Seasonal,Episodic and Targeted Control of Sulfate Deposition

The large differences in seasonal rates of wet sulfate deposition observed at many receptors in eastern North America imply that reducing SO₂ emissions only in the summer half of the year (April-September) would bring about greater annual wet sulfate deposition reductions than reducing emissions by the same amount year-round. Targeting the emission reductions to those source areas which contribute the bulk of summer depositions in ecologically sensitive areas would increase further the gain factor, defined as the ratio of annual fractional deposition decrement to annual fractional emission decrement. In the northeastern U.S., between 10 and 15 rain episodes deposit about 60 percent of the annual wet sulfate; reducing emissions in the dry periods preceding these heavy deposition episodes could further increase the gain factor. However, it is difficult to predict these episodes, and they do not occur simultaneously over large regions of the country.     

The relationship of sulfur emissions to sulfate in precipitation—II. Gas phase processes

A climatological box model is applied to wet deposition of sulfur in eastern North America using the extreme assumption that all wet-deposited sulfates originate in sulfate aerosol. The measured sulfate in precipitation during 1977–1979 constrains the probability that an SO₂ molecule emitted in the eastern United States is oxidized before deposition or outflow from the region to values greater than 0.5 and more likely near 1.0. This result implies that uniform SO₂ emission reductions will produce nearly proportional reductions in wet sulfates originating in those emissions and deposited on land. A similar result was obtained previously using the more likely assumption that oxidation and removal of SO₂ in precipitating clouds is the major source of wet deposition of sulfur. Therefore, it is argued that uniform regional reductions in annual average SO₂ emissions will produce nearly proportional reductions in wet sulfur deposited on land and originating in those emissions. The amount of wet sulfate deposition is demonstrated to constrain the mean scale of transport along air parcel trajectories for the precursor of wet sulfur, whether SO₂ or sulfate aerosol, to values greater than 900 km. An examination of previous microscopic photochemical models indicates that the sulfate aerosol formation rate is roughly proportional to local SO₂ concentrations, an inference which is consistent with the result of the climatological model.     

Concentration and dry deposition of atmospheric sulfur and nitrogen compounds in Hungary

The level of regional air pollution is regularly monitored at three stations in Hungary. The comparison of regional concentrations of SO₂ and NO₂ to those measured in Budapest shows that urban level concentration of SO₂ is ten times higher than the value for background conditions. The corresponding figure for NO₂ is five. An increase eastwards across the country can be observed for NO₂ and SO₂, while particulate sulphate, nitrate and ammonium have practically identical concentrations. The concentration of gaseous ammonia has a summer maximum, while the annual variation of particulate ammonium suggests a winter maximum. The ratio of the level of nitric acid to aerosol nitrate is higher than unity in summer, while in winter it is less than 1. The dry deposition of sulfur and oxidized nitrogen compounds is comparable to their wet deposition. However, in the case of NH_x (x = 3 or 4) the wet deposition exceeds the dry deposition by an order of magnitude.     

Wet and dry deposition of sulfur and nitrogen compounds in Ontario

Measurements have been made of sulfur and nitrogen compounds in precipitation since 1980 and in air since 1981 in Ontario. This paper presents results of the atmospheric deposition measurement program to the end of 1985. As is to be expected from the distribution of emission sources, annual concentrations of SO₄²⁻ andNO₃⁻ in precipitation, and of SO₂,SO₄²⁻ andNO₃⁻ in air are higher in southern Ontario than in northern Ontario. The corresponding distribution pattern for deposition is similar to that of concentration. A wet SO₄²⁻ deposition rate of 20 kg ha¹⁻ y¹⁻, a value considered critical for the acidification of sensitive water bodies, is exceeded in all of central and southern Ontario. On a province-wide basis, sulfur wet deposition is about four times higher than sulfur dry deposition. For nitrogen, wet and dry deposition are more comparable, though the former is still higher. The S- and N-species display different seasonal trends in concentration and deposition reflecting a dependence on meteorological factors, and on the associated chemical transformation rates. On the other hand, year to year variations are small.     

Sulfur processing in the marine atmospheric boundary layer: A review and critical assessment of modeling uncertainties

Sulfur is an extremely motile and vital element in the Earth's biogeochemical environment, one whose active redox chemistry maintains small reservoirs in the atmosphere and biosphere yet large fluxes through both. Essential for life, intimately linked to the climate state, and an important component of air quality, sulfur and its transport and processing in the atmosphere have been the subject of active research for several decades. This review article describes the current state of our understanding of the atmospheric sulfur cycle, focusing on the marine atmospheric boundary layer, with the aim of identifying the largest roots of uncertainty that most inhibit accurate simulation of sulfur cycling in the atmosphere. An overview of the emissions by phytoplankton and shipping, dispersion and entrainment in the marine boundary layer, and chemical processing by aerosols, clouds, and dry deposition is presented. Analysis of 20 contemporary modeling studies suggests that the greatest ambiguity in global sulfur cycling derives from (in descending order) wet deposition of aerosol sulfate, dry deposition of sulfur dioxide to the Earth's surface, and the heterogeneous oxidation of SO₂ in aerosols and clouds.     

A theoretical assessment of pollutant deposition to individual land types during a regional-scale acid deposition episode

A mesoscale model of pollutant transport, transformation and deposition was used to perform a detailed analysis of acidic deposition to the states of New York and Ohio during a 3-day springtime deposition episode. This model can be used to assess the roles of wet and dry deposition to individual land types in the removal of pollutants from the atmosphere. Over two-thirds (67 %, Ohio; 78 %, New York) of the acidic deposition during this rainy period fell as wet deposition, primarily in the form of H₂SO₄. Dry deposition of SO₂ accounted for 70–75 % of the total dry acidic deposition in both areas, and most of the remaining dry deposition occurred as HNO₃. Over both deposition areas, particulate sulfate deposition accounted for <1 % of the total acid deposition. Due to the highly surface-specific nature of the dry deposition process, individual land types displayed unique patterns of pollutant uptake. Water surfaces absorbed primarily SO₂, while rougher forested areas absorbed a larger proportion of HNO₃ vapor. Urban areas, with their associated material surfaces, were found to absorb significantly less acid in the dry form, and during dry periods most of this deposition may occur as HNO₃ vapor, although considerable uncertainty exists regarding the treatment of rainfall-wetted surfaces. These model results suggest that dry pollutant fluxes to individual surface types will show significant variability from any ‘averaged’ flux estimates over larger areas encompassing numerous land types.     

Anthropogenic sulphate aerosol from India: estimates of burden and direct radiative forcing

A one-box chemical-meteorological model had been formulated to make preliminary estimates of sulphate aerosol formation and direct radiative forcing over India. Anthropogenic SO₂ emissions from India, from industrial fuel use and biomass burning, were estimated at 2.0 Tg S yr^-1 for 1990 in the range of previous estimates of 1.54 and 2.55 Tg S yr ^-1 for 1987. Meteorological parameters for 1990 from 18 Indian Meteorological Department stations were used to estimate spatial average sulphate burdens through formation from SO₂ reactions in gas and aqueous phase and removal by dry and wet deposition. The hydrogen peroxide reaction was found dominating for undepleted oxidant-rich conditions. Monthly mean sulphate burdens ranged from 2–10 mg m^-2 with a seasonal variation of winter–spring highs and summer lows in agreement with previous GCM studies. The sulphate burdens are dominated by sulphate removal rates by wet deposition, which are high in the monsoon period from June–November. Monthly mean direct radiative forcing from sulphate aerosols is high (−3.5 and −2.3 W m^-2) in December and January, is moderate (−1.3 to −1.5 W m^-2) during February to April and November and low (−0.4 to −0.6 W m^-2) during May to October also in general agreement with previous GCM estimates. This model, in reasonable agreement with detailed GCM results, gives us a simple tool to make preliminary estimates of sulphate burdens and direct radiative forcing.     

Comparison of surrogate surface techniques for estimation of sulfate dry deposition

The dry deposition rates of sulfate particles to artificial surfaces within and above a mature hardwood forest were measured over an annual range of synoptic weather conditions. Artificial, or ‘surrogate’, surfaces representing both rough and smooth textural types included deposition buckets, petri dishes, filter paper, Teflon configurations and polycarbonate membranes. Ambient concentrations of sulfate and sulfur dioxide were also monitored.The artificial surfaces were evaluated on the basis of the magnitude of the sulfate dry deposition rates and measurement precision. Correlations between techniques and the magnitude of the deposition velocities identified technique similarities. Ambient concentrations of the sulfur oxides and the deposition rates were not well correlated. For diverse reasons, many of the techniques were found to have limited reliability. The petri dish, bucket inside and filter plate surfaces were found to represent the most precise devices for the estimation of dry deposition to smooth, complex and rough artificial surfaces, respectively. Seasonal averages for samplers exposed at all heights were 11.2, 27.7 and 71.2μg SO₄²⁻m⁻²h⁻¹, yielding mean deposition velocities to surfaces exposed within the forest canopy of 0.03,0.11 and 0.14 cm s⁻¹ and an annual estimate of the potential dry deposition to a foliated hardwood forest of 4.0, 11.5 and 21.0 kg SO₄²⁻ha⁻¹ for the petri dish, bucket inside and filter plate surfaces, respectively. The indirect ratio between deposition rates and velocities results from varying concentrations of ambient sulfate between sampling periods. The accuracy of the filter plate data is suspect due to a significant correlation with sulfur dioxide concentrations. Sulfur concentration and deposition rate gradients indicate the forest is providing a net sink for sulfur pollutants during periods with foliage.The wide range of dry deposition rates estimated from the variety of deposition surfaces emphasizes the uncertainty of the artificial surface measurement techniques. In spite of these limitations, surrogate surfaces provide an estimate of sulfate flux rates not currently obtainable from natural surfaces.     

The scavenging of atmospheric sulfate by arctic snow

Scavenging ratios for sulfate on the south-central Greenland Ice Sheet at Dye 3 have been computed for 1982–1984. The ratios are based on measured concentrations in snow and estimated concentrations in air. The snow data have been obtained from snowpit samples which were dated by comparing δ¹⁸O values with meteorological records. The airborne concentrations have been estimated from data collected at coastal Greenland sites. Scavenging ratios resulting from this process are found to be in the range ~ 100–200 in winter and ~ 200–400 in summer. The greater summer values are attributed to increased riming, resulting in scavenging of sulfate as condensation nuclei and possible oxidation of SO₂ in cloudwater droplets. Using the airborne and snowpit concentrations with assumed dry deposition velocities of 0.02–0.05 cms⁻, it is estimated that dry deposition is responsible for roughly 10–30% of the total sulfate deposition on a year-round basis at Dye 3. During portions of the Arctic winter, however, when the snow is unrimed and when there is less precipitation, dry deposition may be dominant.     

Atmospheric deposition of organic carbon to Chesapeake Bay

The organic carbon content of wet and bulk deposition was measured from February through to December 1981 at four stations surrounding Chesapeake Bay. Organic carbon is removed from the atmosphere in association with both wet and dry deposition. Regional yearly wet and dry depositional rates were approximately equal, with a total yearly deposition rate of 11.3 g C m⁻² y⁻¹. Atmospheric deposition provides an important flux of organic material to Chesapeake Bay and must be considered if biogeochemical cycles are to be fully understood.     

An evaluation of sulfate in European precipitation 1955–1982

More than 25 years of data on sulfate in precipitation from the European Air Chemistry Network (EACN) are analysed for seasonal and long-term trends and their spatial variability. The number of stations has varied between about 50 and 100, all of them located in the central and northern parts of western Europe. Despite considerable shortcomings of the data (indicated among other things by poor ionic balance during several years, particularly in the late 1950s and the early 1960s) the following conclusions are drawn.In Norway, Sweden, Denmark and Finland, the sulfate concentration increased by roughly 50 per cent between the late 1950s and the late 1960s. A further increase during the 1970s is indicated at the Danish stations but most stations in Sweden and Norway show a decline by on the average about 20 per cent since the early 1970s. Sulfate data from the U.K. and the European continent seem to exhibit less systematic variations.A comparison between the long-term changes in the Scandinavian data on sulfate in precipitation and the corresponding changes in anthropogenic SO₂ emission in Europe indicates a fair agreement with regard both to the increase in emission during the 1950s and 1960s and, except for the Danish stations, also the subsequent decrease in emission in several of the West European countries.In most parts of the network the annual cycle of concentration of sulfate exhibits a maximum in February through May and a minimum in July to October, whereas the wet deposition normally has its maximum in May to August and minimum in December to March. A comparison with the annual cycle of anthropogenic emission, which has a clear maximum in December to March, indicates that the fraction of sulfur exported out of the region is larger in winter than in summer.     

Relation between estimated dry deposition and throughfall in a coniferous forest exposed to controlled levels of SO2 and NO2

Throughfall was collected in a Scots pine forest exposed to about 14 microg m(-3) of both SO2 and NO2, and in a control forest with 1 microg m(-3) SO2 and < 1 microg m(-3) NO2. Precipitation was collected in a nearby open field. Collection was performed on an event basis during the whole vegetation period. Exposure was made by an open-air release system during the vegetation period, except during rain and at night. Additional sulfate deposition in the exposed forest (compared to control forest) was nearly equal to dry deposition of sulfur dioxide, as estimated with a stomatal conductance model adapted for the particular forest. It is thus concluded that essentially all of the dry deposited sulfur dioxide is eventually extracted and appears in throughfall-including the fraction that has been deposited through stomata. Attempts to relate net throughfall deposition to dry deposition of sulfate in the control forest were inconclusive, since a minor (10%) uncertainty in the water balance had a major influence on calculated deposition velocity for particulate sulfate. Nitrate throughfall deposition is about half of the open field wet deposition, both for the exposed and control forest. Thus, a long-term exposure with about 14 microg m(-3) NO2 decreased nitrate throughfall deposition.     

Analysis of observations relevant to long-range transport and deposition of pollutants

Observations of annual wet deposition of sulfur made during 1980 at 62 stations in northeastern America are interpreted using a statistical long-range transport model. This work is meant to demonstrate the role of an empirical model in the analysis of observations. Our analysis points to the following conclusions:

• 1.(1) The parameters that represent the conversion of SO₂ to SO₄ and the wet and dry removal of sulfur are insensitive to concentration levels,
• 2.(2) the variation of the wet deposition field is closely related to the distribution of sulfur emissions and
• 3.(3) observations demand efficient wet scavenging of SO₂.

     

Observation of SO2 dry deposition velocity at a high elevation flux tower over an evergreen broadleaf forest in Central Taiwan

A 60-m flux tower was built on a 2100 m mountain for the measurement of the air pollutant concentration and the evaluation of dry deposition velocity in Central Taiwan. The tower was constructed in an evergreen broadleaf forest, which is the dominant species of forest in the world. Multiple-level SO₂ concentrations and meteorological variables at the site were measured from February to April 2008. The results showed that the mean dry deposition velocities of SO₂ were 0.61 cm s^?1 during daytime and 0.27 cm s^?1 during nighttime. From the comparison of the monthly data, a tendency was observed that the dry deposition velocity increases with LAI and solar radiation. Furthermore, it was observed that the deposition velocity was larger over wet canopy than over dry canopy, and that higher deposition velocities in the wet season were mainly caused by non-stomatal uptake of wet canopy. Over wet canopy, the mean dry deposition velocities of SO₂ were estimated to be 0.83 cm s^?1 during daytime and 0.47 cm s^?1 during nighttime; and 0.44 cm s^?1 during daytime and 0.19 cm s^?1 during nighttime over dry canopy. There is good agreement between the results of this study and those in other studies and the predictions of Zhang et al. (2003a). The medians (geometric means) of derived r_c during daytime are 233 (266) m s^?1 over dry canopy and 147 (146) m s^?1 over wet canopy. It was found that solar radiation is the critical important meteorological variable determining stomatal resistance during daytime. For non-stomatal resistance, clear dependencies were observed on the friction velocity and relative humidity.     

Spatial patterns in wet and dry deposition of atmospheric mercury and trace elements in central Illinois,USA

An intensive 1-month atmospheric sampling campaign was conducted concurrently at eight monitoring sites in central Illinois, USA, from June 9 to July 3, 2011 to assess spatial patterns in wet and dry deposition of mercury and other trace elements. Summed wet deposition of mercury ranged from 3.1 to 5.4 μg/m² across sites for the total study period, while summed dry deposition of reactive mercury (gaseous oxidized mercury plus particulate bound mercury) ranged from 0.7 to 1.6 μg/m², with no statistically significant differences found spatially between northern and southern sites. Ratios of summed wet to summed dry mercury deposition across sites ranged from 2.2 to 4.9 indicating that wet deposition of mercury was dominant during the study period. Volume-weighted mean mercury concentrations in precipitation were found to be significantly higher at northern sites, while precipitation depth was significantly higher at southern sites. These results showed that substantial amounts of mercury deposition, especially wet deposition, occurred during the study period relative to typical annual wet deposition levels. Summed wet deposition of anthropogenic trace elements was much higher, compared to summed dry deposition, for sulfur, selenium, and copper, while at some sites summed dry deposition dominated summed wet deposition for lead and zinc. This study highlights that while wet deposition of Hg was dominant during this spring/summer-season study, Hg dry deposition also contributed an important fraction and should be considered for implementation in future Hg deposition monitoring studies.     

Wet and dry deposition monitoring in Southeastern Michigan

Wet and dry deposition as collected by a bucket were measured at two sites in southeastern Michigan for two years. The precipitation had an average pH of 4.27 and a SO²⁻₄ to NO⁻₃ ratio of 2.0. Particulate dry deposition velocities of 0.6 cm s⁻¹ for SO²⁻₄ and NO⁻₃ and > 2 cm s⁻¹ for Cl⁻, Ca²⁺, Mg²⁺,Na⁺ and K⁺ were calculated. The ambient particle composition, dry bucket collection and wet deposition were compared at two sites, one urban and the other rural. Higher ambient particle concentrations and dry deposition rates were measured at the urban site than the rural site, indicating the influence of local emissions. However, local emissions had no effect on the wet deposition concentrations. The influence of more distant source regions was examined by separating the precipitation events by wind direction. The events from the south and east had the highest SO²⁻₄ to NO⁻₃ ratios, which corresponded to the areas with the highest sulfur emissions. NO⁻₃ showed no directional dependence.Wet deposition was examined for the effect of storm type and seasonal trends. Contrary to a recent study on Long Island, we found higher concentrations of H⁺, SO²⁻₄ and NH⁺₄ in winter rain compared to snow. The wet deposition concentrations of H⁺, SO²⁻₄, and NH⁺₄ were highest in the summer, while only Na⁺ and Cl⁻ concentrations were highest in the winter, presumably due to winter road salting. The total deposition of acidic ions was highest in the summer and lowest in the winter, due both to lower concentrations and lower precipitation volumes in the winter. The dry deposition as collected by a bucket accounted for 1 % of total H⁺ deposition, 21 % of SO²⁻₄ deposition, 27% of NO⁻₃ deposition, 50% of Cl⁻ deposition and 61 % of Ca²⁺ deposition.     

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.     

Sulfur deposition simulations over China, Japan, and Korea: a model intercomparison study for abating sulfur emission

In response to increasing trends in sulfur deposition in Northeast Asia, three countries in the region (China, Japan, and Korea) agreed to devise abatement strategies. The concepts of critical loads and source?Creceptor (S?CR) relationships provide guidance for formulating such strategies. Based on the Long-range Transboundary Air Pollutants in Northeast Asia (LTP) project, this study analyzes sulfur deposition data in order to optimize acidic loads over the three countries. The three groups involved in this study carried out a full year (2002) of sulfur deposition modeling over the geographic region spanning the three countries, using three air quality models: MM5-CMAQ, MM5-RAQM, and RAMS-CADM, employed by Chinese, Japanese, and Korean modeling groups, respectively. Each model employed its own meteorological numerical model and model parameters. Only the emission rates for SO₂ and NO_x obtained from the LTP project were the common parameter used in the three models. Three models revealed some bias from dry to wet deposition, particularly the latter because of the bias in annual precipitation. This finding points to the need for further sensitivity tests of the wet removal rates in association with underlying cloud?Cprecipitation physics and parameterizations. Despite this bias, the annual total (dry plus wet) sulfur deposition predicted by the models were surprisingly very similar. The ensemble average annual total deposition was 7,203.6?±?370 kt S with a minimal mean fractional error (MFE) of 8.95?±?5.24?% and a pattern correlation (PC) of 0.89?C0.93 between the models. This exercise revealed that despite rather poor error scores in comparison with observations, these consistent total deposition values across the three models, based on LTP group's input data assumptions, suggest a plausible S?CR relationship that can be applied to the next task of designing cost-effective emission abatement strategies.     

Atmospheric particle size distributions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs) and their implications for wet and dry deposition

Concurrent measurements of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs) in different size fractions of atmospheric particulate matter are presented for a winter and a summer sampling period. The PCDD/Fs and PAHs were primarily associated with particles of <1.35 μm aerodynamic diameter. The particle size distributions were similar for the compounds within each substance group and, surprisingly, also between the PCDD/Fs and PAHs. Changes in the particle size distribution of particle mass were reflected in the particle size distributions of the PCDD/Fs and PAHs.The data were employed to identify those particle size fractions dominating the wet and dry particle bound deposition of PCDD/Fs and PAHs and, furthermore, to assess the relative contributions of wet and dry deposition to the total particle bound deposition fluxes. The calculations indicate that coarse particles contribute most to the dry deposition while, in contrast, the wet deposition of the PCDD/Fs and PAHs is dominated by fine particles. Furthermore, it is estimated that in Bayreuth wet deposition dominates the total particle bound deposition of PCDD/Fs and PAHs.     

A long-range air pollution transport model for eastern north america—I. Sulfur oxides

This paper describes a Lagrangian model for simulating long-range air pollution transport over eastern North America. One version (denoted ENAMAP-2S) uses emissions inventories and standard daily weather reports to compute the airborne concentrations of SO₂ and sulfate, and their deposition on the earth's surface. A separate version of the model (ENAMAP-2N) applies to oxides of nitrogen. These versions of the ENAMAP model include the influences of smoothed terrain on the winds, and divide the atmospheric boundary layer into three parts, allowing pollutant emissions to be divided among the layers. Vertical mixing is controlled by diffusion coefficients computed from wind shear, stability, and knowledge of the mixing depth. The transformation and wet and dry deposition rates are based on recently published values.The models have been used to compute monthly average values of concentrations and depositions for January and August 1977. Typical examples are shown. The sensitivity of the output fields of ENAMAP-2S to values selected for input parameters (e.g. transformation rate of SO₂ to sulfate and deposition rates) has been studied; and based on present information, optimum values of the parameters are given. The model results include geographical patterns of pollutant concentration and deposition, and interregional exchange tables that show how much of the pollution in a certain region was emitted locally and how much originated in other specified areas. The simulated monthly airborne concentrations of SO₂ and sulfate are generally within a factor of 2 of measured values.