Study on wet scavenging of atmospheric pollutants in south Brazil

doi:10.1016/j.atmosenv.2010.02.020

Atmospheric Environment

Volume 45, Issue 27, September 2011, Pages 4770-4776

https://doi.org/10.1016/j.atmosenv.2010.02.020 Get rights and content

Abstract

The present paper presents the study of in-cloud and below-cloud SO₂ and SO₄²-scavenging processes by applying numerical models in the Candiota region, located in the state of Rio Grande do Sul, South Brazil. The BRAMS (Brazilian Regional Atmospheric Modeling System) model was applied to simulate the vertical structure of the clouds, and the B.V.2 (Below-Cloud Beheng Version 2) scavenging model was applied to simulate in-cloud and below-cloud scavenging processes of the pollutants SO₂ and SO₄²⁻. Five events in 2004 were selected for this study and were sampled at the Candiota Airport station. The concentrations of SO₂ and SO₄²⁻ sampled in the air and the simulated meteorological parameters of rainfall episodes were used as input data in the B.V.2, which simulates raindrop interactions associated with the scavenging process. Results for the Candiota region showed that in-cloud scavenging processes were more significant than below-cloud scavenging processes for two of the five events studied, with a contribution of approximately 90–100% of SO₂ and SO₄²⁻ concentrations in rainwater. A few adjustments to the original version of B.V.2 were made to allow simulation of scavenging processes in several types of clouds, not only cumulus humilis and cumulus congestus.

Introduction

Mechanisms to scavenge pollutants from the atmosphere can be divided into two processes: dry and wet deposition. The difference between these mechanisms is the phase during which the aerosol or gas leaves the atmosphere. In dry deposition, pollutants are scavenged by absorption on the soil surface, without precipitation. In wet deposition, scavenging occurs due to mass transfer by hydrometeors (raindrops and droplets, ice crystals, snow, etc.)

Wet deposition can occur in-cloud by cloud droplets (condensation, nucleation, and gas dissolution), as well as below-cloud. Both occur continuously during wet precipitation, whose processes may provide variations in the pollutant concentration during precipitation (Seinfeld and Pandis, 1997).

Pollutant scavenging by gases is ruled by the absorption capacity of raindrops and cloud droplets, which, consequently, depend on the chemical composition of the atmosphere. It is known that some gases present in the atmosphere are more hygroscopic than others. For instance, HNO₃ is highly water-soluble. The quantification of this type of scavenging gives more details regarding the contribution of atmospheric pollutants, such as SO₂ and HNO₃.

According to some authors, e.g. Seinfeld and Pandis, 1997, Barthet al., 2001, aerosols are scavenged faster by in-cloud scavenging (by nucleation) than below-cloud scavenging (by interception).

There are studies of scavenging processes by hydrometeors, especially concerning the chemical characterization of atmospheric precipitation, associating these results with the meteorological parameters and data from the source of emission (Frank et al., 1990, Durana et al., 1992.)

In Brazil, some studies simulated the chemical composition of atmospheric precipitation using the ‘Below-Cloud Beheng version 2’ (B.V.2) model. These studies were conducted in northern Brazil and in the state of Sao Paulo, and they include among others Gonçalves et al., 2000, Gonçalves et al., 2002, Gonçalves et al., 2003, Ramos, 2000, Nakaema, 2001, and Silva et al. (2009).

Studies conducted in the state of Rio Grande do Sul on the chemical characterization of atmospheric precipitation were carried out using different types of samplers: bulk, wet-only, wet, and others, associating the results with meteorological parameters. However, there are gaps with regard to studies applying the B.V.2 model to scavenge pollutants from raindrops and cloud droplets.

For the present study, the region of Candiota was chosen. The thermal power plant Usina Termelétrica Presidente Médici (UTPM) is located in this region and has a 446 MW capacity, which is expected to expand to 796 MW. There is a project for a second power plant at the site, with a 500 MW capacity.

The objective of the present study was to analyze the scavenging of atmospheric pollutants (gases and particulates), by cloud droplets and raindrops, in-cloud and below-cloud, comparing the results simulated and observed in atmospheric precipitation. For this, a new methodology is proposed, which will allow the B.V.2 to simulate the scavenging process for any type of cloud, and not only for cumulus clouds.

Section snippets

Area of study

Candiota is located in the southwestern area of the state of Rio Grande do Sul, between 54.183°W/53.310°W and 31.293°S/32.045°S. It is located 420 km from Porto Alegre, the capital of the state, and encompasses the municipalities of Aceguá, Bagé, Candiota, Herval, Hulha Negra, Pedras Altas, and Pinheiro Machado (Fig. 1). The area holds the biggest mineral coal reserves of the country, approximately 12 thousand million tons – approximately 38% of Brazil's coal reserves -, of which almost 30% are

Experimental part

Samples of atmospheric precipitation and SO₂ were collected at the Candiota Airport station, located at 31.495°S/53.694°W, on the following days in 2004: 08/01, 09/08, 10/12, 10/30 and 11/08.

Wet precipitation sampling criteria followed the ASTM D 5012 standard (ASTM, 2008). The wet precipitation sampler was installed 1.5 m above the ground. Sampling and storage procedures are described in Migliavacca et al. (2004).

Atmospheric particles from the Candiota region were collected with HV PM10 at the

Pollutant scavenging by rainwater

The pollutant scavenging process comprises repeated exposures of gases and particulates to cloud droplets or raindrops, which act as collectors of these pollutants. Considering a rainfall of short duration, from minutes to a few hours, that the gases are inert, and that there are no emissions, we will have: $C = C_{o} \cdot \exp (- Λ \cdot t)$ $C = C_{o} \cdot \exp (- β \cdot t)$ where C and C_o are the final and initial concentrations of pollutants (gases and particles) and Λ and β are pollutant scavenging coefficients, below-cloud and

Modeling

The scavenging processes were modeled using B.V.2 and input data supplied by BRAMS.

Aerosol scavenging simulations

In the present study, the species simulated in atmospheric precipitation were selected as a function of available values of SO₂ gases and SO₄²⁻ particulates. Table 1 shows the days selected for modeling and the input dataset (atmospheric concentrations of SO₂ and SO₄²⁻, time of precipitation and accumulated precipitation) for the five events.

Table 2 shows the results obtained for the five events using the adapted B.V.2 model (in-cloud and below-cloud by gas and particle, partial and total

Results and discussion

Table 3 shows that two events presented a higher SO₄²⁻ scavenging by nucleation (in-cloud), while below-cloud scavenging prevailed during the remaining events. Events were in-cloud scavenging prevailed are in agreement with Barth et al. (2001).

The experiments performed in Candiota showed variability in the relative error (ratio of modeled to observed concentration), which showed values between 4.7% and 781%, in the model, where modeled values were greater than the experimental data, except for

Conclusions

In general, the model tends to overestimate sulfate concentration in rainwater. The fact that the model considers the concentration of gases and particulates constant with height from the surface up to the cloud base contributes to this overestimation, although there is a possibility of greater concentration of gases between the middle and the top of the boundary layer, 500 m–1000 m high. The assumption that drop/droplet precipitation and distribution rate remain constant with time, and the

Acknowledgements

We are grateful to the Conselho Nacional de Pesquisa (CNPq) for their financial support.

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Study on wet scavenging of atmospheric pollutants in south Brazil

Abstract

Introduction

Section snippets

Area of study

Experimental part

Pollutant scavenging by rainwater

Modeling

Aerosol scavenging simulations

Results and discussion

Conclusions

Acknowledgements

Nuclear Instruments & Methods in Physics Research. Section B. Beam Interactions with Materials and Atoms

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Environmental Pollution

Atmospheric Environment

Atmospheric Environment

Environmental Pollution