Abstract
Abstract. Aerosols in the middle and upper troposphere have a long enough lifetime for trans-Pacific transport from East Asia to North America to influence air quality on the west coast of the United States (US). Here, we conduct quasi-global simulations (180∘ W–180∘ E and 70∘ S–75∘ N) from 2010 to 2014 using an updated version of WRF-Chem (Weather Research and Forecasting model fully coupled with chemistry) to analyze the spatiotemporal characteristics and source contributions of trans-Pacific aerosol transport. We find that trans-Pacific total aerosols have a maximum mass concentration (about 15 µg m−3) in the boreal spring with a peak between 3 and 4 km above the surface around 40∘ N. Sea salt and dust dominate the total aerosol mass concentration below 1 km and above 4 km, respectively. About 80.8 Tg of total aerosols (48.7 Tg of dust) are exported annually from East Asia, of which 26.7 Tg of aerosols (13.4 Tg of dust) reach the west coast of the US. Dust contributions from four desert regions in the Northern Hemisphere are analyzed using a tracer-tagging technique. About 4.9, 3.9, and 4.5 Tg year−1 of dust aerosol emitted from north Africa, the Middle East and central Asia, and East Asia, respectively, can be transported to the west coast of the US. The trans-Pacific aerosols dominate the column-integrated aerosol mass (∼65.5 %) and number concentration (∼80 %) over western North America. Radiation budget analysis shows that the inflow aerosols could contribute about 86.4 % (−2.91 W m−2) at the surface, 85.5 % (+1.36 W m−2) in the atmosphere, and 87.1 % (−1.55 W m−2) at the top of atmosphere to total aerosol radiative effect over western North America. However, near the surface in central and eastern North America, aerosols are mainly derived from local emissions, and the radiative effect of imported aerosols decreases rapidly. This study motivates further investigations of the potential impacts of trans-Pacific aerosols from East Asia on regional air quality and the hydrological cycle in North America.
Highlights
Atmospheric aerosols, the liquid or solid particulate matter in the atmosphere, are known to be a crucial forcing agent of weather and climate at regional and global scales (Lau et al, 2008; Jimenez et al, 2009; Bond et al, 2013; Huang et al, 2014; Zhao et al, 2011, 2014)
The University of Science and Technology of China (USTC) version of Weather Research and Forecasting (WRF)-Chem used in this study (1) provides a relatively more accurate eight-bin approach to simulate the aerosol mass balance and radiative forcing, which can well simulate particle size distribution and aerosol lifetime during long-range transport (Zhao et al, 2013b); (2) includes complex aerosol processes and interactions between aerosol and radiation, clouds, and snow albedo, which can well resolve aerosol–cloud–precipitation interaction (Zhao et al, 2011, 2012, 2014; Hu et al, 2016); and (3) diagnoses radiative forcing of aerosol composition (Zhao et al, 2013a), which is not included in most global models that treat aerosols as internal mixing
The peak trans-Pacific aerosol mass concentrations occur in spring (MAM) due to the strongest midlatitude westerlies and active extratropical cyclones (Yu et al, 2012), and more emissions in this season (Yu et al, 2008), while the minimum occurs in summer (JJA) because of the greatest aerosol removal induced by summer monsoon precipitation (Holzer et al, 2005; Yu et al, 2008, 2013)
Summary
Atmospheric aerosols, the liquid or solid particulate matter in the atmosphere, are known to be a crucial forcing agent of weather and climate at regional and global scales (Lau et al, 2008; Jimenez et al, 2009; Bond et al, 2013; Huang et al, 2014; Zhao et al, 2011, 2014). The trans-Pacific aerosols can affect atmospheric composition (Chin et al, 2007; Yu et al, 2008), stratospheric ozone depletion (Solomon, 1999), surface air quality (VanCuren, 2003; Heald et al, 2006; Chin et al, 2007; Yu et al, 2012; Tao et al, 2016), regional visibility (Watson, 2002), human health (Pope, 2000; Pope et al, 2002; Schwartz, 1994), regional climate (Eguchi et al, 2009; Huang et al, 2009, 2011; Yu et al, 2012; Fan et al, 2014, 2015), and ecological integrity (Bytnerowicz et al, 1996; Schindler, 1988, 1999) in downwind regions, such as the United States (US)
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