Effects of below-cloud scavenging on the regional aerosol budget in 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).
References (47)
- et al.
Seven-year trend and the time and seasonal dependence of fog water collected near an industrialized area in Japan
Atmospheric Research
(2007) - et al.
Separate chemical characterizations of fog water, aerosol, and gas before, during, and after fog events near an industrialized area in Japan
Atmospheric Environment
(2007) - et al.
Study on the acidification and pollution of precipitation based on a data set collected on a 0.5-mm precipitation basis
Atmospheric Environment
(2008) - et al.
Washout/rainout contribution in wet deposition estimated by 0.5 mm precipitation sampling/analysis
Atmospheric Environment
(2009) - et al.
Development and evaluation of an expression for polydisperse particle scavenging coefficient for the below-cloud scavenging as a function of rain intensity using the moment method
Journal of Aerosol Science
(2006) - et al.
Derivation and verification of an aerosol dynamics expression for the below-cloud scavenging process using the moment method
Journal of Aerosol Science
(2010) - et al.
MICS-Asia II: The model intercomparison study for Asia Phase II methodology and overview of findings
Atmospheric Environment
(2008) Study of scavenging of submicron-sized aerosol particles by thunderstorm rain events
Atmospheric Environment
(2005)- et al.
Effects of Siberian forest fires on air quality in East Asia during May 2003 and its climate implication
Atmospheric Environment
(2008) - et al.
Number size distribution of atmospheric aerosols during ACE-Asia dust and precipitation events
Atmospheric Environment
(2007)
Acid rain and acidification in China: the importance of base cation deposition
Environmental Pollution
In-cloud and below-cloud scavenging of nitric acid vapor
Atmospheric Environment
Precipitation scavenging coefficient: influence of measured particle and raindrop size distributions
Atmospheric Environment
Theoretical estimates of the scavenging coefficient for soluble aerosol particles as a function of precipitation type, rate, and altitude
Atmospheric Environment
A review of parameterizations for modeling dry deposition and scavenging of radionuclides
Atmospheric Environment
The electrical analogy does not apply to modeling dry deposition of particles
Atmospheric Environment
MICS-Asia II: model inter-comparison and evaluation of acid deposition
Atmospheric Environment
Parameterizations of surface resistance to gaseous dry deposition in regional-scale, numerical models
Atmospheric Environment
Formation of nitrate and non-sea-salt sulfate on coarse particles
Atmospheric Environment
Impact of preindustrial biomass-burning emissions on the oxidation pathways of tropospheric sulfur and nitrogen
Journal of Geophysical Research
Estimated variability of below-cloud particle removal by rainfall for observed particle size distributions
Atmospheric Chemistry and Physics
The regional particulate matter model I: model description and preliminary results
Journal of Geophysical Research
Changing trends in sulfur emissions in Asia: implications for acid deposition, air pollution, and climate
Environmental Science and Technology
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