Trans-Pacific transport and evolution of aerosols: spatiotemporal characteristics and source contributions
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.