Elsevier

Atmospheric Environment

Volume 58, October 2012, Pages 14-22
Atmospheric Environment

Effects of below-cloud scavenging on the regional aerosol budget in East Asia

https://doi.org/10.1016/j.atmosenv.2011.08.065Get rights and content

Abstract

We examine the effects of below-cloud scavenging on regional aerosol simulations over East Asia using wet deposition fluxes observed at Acid Deposition Monitoring Network in East Asia (EANET) sites and the Community Multiscale Air Quality (CMAQ) model together with a new below-cloud-scavenging scheme. Typical air quality models, including CMAQ, assume below-cloud scavenging as a simple first-order process with a constant or simple form depending on rain intensity. The scheme used here accounts for the collection efficiency, terminal velocity of raindrops, raindrop-size distributions, and particle-size distributions, which are important factors affecting below-cloud scavenging. We conduct model simulations for spring 2001, including baseline and sensitivity simulations. Our analysis mainly focuses on May 2001 to rule out the effect of dust aerosols. Simulated wet deposition fluxes of SO42−, NO3, and NH4+ by the new scheme are increased by 103, 16, and 108%, respectively, relative to the baseline simulation and show better agreement with observations. The effect of below-cloud scavenging on coarse particles is even greater, producing wet deposition fluxes two orders of magnitude higher than the baseline. The resulting changes in the model indicate the considerable impacts of below-cloud scavenging on regional aerosol simulations over East Asia, where both anthropogenic emissions and natural sources of aerosols are present throughout the year. An accurate wet scavenging simulation is critical to simulate the atmospheric burden and wet deposition fluxes of both fine-mode and coarse-mode aerosols over East Asia.

Highlights

► Use a new mechanistic below-cloud scavenging scheme in CMAQ. ► Effects of below-cloud scavenging are significant for PM wet deposition. ► The effect of below-cloud scavenging on coarse particle is event greater. ► A new scheme alters chemical and physical characteristics of aerosols.

Introduction

Wet deposition, which is divided into in-cloud and below-cloud scavenging processes, can efficiently remove atmospheric aerosols (Seinfeld and Pandis, 2006). In-cloud scavenging involves the encapsulation of aerosols by cloud droplets and is efficient for removing fine aerosols with high solubility (Chang et al., 1987, Seinfeld and Pandis, 2006). Below-cloud scavenging is an aerosol washout process by precipitation and is relatively more important for coarse aerosols. These two processes, also referred to as wet scavenging, are considered critical for determining aerosol concentrations in the atmosphere (Chate, 2005).

Air quality models of aerosol simulations typically compute aerosol wet scavenging using a simple parameterization (Sportisse, 2007). Below-cloud scavenging is traditionally parameterized as a simple first-order process with a constant or a simple form depending on rain intensity (Mircea et al., 2000, Andronache, 2003). In reality, below-cloud scavenging is affected by several factors including the collection efficiency, terminal velocity of raindrops, raindrop-size distributions, and particle-size distributions (Scott, 1982, Levine and Schwartz, 1982). In this study, we use a mechanistic treatment of below-cloud scavenging that accounts for these factors in a comprehensive three-dimensional (3-D) air quality model and examine its effects on regional aerosol simulations over East Asia.

East Asia is one of the most important source regions for aerosols. Rapid economic growth in developing countries such as China and India has resulted in increasing and year-round emissions from anthropogenic aerosol sources (Streets et al., 2003). Dust storms over desert areas of Mongolia and China, such as the Gobi and Taklamakan deserts, and frequent wildfires in Siberia are also important natural sources of aerosols seasonally (Zhang et al., 2003, Jeong et al., 2008). These anthropogenic and natural aerosols are main contributors to regional air quality degradation over East Asia.

The long-range transport of aerosols from East Asia across the Pacific may also have contributed to increased aerosol concentrations in North America in recent decades (Jaffe et al., 1999, Stohl et al., 2010) and thus has important implications for hemispheric pollution (Stohl et al., 2010). Park et al. (2005) previously showed that the export of pollution aerosols out of East Asian boundary layers is critically determined by vertical updraft and accompanied by wet scavenging. This also means that an accurate simulation of wet scavenging is crucial to quantifying the hemispheric pollution using 3-D atmospheric modeling (UNECE, 2007).

Additionally, wet deposition of aerosols causes serious environmental issues including acid rain and soil acidification, and this has been addressed in many modeling studies over East Asia (e.g., Carmichael et al., 2002, Larssen and Carmichael, 2000). Wang et al. (2008) conducted the Model Inter-Comparison Study for Asia (MICS-Asia II) to evaluate regional air quality models, with particular focus on wet deposition fluxes of inorganic sulfate (SO42−), ammonium (NH4+), and nitrate (NO3) aerosols. They found large discrepancies between models and observations over East Asia. Issues related to wet-scavenging parameterization were suggested as a crucial reason for the large disparities among the studied models, and these issues may hinder accurate modeling of acid deposition. In particular, the use of fixed loss rates as a function of rain intensity alone is too simple for accurate simulation of aerosol wet scavenging over East Asia, where physical and chemical characteristics of aerosols are quite diverse (Kim et al., 2007).

In this study, we focus mainly on below-cloud scavenging of aerosols and the effects of this scavenging on regional aerosol simulations over East Asia. Below-cloud scavenging is considered less important than in-cloud scavenging for total aerosol wet deposition. Although this is generally true for fine aerosols, East Asia also has a large abundance of naturally driven aerosols, including soil dust and smoke aerosols in coarse-mode fractions, which are susceptible to below-cloud scavenging. Additionally, some previous studies (Aikawa et al., 2007a, Aikawa et al., 2007b, Aikawa et al., 2008, Aikawa and Hiraki, 2009) demonstrated the importance of below-cloud scavenging even for fine NO3 and SO42− aerosols using measurements at Mt. Rokko and Toyo-oka, Japan.

We use a newly developed below-cloud scavenging scheme by Bae et al. (2010), who explicitly account for Brownian diffusion, interception, impaction, thermophoresis, diffusiophoresis, and electric charging in computing the collection efficiency by raindrops. We implement their scheme in the Community Multiscale Air Quality (CMAQ) model, one of the most widely used 3-D air quality models. The CMAQ model also participated in the MICS-Asia II inter-comparison study (Carmichael et al., 2008) and was found to have the bias in the model for the observed wet deposition fluxes of SO42−, especially in spring (Wang et al., 2008, Carmichael et al., 2008). Here, we discuss possible reasons for the CMAQ bias and test an explicit delineation of wet deposition simulation by CMAQ with a new scheme for wet deposition effects on aerosol budgets and depositions over East Asia.

Section snippets

General description

We use the CMAQ model (version 4.6) driven by meteorological fields from the fifth-generation Penn State/National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) (Grell et al., 1994). National Centers for Environmental Prediction (NCEP) reanalysis data are used to provide boundary and initial conditions for the MM5 simulations. The horizontal resolutions of the models are 45 × 45 km (132 × 97) with 14 vertical layers on a sigma coordinate. The lowest model levels are centered at

Model evaluation

We focus our model evaluation on the surface network of wet-deposition flux observations in East Asia. The observation data are from the Acid Deposition Monitoring Network in East Asia (EANET) and include the concentrations and wet deposition fluxes of SO42−, NO3, Cl, NH4+, Na+, K+, Ca2+, Mg2+, and H+. While the uncertainty of these observations is not quantified, we assume a factor of two is reasonable. In 2001, 47 sites are available in 12 countries in East Asia. Fig. 1 shows the number and

Sensitivity of simulations to wet deposition schemes

In this section we examine the effect of our below-cloud scavenging scheme on the aerosol simulation in CMAQ. Fig. 4 compares observed versus simulated SO42− wet deposition fluxes from the baseline and the sensitivity of simulations at EANET sites in May 2001. Wet deposition fluxes simulated with our below-cloud scavenging in the model generally increase at most sites, especially in China and Vietnam, but a few sites in Japan and South Korea also show slight decreases in wet deposition fluxes.

Effects of below-cloud scavenging on aerosol concentrations

We investigate the effects of below-cloud scavenging on aerosol concentrations using the model results. We examine the change in the simulation by comparing observed and simulated aerosol concentrations from the baseline and sensitivity models with the below-cloud scavenging scheme. Fig. 7 shows a comparison of the observed and simulated daily mean concentrations of SO42− aerosol at Kanghwa and Imsil, South Korea, in spring 2001. First of all, the baseline model appears to reproduce the

Summary

Wet deposition is an efficient removal process of atmospheric aerosols and is divided into in-cloud and below-cloud scavenging. However, air quality models of aerosol typically compute aerosol wet scavenging using simple parameterizations, particularly for below-cloud scavenging. CMAQ does not explicitly separate wet deposition into in-cloud and below-cloud scavenging. The aerosol scavenging computation in CMAQ is also too simple to accurately simulate aerosol loss by raindrops. In this study,

Acknowledgment

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2011-0001282) and the Korea Meteorological Administration Research and Development Program under Grant RACS 2011–2022).

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