Cumulus cloud transport,scavenging and chemistry: Observations and simulations

Observational and numerical investigations of cumulus cloud scavenging, transport and chemical processes are presented. The experimental data set includes surface and aircraft measurements of the chemistry and microphysics of aerosol, cloud and precipitation. To help in the interpretation of these experimental data fully three-dimensional simulations of cloud chemistry and scavenging are performed. After adjusting several unmeasured model parameters, reasonable agreement could be obtained between the simulated and observed cloud chemistry and aerosol distribution in clouds. The rate at which the simulated clouds transported and transformed pollutants did not exceed a few per cent per hour.     

The MICS-Asia study: model intercomparison of long-range transport and sulfur deposition in East Asia

An intercomparison study involving eight long-range transport models for sulfur deposition in East Asia has been initiated. The participating models included Eulerian and Lagrangian frameworks, with a wide variety of vertical resolutions and numerical approaches. Results from this study, in which models used common data sets for emissions, meteorology, and dry, wet and chemical conversion rates, are reported and discussed. Model results for sulfur dioxide and sulfate concentrations, wet deposition amounts, for the period January and May 1993, are compared with observed quantities at 18 surface sites in East Asia. At many sites the ensemble of models is found to have high skill in predicting observed quantities. At other sites all models show poor predictive capabilities. Source–receptor relationships estimated by the models are also compared. The models show a high degree of consistency in identifying the main source–receptor relationships, as well as in the relative contributions of wet/dry pathways for removal. But at some locations estimated deposition amounts can vary by a factor or 5. The influence of model structure and parameters on model performance is discussed. The main factors determining the deposition fields are the emissions and underlying meteorological fields. Model structure in terms of vertical resolution is found to be more important than the parameterizations used for chemical conversion and removal, as these processes are highly coupled and often work in compensating directions.     

Water-soluble main ions in precipitation over the southeastern Adriatic region: chemical composition and long-range transport

Background, aim and scope  

Precipitation samples collected from 1995 to 2000 at meteorological station in the eastern outskirts of Herceg Novi (Montenegro) were analysed on Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^–, SO₄ ^2–, NO₃ ^– and NH₄ ⁺. Four-day backward trajectory simulations were conducted during the precipitation period to investigate the regional transport of main ions and their deposition in the region of the southeastern Adriatic Sea. The air mass trajectories were classified into six trajectory categories by the origin and direction of their approach to Herceg Novi.     

Simulation of long-range transport of acidic pollutants in East Asia during the Yellow Sand event

The atmospheric chemical process was simulated using the Carbon Bond 4 (CB-4) model, the aqueous-phase chemistry in Regional Acid Deposition Model and the thermodynamic equilibrium relation of aerosols with the emission inventories of the Emission Database for Global Atmospheric Research, the database of China and South Korea and the Mesoscale Model version 2 (MM5) meteorological fields to examine the spatial distributions of the acidic pollutant concentrations in East Asia for the case of the long-lasting Yellow Sand event in April 1998. The present models simulate quite well the observed general trend and the diurnal variation of concentrations of gaseous pollutants, especially for O₃ concentration. However, the model underestimates SO₂ and NO_x concentration but overestimates O₃ concentration largely due to uncertainty in NO_x and VOC emissions. It is found that the simulated gaseous pollutants such as SO₂, NO_x, and NH₃ are not transported far away from the source regions but show significant diurnal variations of their concentrations. However, the daily variations of the concentrations are not significant due to invariant emission rates. On the other hand, concentrations of the transformed pollutants including SO₄²⁻, NH₄⁺, and NO₃⁻ are found to have significant daily variations but little diurnal variations. The model-estimated deposition indicates that dry deposition is largely contributed by gaseous pollutants while wet deposition of pollutants is mainly contributed by the transformed pollutants.     

Meteorology of long-range transport

The increasing attention to regional-scale transport of pollution has resulted in numerous air quality networks, models and large scale field studies to relate the sources of pollution to subsequent air quality measurements and thence to the effects of pollution. We compare observed air quality and trajectory data to calculations to quantitatively evaluate the differences. Methods of preparing air trajectories are described with emphasis on the Air Resources Laboratories' trajectory model and an accompanying diffusion model. The comparison of observed atmospheric transport with calculated trajectories shows that large computational errors can occur and more importantly that these may be systematic, but different, depending on the type of advection (cold vs warm) and whether the transport is over land or over the sea. We describe the use of tetroon recovery locations in trajectory analysis and show that such data can be obtained over distances up to 4000 km. Computed trajectories based on the geostrophic assumption and on the observed wind field are compared to observations to determine the adjustments required to obtain the best comparisons. Directional adjustments of up to 40° and changes in speed by a factor of two are sometimes necessary. We make suggestions for studies to improve the capability of calculating trajectories, including experiments using balloons and controlled tracer releases. Both of these techniques are applicable over regional scales. Finally we show the global distribution of sulfur from the industrial areas of the Northern Hemisphere, as calculated by an efficient computer model, as a step in the determination of the global sulfur budget.     

Nonlinear regression and ARIMA models for precipitation chemistry in East Central Florida from 1978 to 1997

Continuous monitoring of precipitation in East Central Florida has occurred since 1978 at a sampling site located on the University of Central Florida (UCF) campus. Monthly volume-weighted average (VWA) concentration for several major analytes that are present in precipitation samples was calculated from samples collected daily. Monthly VWA concentration and wet deposition of H(+), NH(4)(+), Ca(2+), Mg(2+), NO(3)(-), Cl(-) and SO(4)(2-) were evaluated by a nonlinear regression (NLR) model that considered 10-year data (from 1978 to 1987) and 20-year data (from 1978 to 1997). Little change in the NLR parameter estimates was indicated among the 10-year and 20-year evaluations except for general decreases in the predicted trends from the 10-year to the 20-year fits. Box-Jenkins autoregressive integrated moving average (ARIMA) models with linear trend were considered as an alternative to the NLR models for these data. The NLR and ARIMA model forecasts for 1998 were compared to the actual 1998 data. For monthly VWA concentration values, the two models gave similar results. For the wet deposition values, the ARIMA models performed considerably better.     

Implications for long-range atmospheric transport of polycyclic aromatic hydrocarbons in Lhasa, China

The Tibetan Plateau is suggested to be an important indicator region to study the global long-range atmospheric transport of persistent organic pollutants. In this study, atmospheric polycyclic aromatic hydrocarbons (PAHs) were studied in Lhasa City in the Tibetan Plateau, China. Air samples in gas and particle phases were concurrently collected by a modified high-volume air sampler from 5 August 2008 to 13 July 2009. The concentration of ∑₁₆PAHs ranged from 18 to 160 ng?m^?3 (with a geometric mean of 68 ng?m^?3). The most abundant PAHs were phenanthrene and benzo(b)fluoranthene in gas and particle phases, respectively. Compared with other two similar studies in Beijing and Harbin, different temporal trends were found between gas and particle phases PAHs in Lhasa. The influences of meteorological parameters (ambient temperature and relative humidity) and air masses from China, India, Southeast Asia, and West Asia were the two important reasons for explaining the difference, which was confirmed by the 5-day backward trajectories. This is the first comprehensive study to provide the evidence for the different influences of long-range atmospheric transport on gas and particle phases PAHs pollution in the Tibetan Plateau.     

Study of aerosol transport through precipitation chemistry over Arabian Sea during winter and summer monsoons

Precipitation samples over the Arabian Sea collected during Arabian Sea Monsoon Experiment (ARMEX) in 2002–2003 were examined for major water soluble components and acidity of aerosols during the period of winter and summer monsoon seasons. The pH of rain water was alkaline during summer monsoon and acidic during winter monsoon. Summer monsoon precipitation showed dominance of sea-salt components (∼90%) and significant amounts of non-sea salt (nss) Ca²⁺ and SO₄²⁻. Winter monsoon precipitation samples showed higher concentration of NO₃⁻ and NH₄⁺ compared to that of summer monsoon, indicating more influence of anthropogenic sources. The rain water data is interpreted in terms of long-range transport and background pollution. In summer monsoon, air masses passing over the north African and Gulf continents which may be carrying nss components are advected towards the observational location. Also, prevailing strong southwesterly winds at surface level produced sea-salt aerosols which led to high sea-salt contribution in precipitation. While in winter monsoon, it was observed that, air masses coming from Asian region towards observational location carry more pollutants like NO₃⁻and nss SO₄²⁻ that acidify the precipitation.     

An anticyclonic point of view on low-level tropospheric long-range transport

Research on Arctic haze has provided an example when anticyclones may play a dominant role in carrying out low-level tropospheric long-range transport. This dominant role of anticyclones in transporting Arctic haze may be the result of the unique geographic and climatological situation existing during winter/spring in which both the huge Eurasian continent and the adjacent ice-covered Arctic Ocean tend to be regions where anticyclones form and exist over long periods of the winter and spring seasons. It is assumed that the seasonal variation of transport mechanisms provided by anticyclones is the primary cause for the seasonal variation of Arctic haze. Centers of anticyclones are the regions where air masses form and obtain their characteristics, both meteorological and chemical, due to the aerosols and gases released into the air. Transport within an air flow along the edges of quasi-stationary anticyclones will remain under stable atmospheric conditions, hence, dilution, lifting and removal of aerosols and gases will be less compared to a transport within the influence of a cyclonic pressure system. According to the concept of isentropic flow, anticyclones may dominate only low-level transport, whereas cyclones may be more important in controlling transport at upper tropospheric levels.     

Effects of below-cloud gas scavenging on raindrop chemistry over remote ocean regions

A particularly simple case in which raindrop chemistry may be studied is in rain over remote ocean regions, where natural spatial and temporal chemical concentrations are relatively unperturbed. Recent observations of the stable molecules and ions in marine air and marine rain have been used as initial conditions for a computational model of raindrop chemistry over the North Atlantic and Equatorial Pacific Oceans. The model incorporates below-cloud gas scavenging as well as an extensive set of inorganic and organic chemical reactions. The results include the following:Photochemical processes are central to the free radical chemistry of raindrops over the oceans.Within raindrops, the concentrations of species may be controlled by their initial concentrations prior to fall (as is the case for NO₃⁻), by their rate of incorporation from the gas phase (as with RO₂·), or by chemical equilibria or reactions (as with O₂⁻). This control sometimes differs in different geographical areas; for example, aqueous organic acids are produced about equally by incorporation and by reaction in the Equatorial Pacific calculations but are produced primarily by reaction in the North Atlantic calculations.Of the inorganic ions in these calculations, ammonium changes the most (as much as 20 %) during raindrop fall. However, differences in solar flux and gas concentration influences on raindrop chemistry are insufficient to explain observed differences in inorganic ion concentrations in North Atlantic and Equatorial Pacific raindrops.Organic thermochemical and photochemical processes have the potential to play interesting and important roles in raindrop chemistry over remote ocean regions, particularly if formaldehyde and alkyl hydroperoxide concentrations in the gas phase are large.     

Sensitivity study of air trajectory long-range transport modelling

A forward trajectory model for long-term regional scale simulation of transport and deposition of sulphur from a single elevated point source is used for sensitivity tests of the deposition pattern and suspension ratio, subject to a spatial and temporal variable mixing depth, contra constant mixing depth different oxidation rates of SO₂, wet deposition rates and different source heights. The deposition is studied up to 1800 km from the source using meteorological data measured every 6 h covering Europe. For dry conditions, the inclusion of a diurnally variable mixing depth causes longer residence times of the airborne S and larger deposition far from the source. An effective wet deposition rate, however, has a tendency to suppress the change in the dry deposited mass induced by variable mixing height. A change from an oxidation rate of about 1 to 3%h⁻¹ increases both the suspension ratio and the deposition significantly at large distances from the source, while a change to a low value, 0.5 % h⁻¹ has the opposite effect. Halving the wet removal rates results in about 30% larger deposition at remote locations and 30% lower deposition occurs when the removal rates are doubled. Similar changes occur in the suspension ratio. A low source causes about 60% higher local depositions and consequently reduced suspension ratios and distant depositions of about 30%. These results support the assumption that the uncertainty in predictions of total deposition and residence times is within about ±50% when the uncertainty in the parameters considered is taken into account.     

Assessment of the environmental persistence and long-range transport of endosulfan

Concentrations of the insecticide endosulfan (α- and β-isomer) and its degradation product endosulfan sulfate in air, seawater and soil are calculated with the global environmental fate model CliMoChem. As model input, physicochemical properties of all three compounds were assembled and a latitudinally and temporally resolved emission inventory was generated. For concentrations in air, model and measurements are in good agreement; a bimodal seasonality with two peaks in spring and fall as it is observed in Arctic air is reproduced by the model. For seawater, the agreement of model and measurements depends on the values of the hydrolysis activation energy of endosulfan used in the model; with relatively high values around 100 kJ/mol, model results match field data well. The results of this assessment of the levels, persistence, and global distribution of endosulfan are also relevant for the evaluation of endosulfan as a Persistent Organic Pollutant under the Stockholm Convention.     

Satellite detection of long-range pollution transport and sulfate aerosol hazes

While not designed for such a task, meteorological satellites now play a growing role in our understanding of long range transport of secondary pollutants. This paper reports on a demonstration project showing that currently available synchronous satellite data can detect the aerial extent and motion of large-scale “hazy” air masses associated with sulfate and ozone episodes. An interactive computer graphics system is utilized showing that digital satellite data can obtain precise measurements of upward scattered solar radiation which is correlated to aerosol optical thickness and therefore to sulfate concentrations. Measurements over Lake Michigan for instance, reveal over-water image brightness enhanced fully 60–70% as visibility estimates of fb_scatincreasedfrom 1.8 × 10⁴m⁻¹to 5.7 × 10⁻⁴m⁻¹.Digital satellite data is shown to have great promise in mapping sulfate haze areas, especially over water.     

The oxidation and long-range transport of sulphur dioxide in a remote region

The flux of sulphur dioxide has been measured in the plume of a remotely situated smelter (Mount Isa, Australia) at distances of up to 1000 km from the source. These measurements were made with an airborne correlation spectrometer in plumes ranging in age from 1.0 to 42.5 h, the latter corresponding to a photolytic age of almost two periods of daylight. Ground-based experiments were also performed to determine the rate of dry deposition of sulphur dioxide in the area situated about 500 km from the source.The implications of an error in the deposition velocities, reported by Milne J.W., Roberts D.B. and Williams D.J. (Atmos. Envir. 13, 373–380, 1979.), are considered. The average velocity for the Mount Isa region is higher than that previously employed by Roberts D. B and Williams D.J. (Atmos. Envir. 13, 1485–1489, 1979). Along with a reassessment of the diurnal model and the average mixing height encountered by Roberts and Williams, this necessitates a reduction in the estimate of the photochemical oxidation rate of sulphur dioxide from 0.25 to 0.15%h⁻¹, when averaged over 24 h.It is shown in this study that the rate of loss of sulphur dioxide from the plume out to the furthest distances measured can be accounted for by the combined processes of atmospheric oxidation and dry deposition, as determined by Roberts and Williams for much shorter distances (< 260 km). The lifetime of sulphur dioxide in the region under study is shown to be fourteen days, with about half of the loss being due to atmospheric oxidation.     

Acid rain: chemistry and transport

This review describes the more important features of the emission, chemistry, transport and deposition of pollutants involved in acid deposition. Global emissions, both natural and man-made, of sulphur and nitrogen oxides are discussed and examples of spatial distributions and trends over the last century presented. The more significant chemical and physical processes involved in the transformation of the primary emissions into their acidic end products are described, including a summary of the approximate timescales of the processes involved. Measurements and modelled calculations of spatial and temporal patterns in the deposition of acidic pollutants by both wet and dry pathways are presented.     

Trend and seasonal variation of precipitation chemistry data in the Netherlands

A multiple regression model is introduced to describe temporal variations in precipitation chemistry data. The model considers the effects of the annual cycle, a linear trend and precipitation-quantity simultaneously. The paper discusses the application of the model to concentrations and depositions of hydronium, ammonium, nitrate and sulfate for monthly bulk samples in The Netherlands for the period 1978–1984.Statistical conclusions about the annual cycle and the trend are hardly influenced by the choice of the dependent variable (depositions, concentrations or logarithms of concentrations). With the exception of H⁺ concentrations, a large part of the temporal variations was due to precipitation-quantity effects. Significant annual cycles were found for nitrate, ammonium and sulfate. There was statistical evidence of a downward trend for sulfate and nitrate. A complex, non-linear trend was observed for H⁺ which resulted in a significant autocorrelation of the residuals from the regression equation.Much attention is paid to the detectability of trend. For ammonium, nitrate and sulfate it is possible to discriminate small systematic changes in quite short records (a mean annual change of 4–6% in a 5-year record). This is not the case for H ⁺, because temporal variations of this component are insufficiently explained by the systematic annual cycle and precipitation-quantity.     

Modelling long-range transport of primary particulate material over Europe

This paper uses a simple model of atmospheric transport and an emissions inventory prepared by TNO to estimate the contribution of primary particulate material to PM₁₀ and PM_2.5 concentration across Europe. The resulting population exposure is compared with that of secondary particulates, and it is noted that both primary and secondary contributions will be significantly reduced with the implementation of new protocols under the Convention on Long-Range Transboundary Air Pollution (CLRTAP). Since concentrations of primary PM₁₀ can become elevated in episodic situations, when long-range transport of particulate could, on its own, exceed 24 h average targets of 50 μg m⁻³ over large areas of Europe, such reduction is important for achievement of current air quality standards to control exposure to atmospheric particulate PM₁₀.     

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.