A climatology of 7Be at four high-altitude stations at the Alps and the Northern Apennines

The ⁷Be activity concentrations measured from 1996 to 1998 at four high-altitude stations, Jungfraujoch—Switzerland, Zugspitze—Germany, Sonnblick—Austria and Mt. Cimone—Italy, were analyzed in combination with a set of, meteorological and atmospheric parameters such as the tropopause height, relative and specific humidity and also in conjunction with 3D back-trajectories in order to investigate the climatological features of ⁷Be. A frequency distribution analysis on ⁷Be activity concentrations revealed the existence of two concentration classes around 1.5 and 6 mBq m⁻³ and a transition class between the two modes of the distribution at 3–4 mBq m⁻³. Cross-correlation analysis performed between ⁷Be and a number of meteorological and atmospheric parameters at the first three stations showed a strong negative correlation with relative humidity (−0.56, −0.51, −0.41) indicating the importance of wet scavenging as a controlling mechanism. Also, the positive correlation with the height of 3-days back-trajectories and tropopause height (+0.49/+0.43, +0.59/+0.36, +0.44/+0.38) shows that downward transport from the upper or middle to lower troposphere within anticyclonic conditions plays also an important role. Trajectory statistics showed that low ⁷Be concentrations typically originate from lower-altitude subtropical ocean areas, while high concentrations arrive from the north and high altitudes, as is characteristic for stratospheric intrusions. Although the ⁷Be activity concentrations are highly episodic, the monthly means indicate an annual cycle with a late-summer maximum at all stations. The correlation coefficients calculated for monthly means of the ⁷Be and atmospheric data suggest that the main predictor controlling the seasonality of the ⁷Be concentrations is tropopause height (+0.76, +0.56, +0.60), reflecting more vertical transport from upper tropospheric levels into the lower troposphere during the warm season than during the cold season.     

A climatology of Be at four high-altitude stations at the Alps and the Northern Apennines

The ⁷Be activity concentrations measured from 1996 to 1998 at four high-altitude stations, Jungfraujoch—Switzerland, Zugspitze—Germany, Sonnblick—Austria and Mt. Cimone—Italy, were analyzed in combination with a set of, meteorological and atmospheric parameters such as the tropopause height, relative and specific humidity and also in conjunction with 3D back-trajectories in order to investigate the climatological features of ⁷Be. A frequency distribution analysis on ⁷Be activity concentrations revealed the existence of two concentration classes around 1.5 and 6 mBq m⁻³ and a transition class between the two modes of the distribution at 3–4 mBq m⁻³. Cross-correlation analysis performed between ⁷Be and a number of meteorological and atmospheric parameters at the first three stations showed a strong negative correlation with relative humidity (−0.56, −0.51, −0.41) indicating the importance of wet scavenging as a controlling mechanism. Also, the positive correlation with the height of 3-days back-trajectories and tropopause height (+0.49/+0.43, +0.59/+0.36, +0.44/+0.38) shows that downward transport from the upper or middle to lower troposphere within anticyclonic conditions plays also an important role. Trajectory statistics showed that low ⁷Be concentrations typically originate from lower-altitude subtropical ocean areas, while high concentrations arrive from the north and high altitudes, as is characteristic for stratospheric intrusions. Although the ⁷Be activity concentrations are highly episodic, the monthly means indicate an annual cycle with a late-summer maximum at all stations. The correlation coefficients calculated for monthly means of the ⁷Be and atmospheric data suggest that the main predictor controlling the seasonality of the ⁷Be concentrations is tropopause height (+0.76, +0.56, +0.60), reflecting more vertical transport from upper tropospheric levels into the lower troposphere during the warm season than during the cold season.     

Simulation of the atmospheric concentrations of 210Pb and 7Be and comparison with daily observations at three surface sites

The atmospheric transport of ²¹⁰Pb and ⁷Be is simulated by the Laboratoire de Météorologie Dynamique general circulation model, LMDz, driven by ECMWF reanalyses. Daily averaged concentrations collected at three surface stations are compared with numerical results for a 1 year (2004) global simulation. The model, with a resolution of 3.75°×2.5° and 19 levels, succeeds in reproducing daily variations of ²¹⁰Pb and ⁷Be concentrations. Figures of merit in time (FMT), quantifying the overlapping of the predicted and observed time series over each month, range from 55% to 70%. Sensitivity studies as well as the analysis of numerical signals allow determining the main physical atmospheric processes characterizing the stations on a daily basis. Concentrations of ²¹⁰Pb at the three stations are particularly sensitive to scavenging in convective updrafts.     

A model of wet deposition to complex terrain

A two dimensional model of the seeder-feeder mechanism of orographic rainfall enhancement has been developed. The model has been extended to include the deposition of aerosol material incorporated into the orographic feeder cloud by nucleation scavenging. Parameterizations of any changes in the concentration of SO₄²⁻ in the cloud due to chemical reactions have also been included. The model is used to predict the rainfall enhancement and SO₄²⁻ deposition over terrain consisting of two parallel ridges oriented perpendicular to the wind. A wide range of spatial scales has been used of up to 150 km. It is found that the patterns of rainfall enhancement and deposition are strongly dependent on the spatial scales, the atmospheric structure and the cloud chemistry.     

Impact of the tropical storm Delta on the gross alpha,gross beta, 90Sr, 210Pb, 7Be, 40K and 137Cs activities measured in atmospheric aerosol and water samples collected in Tenerife (Canary Islands)

On 28 and 29 November 2005, the tropical storm Delta struck the Canary Islands (Spain) and the western shores of Morocco. Gravimetric and radiometric measurements carried out in atmospheric aerosol and water samples, collected after the storm, showed increased levels of total suspended particles (TSP) in the atmosphere and gross alpha, gross beta as well as ⁹⁰Sr activities in both the atmosphere and drinking water. These variations were most likely produced by local re-suspension of soil material. However, ²¹⁰Pb and ⁷Be activities, measured in atmospheric aerosols, did not increase until a week after the storm had passed. ⁴⁰K and ¹³⁷Cs activities, also measured in atmospheric aerosols, did not vary significantly with respect to previous weeks indicating that the slightly higher levels of TSP, measured during the week when the storm occurred, were not produced by the long-range transport of re-suspended aerosols from the African continent, as it has been observed in other occasions at this site. Gross alpha, gross beta, ⁴⁰K and ⁹⁰Sr levels in drinking water samples increased after the storm over their average values by approximately 245%, 245%, 130% and 440%, respectively. These results indicate how important the local re-suspension and later deposition/scavenging of aerosols may be on the water supply in Tenerife.     

Estimating contribution of precipitation scavenging of atmospheric particulate mercury to mercury wet deposition in Japan

Mercury wet deposition is dependent on both the scavenging of divalent reactive gaseous mercury (RGM) and atmospheric particulate mercury (Hg(p)) by precipitation. Estimating the contribution of precipitation scavenging of RGM and Hg(p) is important for better understanding the causes of the regional and seasonal variations in mercury wet deposition. In this study, the contribution of Hg(p) scavenging was estimated on the basis of the scavenging ratios of other trace elements (i.e., Cd, Cu, Mn, Ni, Pb and V) existing entirely in particulate form. Their wet deposition fluxes and concentrations in air, which were measured concurrently from April 2004 to March 2005 at 10 sites in Japan, were used in this estimation. The monthly wet deposition flux of mercury at each site correlated with the amount of monthly precipitation, whereas the Hg(p) concentrations in air tended to decrease during summer. There was a significant correlation (P<0.001) among the calculated monthly average scavenging ratios of trace elements, and the values in each month at each site were similar. Therefore, it is assumed the monthly scavenging ratio of Hg(p) is equivalent to the mean value of other trace elements. Using this scavenging ratio (W), the wet deposition flux (F) due to Hg(p) scavenging in each month was calculated by F=WKP, where K and P are the Hg(p) concentration and amount of precipitation, respectively. Relatively large fluxes due to Hg(p) scavenging were observed at a highly industrial site and at sites on the Japan Sea coast, which are strongly affected by the local sources and the long-range transport from the Asian continent, respectively. However, on average, at the 10 sites, the contribution of Hg(p) scavenging to the annual mercury deposition flux was 26%, suggesting that mercury wet deposition in Japan is dominated by RGM scavenging. This RGM should originate mainly from the in situ oxidation of Hg⁰ in the atmosphere.     

A 3-mode parameterization of below-cloud scavenging of aerosols for use in atmospheric dispersion models

Atmospheric aerosols are subject to below-cloud scavenging by precipitation. The scavenging coefficient depends on the aerosol size significantly. The traditional bulk parameterization represents the mean wet scavenging coefficient for the whole aerosol size range. This parameterization significantly overestimates the scavenging of aerosol mass by a heavy rain or a long-duration medium rain. In this study, we present a 3-mode parameterization of the mean scavenging coefficient for each aerosol mode instead of representation for the whole aerosol size range. The new parameterization takes into account the aerosol number size distribution, the rain droplet size distribution and the spectral collision efficiency between the aerosol particle and the rain droplet. Comparing the calculation of mass depletion due to below-cloud scavenging, the 3-mode parameterization agrees well with the size-resolved explicit method. The new parameterization can be easily implemented in atmospheric dispersion models.     

The effect of improved nowcasting of precipitation on air quality modeling

The predictive potential of air quality models and thus their value in emergency management and public health support are critically dependent on the quality of their meteorological inputs. The atmospheric flow is the primary cause of the dispersion of airborne substances. The scavenging of pollutants by cloud particles and precipitation is an important sink of atmospheric pollution and subsequently determines the spatial distribution of the deposition of pollutants. The long-standing problem of the spin-up of clouds and precipitation in numerical weather prediction models limits the accuracy of the prediction of short-range dispersion and deposition from local sources. The resulting errors in the atmospheric concentration of pollutants also affect the initial conditions for the calculation of the long-range transport of these pollutants. Customary the spin-up problem is avoided by only using NWP (Numerical Weather Prediction) forecasts with a lead time greater than the spin-up time of the model. Due to the increase of uncertainty with forecast range this reduces the quality of the associated forecasts of the atmospheric flow.In this article recent improvements through diabatic initialization in the spin-up of large-scale precipitation in the Hirlam NWP model are discussed. In a synthetic example using a puff dispersion model the effect is demonstrated of these improvements on the deposition and dispersion of pollutants with a high scavenging coefficient, such as sulphur, and a low scavenging coefficient, such as cesium-137. The analysis presented in this article leads to the conclusion that, at least for situations where large-scale precipitation dominates, the improved model has a limited spin-up so that its full forecast range can be used. The implication for dispersion modeling is that the improved model is particularly useful for short-range forecasts and the calculation of local deposition. The sensitivity of the hydrological processes to proper initialization implies that the spin-up problem may reoccur with changes in the model and increased model resolution. Spin-up should be an ongoing concern for atmospheric modelers.     

Preliminary estimation of the rainfall chemical composition evaluated through the scavenging modeling for north-eastern Amazonian region (Amapá state,Brazil)

Numerical modeling of scavenging processes has been compared with data obtained for rainwater and aerosol chemistry at Serra do Navio, in the state of Amapá in the Brazilian Amazon region. Sulfate, nitrate and ammonium concentrations were determined in rainwater samples collected from May 1995 until June 1997. The levels of these same chemicals were also determined in aerosols for the same period and region. Scavenging processes have been evaluated on a rainfall event basis, via numerical modeling, in order to simulate the rainwater concentrations and compare them with the observed data. RAMS (Regional Atmospheric Modeling System) was used to simulate cloud structures. A model of below-cloud scavenging was evaluated, as well. The determinations made from the results of the scavenging model are the following: a) aerosol vertical profiles are quite important to rainwater concentrations; b) modeled sulfate in rainwater is a better fit to the observed data values than ammonium and nitrate; c) the obtained sulfate aerosol concentrations samples are similar to ones found in the literature, although the sulfate concentrations in rainwater are much lower than other studies in the literature; d) the in-cloud scavenging process dominates, e) our modeled results, using an input gas vertical profile extracted from the ABLE2B experimental data set, present a smaller ratio between gas and aerosol scavenging than found in other studies in the literature, other studies may have had larger rainfall times, which increase the importance of gas phase scavenging.     

Analyses of airborne Be concentrations in Hong Kong using back-trajectories

A total of 71 air samples were collected in Hong Kong area from November 2001 to February 2003 using a high-volume air sampler and a high-volume cascade impactor with five atmospheric pressure stages. The ⁷Be radioactivity on each stage was measured using a high-efficiency germanium gamma-ray spectrometer. From the radioactivity of stages, the total airborne ⁷Be radioactivity was determined. The activity median aerodynamic diameter (AMAD) of ⁷Be-associated atmospheric aerosols was found to be 0.22–1.11 μm and the geometric standard deviation (GSD) was found to be 1.2–10.5. With the assumed mean growth rate (MGR) of atmospheric aerosols of 0.004–0.005 μm h⁻¹ and the size of Aitken nuclei of 0.015 μm, the residence times of ⁷Be-associated atmospheric aerosols were also found from the AMAD.Three-dimensional 4-day back-trajectories were obtained using the HYSPLIT model from NOAA Air Resources Laboratory. These trajectories were used with the measured ⁷Be radioactivity to construct regional ⁷Be intensity fields for four different altitude levels (less than 1000, 1000–2000, 2000–3000 and above 3000 m) with a Geographic Information System (GIS). Low ⁷Be intensities were found to have advected from low altitudes (less than 1000 m) and oceanic areas. The ⁷Be intensities increased for the higher intensity field layers.By comparing the time taken for air masses to come from the ⁷Be source to Hong Kong and the residence time determined from the AMAD of ⁷Be-associated atmospheric aerosols, good agreement was found if the mean growth rate of 0.005 μm h⁻¹ for atmospheric aerosols was used, and the use of back-trajectories was shown to be satisfactory even up to about 6.5 d. By using the residence time with a MGR of 0.005 μm h⁻¹, the ⁷Be source was found to be relatively well confined in the areas of Mongolia and southeastern Siberia, which further supported that the association of ⁷Be source with the Siberian anticyclone.     

A simulation of sulfur wet deposition and its dependence on the inflow of sulfur species to storms

Numerical precipitation scavenging models are used to investigate the relationship between the inflow concentrations of sulfur species to precipitation systems and the resulting sulfur wet deposition. Simulations have been made for summer and winter seasons using concentration ranges of SO₂, aerosol SO₄²⁻, H₂O₂ and O₃ appropriate for the eastern U.S. summer simulations use one-dimensional timedependent convective cloud and scavenging models; winter simulations use two-dimensional steady-state warm-frontal models. Sulfur scavenging mechanisms include nucleation scavenging of aerosol, aqueous reactions of H₂O₂, O₃ and HCHO with S(IV), and nonreactive S(IV) scavenging. Over the wide range of conditions that have been examined, the relation between sulfur inflow and sulfur wet deposition varies from nearly linear to strongly nonlinear. The degree of nonlinearity is most affected by aerosol SO₄²⁻ levels and relative levels of SO₂ vs H₂O₂. Higher aerosol SO₄²⁻ levels (as found in summer) produce a more linear relation. The greatest nonlinearity occurs when SO₂ exceeds H₂O₂. Winter simulations show more nonlinearity than summer simulations.     

Sensitivity of shortwave radiative fluxes to the vertical distribution of aerosol single scattering albedo in the presence of a desert dust layer

The aim of this work is to quantify the sensitivity of shortwave radiative fluxes to changes in the vertical distribution of aerosol absorption, taken into account through the aerosol Single Scattering Albedo (SSA). The case study represents a real atmospheric situation with a desert dust layer (DDL) in the mid troposphere over an urban Boundary Layer (BL) observed at Rome on 20 June 2007. A moderately high aerosol optical depth (AOD), 0.292 at 550 nm, and low Ångström exponent of 0.30 were measured. The observed case was reconstructed with a radiative transfer model, in which the SSA of the boundary layer aerosols was varied from that of a highly absorbing aerosol type (urban) to a highly scattering one (clear marine). The SSA of the DDL is determined keeping fixed the measured SSA of the whole atmospheric column. The simulations show notable changes in the surface and top of the atmosphere (TOA) diffuse fluxes depending on the boundary layer aerosol properties. The aerosol radiative forcing (ARF) at the surface changes by 6–19 W m^?2, depending on the solar zenith angle, when urban or clean marine particles are included in the boundary layer. The ARF differences observed at TOA are between 1 and 5 W m^?2 when urban and clean marine aerosol types in the BL are respectively used, showing a smaller dependency on the solar zenith angle than at the surface.     

Land-surface exchange in a chemically-reactive system; surface fluxes of HNO3, HCl and NH3

Vertical gradients from 0.25 to 2 m of NH₃, HNO₃ and HCl and associated aerosol components have been measured in the field above various surfaces in eastern England. The data have been examined to identify the effect, if any, of chemical reaction processes upon the observed vertical profiles. It is concluded that chemical transformations are too slow to influence concentration gradients and thus the surface exchange process. Assuming chemically conservative behaviour, deposition velocities for HNO₃ and HCl have been calculated; these lie within the range 0.4–7.7 cms⁻¹ and 0.4–6.9 cms⁻¹ for HNO₃ and HCl, respectively. Estimation of resistances to deposition indicates a negligible surface resistance for both species. Fluxes of ammonia were predominantly upward from the ground with a mean value of 0.031 μg m⁻²s⁻¹ which is consistent both with an emission inventory of the U.K. and with the measured atmospheric concentration of NH₃.     

Major ions and scavenging of sulphate in the norwegian arctic

Precipitation samples at eight sites in the Norwegian Arctic and Northern Norway have been collected and analyzed for the period June 1982–June 1984. The results are compared with data for airborne SO²⁻₄ and SO₂ measured at ground level, and information on concentrations at higher levels from aircraft measurements and model calculations.The scavenging coefficients are generally low under Arctic winter conditions, and high during summer. This can be explained by the different vertical distributions of scavenged material in summer and winter, and differences in precipitation-forming processes. In addition, local climatic conditions with high relative humidity and high concentrations of sea-spray particles result in locally enhanced scavenging efficiencies at the island stations Bjørnøya and Jan Mayen. High NH⁺₄ concentrations in precipitation occur near bird colonies during the breeding period.The wet deposition in the Arctic is small, i.e. an order of magnitude lower than in Southern Norway.     

Atmospheric deposition of polycyclic aromatic hydrocarbons to water surfaces: A mass balance approach

A mass balance model was developed to explain the movement of polycyclic aromatic hydrocarbons (PAH) into and out of Siskiwit Lake, which is located on a wilderness island in northern Lake Superior. Because of its location, the PAH found in this lake must have originated exclusively from atmospheric sources. Using gas Chromatographie mass spectrometry, 11 PAH were quantified in rain, snow, air, lake water, sediment core and sediment trap samples. From the dry deposition fluxes, an aerosol deposition velocity of 0.99 ± 0.15 cm s⁻¹ was calculated for indeno[1,2,3-cd]pyrene and benzo[ghi]perylene, two high molecular weight PAH which are not found in the gas phase. The dry aerosol deposition was found to dominate the wet removal mechanism by an average ratio of 9:1. The dry gas flux was negative, indicating that surface volatilization was taking place; it accounted for 10–80 % of the total output flux depending on the volatility of the PAH. The remaining PAH were lost to sedimentation. From the dry gas flux, an overall mass transfer coefficient for PAH was calculated to be 0.18 ± 0.06 m d⁻¹. In this case, the overall mass transfer is dominated by the liquid phase resistance.     

Beryllium-7 Measurements in the Houston and Phoenix Urban Areas: An Estimation of Upper Atmospheric Ozone Contributions

Abstract

Natural radionuclides have been proposed as a means of assessing the transport of ozone (O₃) and aerosols in the troposphere. Beryllium-7 (⁷Be) is produced in the upper troposphere and lower stratosphere by the interaction of cosmogenic particles with atmospheric nitrogen and oxygen. ⁷Be has a 53.29-day half-life (478 keV γ) and is known to attach to fine particles in the atmosphere once it is formed. It has been suggested that O₃ from aloft can be transported into rural and urban regions during stratospheric–tropospheric folding events leading to increased background levels of O₃ at the surface. ⁷Be can be used as a tracer of upper atmospheric air parcels and the O₃ associated with them. Aerosol samples with a 2.5-µm cutoff were collected during 12-hr cycles (day/night) for a 30-day period at Deer Park, TX, near Houston, in August– September of 2000, and at Waddell, AZ, near Phoenix, in June–July of 2001. A comparison of ⁷Be levels with 12-hr O₃ averages and maxima shows little correlation. Comparison of nighttime and daytime O₃ levels indicate that during the day, when mixing is anticipated to be higher, the correlation of ⁷Be with O₃ in Houston is approximately twice that observed at night. This is consistent with mixing and with the anticipated loss of O₃ by reaction with nitric oxide (NO) and dry deposition. At best, 30% of the O₃ variance can be explained by the correlation with ⁷Be for Houston, less than that for Phoenix where no significant correlation was seen. This result is consistent with the intercept values obtained for ⁷Be correlations with either O₃ 24-hr averages or O₃ 12-hr maxima and is also in the range of the low O₃ levels (25 ppb) observed at Deer Park during a tropical storm event where the O₃ is attributable primarily to background air masses. That is, maximum background O₃ level contributions from stratospheric sources aloft are estimated to be in the range of 15–30 ppb in the Houston, TX, and Phoenix, AZ, area, and levels above these are because of local tropospheric photochemical production.     

Deposition processes of ionic constituents to snow cover

Atmospheric deposition is an important removal process of aerosol particles and gases from the atmosphere. To elucidate the relative contributions of wet and dry processes and in-cloud and below-cloud scavenging based on deposition amounts in winter at Mt. Tateyama, central Japan, we obtained daily samples (December, 2006–March, 2007) of size-segregated aerosol particles and precipitation at Senjyugahara (SJ; 475 m a.s.l.) and vertical samples of spring snow cover at Murododaira (MR, 2450 m a.s.l., 13 km distance from SJ) on the western flank of Mt. Tateyama. The NH₄⁺ and nssSO₄^2? in aerosols were mostly found in the fine fraction (<2 μm), although Na⁺, NO₃^?, and nssCa²⁺ were mainly detected in the coarse fraction (>2 μm). Average ionic concentrations (μg g^?1) in precipitation at SJ were higher about 3.8 for Na⁺ and nssCa²⁺, 3.4 for NO₃^?, 3.7 for NH₄⁺, 2.5 for nssSO₄^2? than those at MR, whereas cumulative precipitation amounts at SJ and MR were, respectively, 84 and 175 cm of water equivalent. Wet and dry deposition amounts during the study period were estimated for sites using size-segregated aerosol data, winter averages of HNO₃, NH₃, and SO₂ concentrations, and dry deposition velocities. Particle-dry deposition comprised about 3% (Na⁺) to 11% (NH₄⁺) of the total deposition at MR. The maximum amounts of gas dry deposition were estimated, respectively, as 4, 13, and 3% of the total deposition at MR for NH₄⁺, NO₃^?, and nssSO₄^2?. The relative contributions of below-cloud scavenging (BCS) between MR and SJ were estimated as considering the wet only deposition amount at MR. Higher contributions of BCS were obtained for Na⁺ (56%) and nssCa²⁺ (45%), whereas BCSs for NH₄⁺, NO₃^?, and nssSO₄^2? were lower than 28%. Ionic constituents existing predominantly in the coarse fraction showed a large contribution of BCS.     

The aging process of naturally emitted aerosol (sea-salt and mineral aerosol) during long range transport

The aging processes of two representative natural aerosol, sea-salt and mineral aerosol, are investigated by using a box model equipped with a thermodynamic module (SCAPE). The model is shown to successfully describe the aging processes between the gas-phase anthropogenic pollutants (SO₂, NO_x, and NH₃) and primary aerosol particles, including self-neutralization process/chlorine depletion in the sea-salt aerosol; formation/dissipation of carbonate and bicarbonate ions in the mineral aerosol; irreversible dynamic deposition of SO₂ and H₂SO₄; and reversible thermodynamic distribution of inorganic volatile species. It is found that SO₂ and H₂SO₄ tend to deposit onto the mode with the largest surface area, and that ammonia deposition is controlled by preceding SO₂/H₂SO₄ deposition. During the SO₂/H₂SO₄ deposition, chloride and carbonate are continuously released from the sea-salt and mineral dust particles, respectively. The findings by the model predictions are consistent with field and observational studies.     

Transport and transformation of sulfur compounds over East Asia during the TRACE-P and ACE-Asia campaigns

On the basis of the recently estimated emission inventory for East Asia with a resolution of 1×1°, the transport and chemical transformation of sulfur compounds over East Asia during the period of 22 February through 4 May 2001 was investigated by using the Models-3 Community Multi-scale Air Quality (CMAQ) modeling system with meteorological fields calculated by the regional atmospheric modeling system (RAMS). For evaluating the model performance simulated concentrations of sulfur dioxide (SO₂) and aerosol sulfate (SO₄²⁻) were compared with the observations on the ground level at four remote sites in Japan and on board aircraft and vessel during the transport and chemical evolution over the Pacific and Asian Pacific regional aerosol characterization experiment field campaigns, and it was found that the model reproduces many of the important features in the observations, including horizontal and vertical gradients. The SO₂ and SO₄²⁻ concentrations show pronounced variations in time and space, with SO₂ and SO₄²⁻ behaving differently due to the interplay of chemical conversion, removal and transport processes. Analysis of model results shows that emission was the dominant term in regulating the SO₂ spatial distribution, while conversion of SO₂ to SO₄²⁻ in the gas phase and the aqueous phase and wet removal were the primary factors that controlled SO₄²⁻ amounts. The gas phase and the aqueous phase have the same importance in oxidizing SO₂, and about 42% sulfur compounds (25% in SO₂) emitted in the model domain was transported out, while about 57% (35% by wet removal processes) was deposited in the domain during the study period.