Modeling wet deposition of inorganics over Northeast Asia with MRI-PM/c and the effects of super large sea salt droplets at near-the-coast stations
1Meteorological Research Institute, Japan Meteorological Agency, 1-1 Nagamine, Tsukuba, 305-0052, Japan
2Pacific Northwest National Laboratory, P.O. Box 999 Richland, WA 99352, USA
3Asia Center for Air Pollution Research, 1182 Sowa, Nishi, Niigata, 950-2144, Japan
4Toyohashi Institute of Technology, 1-1 Hibarigaoka, Tempaku, Toyohashi 441-8580, Japan
Abstract. We conducted a regional-scale simulation (with grid spacing = 60 km) over Northeast Asia for the entire year of 2006 by using an aerosol chemical transport model, the lateral and upper boundary concentrations of which we predicted with a global stratospheric and tropospheric chemistry-climate model, with a horizontal resolution of T42 (grid spacing ~300 km) and a time resolution of 1 h. The present one-way nested global-through-regional-scale model is called the Meteorological Research Institute – Passive-tracers Model system for atmospheric Chemistry (MRI-PM/c). We evaluated the model performance with respect to the major inorganic components in rain and snow measured by stations of the Acid Deposition Monitoring Network in East Asia (EANET). Through statistical analysis, we show that the model successfully reproduced the regional-scale processes of emission, transport, transformation, and wet deposition of major inorganic species derived from anthropogenic and natural sources, including SO42−, NH4+, NO3−, Na+ and Ca2+. Interestingly, the only exception was Na+ in precipitation at near-coastal stations (where the distance from the coast was from 150 to 700 m), concentrations of which were significantly underestimated by the model, by up to a factor of 30. This result suggested that the contribution of short-lived, super-large sea salt droplets (SLSD; D > 10–100 μm) was substantial in precipitation samples at stations near the coast of Japan; thus samples were horizontally representative only within the traveling distances of SLSD (from 1 to 10 km). Nevertheless, the calculated effect of SLSD on precipitation pH was very low, a change of about +0.014 on average, even if the ratio of SLSD to all sea salt in precipitation was assumed to be 90%.