; PAHs; Nanoparticles; Indoor air; Secondhand smoke; Cigarettes; Cigars; Pipes; Incense; Candles; Cooking; Wood smoke; Motor vehicles

External influences on Europe's air quality: Baseline methane, carbon monoxide and ozone from 1990 to 2030 at Mace Head, Ireland by R.G. Derwent; P.G. Simmonds; S. O’Doherty; D.S. Stevenson; W.J. Collins; M.G. Sanderson; C.E. Johnson; F. Dentener; J. Cofala; R. Mechler; M. Amann (pp. 844-855).

A global 3-D chemistry-transport model STOCHEM has been employed to study trends in the mole fractions of methane, carbon monoxide and ozone in baseline air masses entering Europe from the Atlantic Ocean over the period from 1990 to 2030. With a range of emission scenarios for man-made ozone precursor emission sources available, a wide range of model trends were predicted for the period up to 2030. In the scenario based on current planned air pollution controls, IIASA CLE, methane shows a strong upward trend, ozone shows a weaker upward trend, and carbon monoxide is approximately flat in baseline air masses. In one of the more pessimistic IPCC SRES scenarios, A2, all three gases show future increases. However, in the scenario based on maximum feasible emission reductions, IIASA MFR all three trace gases decline. By 2030, projected climate change reduces the growth in CH₄, but has insignificant effects on baseline CO and O₃ in these simulations. Global or hemispheric ozone precursor emissions and their controls exert a potentially large external influence on Europe's air quality. This influence is currently not taken into account in future European air quality policy formulation.

Keywords: 3-D CTM; Tropospheric ozone; Air quality; Ozone precursors

Estimation of Hg⁰ exchange between ecosystems and the atmosphere using²²²Rn and Hg⁰ concentration changes in the stable nocturnal boundary layer by Daniel Obrist; Franz Conen; Roland Vogt; Rolf Siegwolf; Christine Alewell (pp. 856-866).

The goal of this study was to test a²²²Rn/Hg⁰ method to measure exchange fluxes of Hg⁰ between ecosystems and the atmosphere complementing gradient-based micrometeorological methods which are difficult to apply when exchange fluxes are very low, during calm nights, and over heterogeneous surfaces. The method is based on absolute concentration changes of Hg⁰ and of the trace gas²²²Rn over several hours in the stable nocturnal boundary layer (NBL) when absolute gas concentrations change according to the source or sink strength of the underlying landscape.²²²Rn accumulations were observed in 28 of 66 measured nights in an urban area and in 14 of the 40 nights at a subalpine grassland. Concurrent and significant increases in atmospheric Hg⁰ concentrations were observed 22 times in the urban area and calculated Hg⁰ emissions—for the first time measured over a city area—averaged 6.4±0.9ngm⁻²h⁻¹. Concurrent changes in Hg⁰ in the stable NBLs at the subalpine site were very small and significant only nine times with calculated Hg⁰ fluxes averaging −0.2±0.3ngm⁻²h⁻¹, indicating that the flux of Hg⁰ in the grassland was a very small net deposition of atmospheric Hg⁰ to the ecosystem. At the subalpine grassland the²²²Rn/Hg⁰ method compared reasonably well to a modified Bowen ratio (MBR) method applied during turbulent conditions (−1.9±0.2ngm⁻²h⁻¹ [or −1.7±0.4ngm⁻²h⁻¹ during nights only]). The MBR, however, was not applicable in the urban area due to high surface roughness. We conclude that the²²²Rn/Hg⁰ method—although limited to nighttime periods in our study—can complement gradient-based methods during stable NBL periods and can be used over heterogeneous surfaces when conventional micrometeorological approaches are not applicable.

Keywords: Elemental atmospheric mercury; Flux measurements; ²²²; Rn accumulation

Dispersion of particles and trace gases nearby a city highway: Mobile laboratory measurements in Finland by L. Pirjola; P. Paasonen; D. Pfeiffer; T. Hussein; Hameri K. H�meri; T. Koskentalo; A. Virtanen; Ronkko T. R�nkk�; J. Keskinen; T.A. Pakkanen; R.E. Hillamo (pp. 867-879).

Traffic particles, total number concentration and size distribution in the range 3nm–10μm, as well as concentrations of carbon monoxide and nitrogen oxides were measured by a mobile laboratory at a major highway and in its vicinity, 0–140m from the road side, in Helsinki, during the summer and winter campaigns in 2003 and 2004. The data have been analysed according to the sampling point and temperature, and classified to three wind sectors; wind blowing perpendicular to the road from northwest (S1) or to the opposite direction (S3), and wind blowing along the road (S2). The average concentrations in winter were 2–3 times higher than the concentrations in summer. In winter, 90–95%, and in summer, 86–90% of particles were smaller than 50nm. The equations for the concentration curves in sectors S1 and S2 were derived, starting point being at the roadside. When moving 65m from the roadside, the average concentration reduced to 39% in winter and to 35% in summer in wind sector S1 whilst in sector S2, it already decreased to 19% in summer. The size distributions showed 2–3 modes peaking at 14–23nm (nucleation mode), 40–50nm (Aitken mode) and 120–125nm (accumulation mode). In wind sector S1, in winter, the nucleation mode shifted to larger sizes when moving away from the road.

Keywords: Ultrafine particles; Size distribution; Aerosol dynamics; Dispersion; Traffic emissions

Distributions of C₂–C₅ NMHCs and related trace gases at a tropical urban site in India by L.K. Sahu; S. Lal (pp. 880-891).

Simultaneous surface measurements of C₂–C₅ non-methane hydrocarbons (NMHCs), O₃, CO and CH₄ were made during the year 2002 at a tropical urban site, Ahmedabad. This is the first time that NMHCs levels have been characterized in detail in India. The diurnal distributions of these species show pronounced variations in the winter months and less during the summer months. The seasonal variations of all these species show substantially higher levels during the winter and lowest during the summer season. The strength (winter to summer ratios) of seasonal variations in NMHCs are observed to be higher than other reported measurements elsewhere. The seasonal changes in transport patterns, boundary layer height and OH concentrations, all contribute in the seasonal variations of these trace gases. The correlation studies of various NMHCs and CO indicate dominant role of local emissions in the observed distributions of trace gases. The natural gas emission and leakage of liquid petroleum gas contribute to elevated levels of ethane and propane. While emissions from vehicular exhaust are found to be dominant sources of ethene, propene and acetylene. The higher C₂H₂/CO ratio of about 6.4pptv/ppbv indicates influences of fresh emissions at Ahmedabad.

Keywords: NMHCs; Ozone; Seasonal variations; Pollution; Vehicular exhaust

Volatile organic compounds in ambient air of Mumbai—India by Anjali Srivastava; A.E. Joseph; S. Devotta (pp. 892-903).

Volatile organic compounds (VOCs) are a major group of air pollutants which play a critical role in atmospheric chemistry. These contribute to toxic oxidants which are harmful to ecosystem, human health and atmosphere. The variability of pollutants is an important factor in determining human exposure to these chemicals. Data on levels of VOCs in developing countries, including India, are lacking.The present work deals with the estimation of target VOCs at 15 locations of five categories in Mumbai. The categories are residential, industrial, commercial, traffic intersections and petrol refueling stations. The monitoring was carried out during peak hours in the morning and evening, once every month, during May 2001 to April 2002. The study focused on target VOCs as defined by USEPA. Concentrations of benzene, at all the locations, were found to be much above the guidelines values prescribed by World Health Organization (WHO) for ambient air quality. All other VOCs were observed to be below the WHO guideline values. The results show that levels of VOCs in Mumbai were high. There is need for a regular monitoring schedule of VOCs in the urban environment.Variability studies are important to assess the exposure potential of pollutants which are an important parameter for health impact studies. This study also presents the variability of VOCs in the urban area of Mumbai. Variability was divided into measurement spatial, temporal and temporal–spatial interaction components. The temporal component along with temporal–spatial interaction component were the major contributors to variability. VOCs associated with mobile source emissions and emissions from marine source were found to be distributed uniformly in the urban atmosphere in Mumbai. the need for continuous monitoring, to capture short term peak concentrations and averages, is evident.

Keywords: VOC; Mumbai; Directional dependence; Variance; Benzene; Toluene; Xylene

Costs and benefits of an enhanced reduction policy of particulate matter exhaust emissions from road traffic in Flanders by Liesbeth Schrooten; Ina De Vlieger; Filip Lefebre; Rudi Torfs (pp. 904-912).

We demonstrate that accelerated policies beyond the steady improvement of technologies and the fleet turnover are not always justified by assumptions about health benefits. Between the years 2000 and 2010, particulate matter (PM) exhaust emissions from traffic in Flanders, a region of Belgium, will be reduced by about 44% without taking any extra reduction measures (baseline scenario). The PM emissions from road traffic were calculated using the MIMOSA model. Furthermore, we explored a range of options to increase attempts to reduce PM exhaust emission from traffic in 2010. When installing particle filters on heavy-duty trucks and buses, introducing biodiesel and diesel/hybrid cars, as well as slowing down the increase of private diesel cars, only an extra reduction of about 8% PM can be achieved in Flanders. The costs to achieve this small reduction are very high. To justify these costs, benefits for public health have been calculated and expressed in external costs. We demonstrate that only an enhanced effort to retrofit trucks and buses with particle filters has a net benefit. We have used Monte Carlo techniques to test the validity of this conclusion. It is concluded that a local or national policy that goes beyond European policies is not always beneficial and that additional measures should be assessed carefully.

Keywords: Particulate matter; Emission reduction strategies; Marginal costs; External costs; Air quality management

Forecasting of ozone level in time series using MLP model with a novel hybrid training algorithm by Dong Wang; W.-Z. Wei-Zhen Lu (pp. 913-924).

As far as the impact of tropospheric ozone (O₃) on human heath and plant life are concerned, forecasting its daily maximum level is of great importance in Hong Kong as well as other metropolises in the world. This paper proposed a multi-layer perceptron (MLP) model with a novel hybrid training method to perform the forecasting task. The training method synergistically couples a stochastic particle swarm optimization (PSO) algorithm and a deterministic Levenberg–Marquardt (LM) algorithm, which aims at exploiting the advantage of both. The performance of such a hybrid model is further compared with ones obtained by the MLP model trained individually by these two training methods mentioned above. Based on original data collected from two typical monitoring sites with different O₃ formation and transportation mechanism, the simulation results show that the hybrid model is more robust and efficient than the other two models by not only producing good results during non-episodes but also providing better consistency with the original data during episodes.

Keywords: Daily maximum ozone level; Multi-layer perceptron model; Particle swarm optimization algorithm; Levenberg–Marquardt algorithm; Performance comparison

Large-eddy simulations of turbulent flow over complex terrain using modified static eddy viscosity models by Satoru Iizuka; Hiroaki Kondo (pp. 925-935).

The performances of the modified static eddy viscosity model proposed by Inagaki et al. [2005. A mixed-time-scale SGS model with fixed model-parameters for practical LES. Transactions of the American Society of Mechanical Engineers: Journal of Fluids Engineering 127, 1–13] and of its variations were examined in the large-eddy simulation (LES) of a turbulent flow over a two-dimensional steep hill under a neutral condition. Unlike the standard Smagorinsky model, the modified static eddy viscosity models require no explicit wall damping function. Moreover, the modified static models showed excellent computational stability. The accuracy of the modified static models was assessed by comparing the results with those obtained from a wind tunnel experiment conducted by Ishihara and Hibi [1998. An experimental study of turbulent boundary layer over a steep hill. Proceedings of the 15th National Symposium on Wind Engineering, 61–66 (in Japanese)] and Ishihara et al. [2001. A wind tunnel study of separated flow over a two-dimensional ridge and a circular hill. Journal of Wind Engineering 89, Proceedings of the Fifth Asia-Pacific Conference on Wind Engineering, Kyoto, Japan, pp. 573–576] and the standard Smagorinsky model. The original model of Inagaki et al. (2005) produced unacceptable predictions, mainly due to the inaccurate estimation of the turbulence characteristics near the ground surface of the flow approaching the hill. On the other hand, the modification of the original model of Inagaki et al. (2005), in which the modified Leonard term was adopted for the parameterization of the square of the velocity scale in the expression of the subgrid-scale (SGS) eddy viscosity, gave more accurate predictions and produced the best results among the modified static models introduced in this study.

Keywords: Large-eddy simulation; Complex terrain; Turbulent flow; Static eddy viscosity model; Wall damping function

Recent changes in the production and global atmospheric emissions of chlorodifluoromethane (HCFC-22) by A. McCulloch; P.M. Midgley; A.A. Lindley (pp. 936-942).

The atmospheric concentrations of chlorodifluoromethane (HCFC-22) have been continuously measured for over a decade in long-term monitoring experiments. Comparison of these data with the industrial and regulatory databases of production and sales indicates that containment of this material is improving and its rate of emission, relative to the quantities produced and in service, is falling. In this paper, we reconcile the results of long-term monitoring with emissions calculated from activity (as global sales into the categories of end-use) and identify the principal changes that have occurred in the overall emission functions.

Keywords: Chlorodifluoromethane; China; India; Korea; Russia; AFEAS; UNEP; Emission

Influence of different emission sources on atmospheric organochlorine patterns in Germany by K.-D. Klaus-Dieter Wenzel; Andreas Hubert; Ludwig Weissflog; Kuhne Ralph K�hne; Peter Popp; Annegret Kindler; Schuurmann Gerrit Sch��rmann (pp. 943-957).

The concentrations of organochlorine parent substances such as p,p′-DDT (2,2-bis(chlorophenyl)-1,1,1-trichloroethane) and lindane ( γ-hexachlorocyclohexane (HCH)) as well as of their metabolites and conversion products, chlorobenzenes (CBz) and polychlorinated biphenyl congeners (PCBs), were determined both in the gas phase and in the particle bound fraction at 10 locations in Germany. The ratios between parent substances and possible degradation products were influenced by different gaseous point-source emissions. Factors of site-related degradation products are dependent on the emission source. Surprisingly, the highest degradation ratios of p,p′-DDT to DDE and DDD were not calculated at +20°C, but at −19°C. This indicates that heavy metals, black carbon and other organic substances such as PAHs may catalyse degradation reactions on particles because of higher condensation of all these substances at lower temperatures.To detect hidden characteristic of pollutant patterns that are dependent on the specific emission source and on typical degradation processes, the principal component analysis (PCA) results suggested that the organochlorines appear to be associated. Comparatively higher concentrations of DDX and HCH isomers mean at some sites also higher concentrations of CBz and PCBs, without an additional source being recognizable.

Keywords: Organochlorines; Air concentrations; Emission sources; Degradation behaviour

Evaluation of a turbulent flow and dispersion model in a typical street canyon in York, UK by N.S. Dixon; J.W.D. Boddy; R.J. Smalley; A.S. Tomlin (pp. 958-972).

It is likely that the use of computational fluid dynamics (CFD) modelling approaches to represent the dispersion of pollutants within urban areas will become increasingly common in the future. If such models are to be used within a decision-making context, their evaluation for a range of urban geometries should be undertaken. In this work a k–ε flow model coupled with a Lagrangian stochastic dispersion model are used to model the flow and dispersion of a traffic-related pollutant in a complex urban street canyon within the city of York, UK. The results are compared with measurements of mean wind, turbulence and carbon monoxide concentrations from a field experiment. For comparison, measured concentrations were normalised by the background wind speed and an estimate of the traffic emissions based on traffic flows. The normalisation was found to be inapplicable at background wind speeds below 2ms⁻¹ and therefore comparisons are restricted to higher wind speeds. The mean flow is found to be well predicted by the model and helps to interpret complex flow structures in the street, which contained vertical recirculation, corner vortices, channelling and convergence. The modelled worst case concentrations are generally higher than the measurements which could potentially be due in part to the reduced vertical velocities in the model and lower levels of predicted turbulence for certain background wind conditions when compared to the field data. Sensitivity analysis suggests that the modelled concentrations are fairly robust with respect to changes in grid resolution and upwind velocity profiles. The variability of the normalised measured concentrations was high, which is thought to be due to variability within the traffic emissions due to the presence of different traffic regimes, as well as large-scale turbulence within the background flow.

Keywords: MISKAM; Lagrangian; Stochastic; k; –; ε; Traffic; Pollution; Urban

Improved model for mercury emission, transport and deposition by Philip K. Gbor; Deyong Wen; Fan Meng; Fuquan Yang; Baoning Zhang; James J. Sloan (pp. 973-983).

Most regional models that have been developed to study the behaviour of mercury (Hg) in the environment do not include detailed treatment of natural (i.e. non-anthropogenic) Hg emissions. We have developed a Hg emission model including natural sources; incorporated it into the CMAQ-Hg model (US EPA) and used it to study the behaviour of Hg in the atmosphere.Our natural Hg emission model uses an algorithm based on meteorology to calculate the emission from vegetation, soil and water using Hg concentrations in soil and water. Using our new model, we carried out simulations for the months of May to September 2000 inclusive on a domain covering eastern North America containing 75 by 70 grid cells at a resolution of 36km. The simulated daily average natural Hg emission flux varied between 1.8 and 3.7ngm⁻²h⁻¹, with the maximum occurring in July. The simulation found that the total natural emission averaged across the domain is about twice the anthropogenic emission. Hg concentration in the air and wet deposition amounts were higher in the eastern part of the domain than in the western part and reflected both local sources and transport. We report for the first time comparisons of hourly and daily Hg concentrations measured by CAMNet monitoring stations with model simulations done at the same timescales. The correlation coefficients ( R) between modelled daily average total gaseous Hg concentrations and measurements for Point Petre, Egbert and Burnt Island are between 0.74 and 0.82. Neglecting natural emissions led to poorer correlations (between 0.22 and 0.51), indicating the importance of including an accurate and detailed treatment of natural Hg emissions in atmospheric Hg models.

Keywords: Mercury; Modelling; Natural emission; CMAQ; Mercury measurements

Investigating the mixture of air pollutants associated with adverse health outcomes by Steven Roberts; Michael A. Martin (pp. 984-991).

Most investigations of the adverse health effects of multiple air pollutants analyse the time series involved by simultaneously entering the multiple pollutants into a Poisson log–linear model. Concerns have been raised about this type of analysis, and it has been stated that new methodologies or models need to be developed for investigating the adverse health effects of multiple air pollutants. To this end, it has recently been stated that it may be more reasonable to assume that there is a mixture of pollutants considered harmful to health and that assessing the adverse health effects of an air pollution mix may be both more meaningful and more tenable than attempting to isolate the effects of individual pollutants. In this paper, a new model is introduced that is able to reveal the mixture of pollutants associated with an adverse health outcome and the effect of this mixture on the adverse health outcome. The model is shown to have a number of advantages over the traditional method of estimating the adverse health effects of multiple air pollutants. In addition, the model is also shown to be an improvement over a previously proposed, and somewhat ad-hoc, method for estimating the mixture of pollutants associated with an adverse health outcome.

Keywords: Air pollution; Mortality; Time series; Multiple pollutants

Association of airmass transport patterns and particulate sulfur concentrations at Big Bend National Park, Texas by Bret A. Schichtel; Kristi A. Gebhart; Michael G. Barna; William C. Malm (pp. 992-1006).

The Big Bend Regional Aerosol and Visibility Observational (BRAVO) study was initiated to understand the causes of haze at Big Bend National Park. BRAVO included the measurement of aerosols throughout Texas from July to October 1999 and extensive modeling of these aerosols. In support of BRAVO, the potential contributions from source regions to particulate sulfur at Big Bend during the BRAVO period were examined via an airmass history analysis. This was done using residence time analysis and a new technique of decomposing the residence time probability density function into its basic components, an airmass transport directional frequency and inverse characteristic transport speed. Trajectory heights over potential source regions were also examined. The system was validated using inert perfluorocarbon tracers that were released from four Texas sites. Airmass transport to Big Bend was examined on days with high (>80th percentile), and days with low (<20th percentile), particulate sulfur. High particulate sulfur concentrations were associated with low-level and low-speed airmass transport from the eastern United States, eastern Texas, and northeastern Mexico. All three of these regions have high SO₂ emissions that could contribute to Big Bend's haze. Examination of individual trajectories showed that the highest particulate sulfur concentrations occurred when transport over several of these regions coincided. Low particulate sulfur concentrations coincided with low-level but high-speed airmass transport from the Gulf of Mexico and along the Mexico–Texas border. Precipitation often occurred along these trajectories. Low sulfur was also associated with transport from low SO₂ emission regions north and west of Big Bend. Days with high SO₂ or selenium concentrations were also examined. High SO₂ concentrations were associated with prior transport from nearby sources, particularly the Carbón power plants located in Mexico ∼230km southeast of Big Bend. High selenium concentrations were associated with prior transport over Carbón and eastern Texas.

Keywords: BRAVO; Sulfate; Regional haze; Long-range transport; Receptor model

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