Atmospheric Scavenging Processes Research Articles

In-cloud and below-cloud numerical simulation of scavenging processes at Serra Do Mar region, SE Brazil

Atmospheric scavenging processes have been investigated, taking into consideration a numerical simulation through the model Regional Atmospheric Modeling System (RAMS), the below-cloud scavenging model, local atmospheric conditions and local emissions in the Serra do Mar region in southeastern Brazil. The RAMS modeling was coupled with a one-dimensional (1-D) below-cloud scavenging model in order to simulate the in-cloud and below-cloud scavenging processes. RAMS modeling was also used in order to simulate the cloud structures. The aim of the modeling was to predict the average concentration of three chemical species found in rainwater: SO 4 =, NO 3 − and NH 4 +, scavenged from the atmosphere. The concentrations of gases and particles in the samplings, as well as the meteorological parameters obtained during the March 1993 Campaign, were the input data in both models. Another objective was to compare the modeled and the observed rainwater and determine the variability in concentration. Rainwater was obtained by using fractionated rain samplers. Variability was determined through chemical analysis. Urban and rural aerosol spectra modeling were also used in order to compare the rainwater concentration species variability. When both in-cloud and below-cloud processes are included, the general result of the March 1993 events presents a better agreement between modeled and observed data sets than only below-cloud . Preliminary results lead us to conclude that the rainwater variability of nitrate is explained by the scavenging of particles from urban spectrum size distribution, whereas rural spectra explain ammonium rainwater variability—indicating the different sources of those species. Comparing to the March 1992 events, these case studies present a significant contribution from the in-cloud scavenging, supported by the Weather Radar maps and RAMS modeling. In particular, the almost constant rainwater concentrations on 16 March (an indication of strong in-cloud contribution) are related to the rainfall event, which crossed the study area on that day. These results add an important understanding to the atmospheric wet removal processes in the region studied.

Preliminary analysis of atmospheric scavenging processes in the industrial region of Cubatao, Southeastern Brazil

During a German-Brazilian research project on vegetation damage, atmospheric scavenging processes were investigated, considering transport and local emissions. For this goal, radar precipitation data was evaluated together with analyses of fractionated rain samples of one stratiform and one convective rain event. The temporal variation of pollutant concentration in rain water and the meteorological conditions were compared for both rain events at two stations: one at the top of the Serra do Mar mountain ridge (∼ 900 m) with minor influence of local emissions and one at sea level, close to the high indutrialized area of Cubatao. At the station close to the industrial plants, major influence of the local sources on wet deposition was determined. During the stratiform rain event, an additional influence of transport could be observed. On the top of the Serra do Mar, major influence of the local sources could only be observed during the convective rain event, while transport is mainly responsible for wet deposition during the stratiform event.

Atmospheric scavenging processes over the North Sea

The results of parallel aerosol and rain sampling programmes for trace and major ions at a coastal site in southeast England and on the southern North Sea are presented. The two data sets agree reasonably well, suggesting that extrapolation from coastal sites to adjacent marine areas does not introduce large errors. However, detailed consideration provides evidence of somewhat different scavenging processes at the two sites which can in part be rationalized in terms of different transport processes and atmospheric chemistry in marine and terrestrial settings.

The pluviospectrometer: classification of falling hydrometeors via digital image processing

Characteristic properties of precipitation as intensity, raindrop size distribution, velocity of raindrops and morphology of the hydrometeors (especially snow flakes) are important parameters for the atmospheric scavenging processes. In order to measure these parameters without any disturbance, optical methods are suitable. The Pluviospectrometer is a new optical device that has been developed to classify raindrops and other hydrometeors by means of digital image processing at a low cost level. Using a commercial video system and a personal computer equipped with a frame grabber card, it is possible to measure raindrops size distributions automatically. For rain drops the considered size range reaches from 0.1 mm to 6.0 mm. There is the possibility to modify this measuring range by using different camera optics for example to analyse snowflake structures in the submillimetre range. To get high quality pictures of falling hydrometeors a special illumination unit has been developed on the principle of projection illumination. The image acquisition system is composed to meet the requirements given by the characteristics of the falling raindrops. The analysing image processing software measures the sizes of the objects and separates them into different size classes. At present only a few raindrop size distributions have been analysed. They show a behaviour similar to the corresponding Marshall-Palmer distributions, but droplets with size below 0.3 mm seem to be underestimated significantly by the approximation of Marshall and Palmer. A comparison of the rainfall rate calculated from the drop size distributions to values measured by an Ombrometer hasn't been satisfying in all cases considered. In principle there is the possibility for more sophisticated hydrometeor analysis with the Pluviospectrometer. Future developments may allow analysis of the velocities of fall or snowflake classification.

Effects of land-use and atmospheric input on stream and soil chemistry: field results and long-term simulation at Mont-Lozere (Cevennes National Park, Southern France)

The main results of a 9-year study of three mountain Mediterranean ecosystems (grassland, beech and spruce forests) are presented. Atmospheric inputs are influenced both by anthropogenic acidic oxides and by Saharan alkaline dust. The streams are moderately acidic (pH 5.8), and the concentrations show irregular but limited variations with flow. The stream draining the spruce catchment exhibits the highest concentrations. The MAGIC model has been applied to evaluate the long-term changes in soil and water quality with forest development and changing SO 2 emissions. The simulations suggest that the forested catchments are currently acidifying much more rapidly than the grassland catchment, In the broadleaf ecosystem, the acidification is mainly due to a high biological uptake: a rapid recovery is predicted even if proposed reductions in SO 2 emissions are not enacted. For the conifer catchment, the model shows a prolonged decline of soil and water quality without emission reduction: a slow recovery is implied with 60% emission reduction. The results show the relative importance of biological uptake and atmospheric scavenging processes for beech and spruce vegetation, respectively.

Evidence for S(IV) compounds other than dissolved SO2 in precipitation

Preliminary results from a study characterizing S(IV) compounds in wintertime precipitation samples indicate that bisulfite ion is not the primary form of S(IV), as previously believed. By employing a differencing technique that permits estimation of both SO2 · aq and non‐SO2 · aq compound concentrations, it was found that, on an average, more than 60% of the total S(IV) is present in a form other than dissolved SO2. Formaldehyde analyses on selected samples suggest that the most likely form of the S(IV) is hydroxymethanesulfonate, although other aldehyde‐S(IV) adducts may also be present. The non‐SO2‐compounds represented a significant portion of the total sulfur concentrations present in the samples analyzed, with contributions ranging from 1.2 to 27%. Because of the stability and oxidation resistance of these S(IV) compounds, sulfur deposition estimates that are based solely on sulfate measurements are undoubtedly low, especially for wintertime events. The study underscores the importance of S(IV) compounds in atmospheric scavenging processes.

Atmospheric scavenging of soluble and insoluble matter

Retention efficiency of insoluble particle scavenging by rainfall and in-cloud and below-cloud fractions of scavenging for both soluble and insoluble particles were experimentally determined for rainfall rates of about 1 mm/h. The retention efficiency was found to be 0.90, which is in good agreement with the theoretical work of McDonald (1963). In-cloud scavenging was found to be a dominant factor in atmospheric scavenging processes.

Simultaneous measurements of Rn222, Pb214, and Bi214in air near the ground

Measurement of the rate of decay of Pb314 + Bi214 β-activity collected on a filter provides a means of determining the extent of secular equilibrium between radon and its daughter products in the air; the extent of equilibrium is expressed by the derived atom concentration ratio ρ = Bi214/Pb214 in the air. This parameter is used to obtain a correction factor for relating the β-counting rates on the filter to radon concentration, by comparing the total (corrected) β-count against a known β-count from 100 pc of radon per unit volume. To obtain a calibration of this counting technique simultaneous air sampling, over 20-min durations, for Pb214 + Bi214 β-activity by filtration and for radon by α-counting of grab samples, was conducted at elevations of 1 and 91 m above ground. The comparative sampling indicates that for normal weather conditions, by applying a ρ correction for indicated equilibrium departure, the β-counting technique yields radon concentrations equivalent to those obtained by grab samples at an elevation of 91 m; for the 1-m elevation computed concentrations are about 80% of those obtained by grab sample. The depletion of radon daughter product β-activity from the air stream, through impaction or electrostatic effects by polyethylene tubing, is shown to be negligible. Natural atmospheric scavenging processes and dust-loading on filters are discussed as possible mechanisms leading to errors in the comparison of β-counting rates with directly measured radon concentrations.