Satellite observations of aerosol transport from East Asia to the Arctic: three case studies
Abstract. Vertical profiles of aerosol extinction obtained with the CALIOP lidar onboard CALIPSO are used in conjunction with the GEOS-Chem chemical transport model and NOAA's HYSPLIT trajectory model to document three aerosol export events from East Asia to the Arctic in the year 2007. During each of these events CALIOP sampled the pollution plumes multiple times over periods of five to seven days. Midlatitude cyclones lifted the pollution to the free troposphere with net diabatic heating of ~5 °C day−1 and precipitation in this initial ascending stage. Rapid meridional transport to the Arctic took place at 3–7 km altitude, and was mediated by either a blocking high pressure system in the NW Pacific or a trough-ridge configuration. Once in the Arctic transport was nearly isentropic with slow subsidence and radiative cooling at a rate of 1–1.5 °C day−1. We find good agreement between modeled and observed plumes in terms of length, altitude, thickness and, within the measurement uncertainties, extinction coefficient. In one event the satellite algorithm misclassifies the aerosol layer as ice clouds as a result of the relatively high depolarization ratio (0.06), likely caused by a high dust component in the aerosol mixture. Using 500 hPa geopotential height anomalies for these three events along with eight other export events observed by CALIOP in 2007–2009, we develop a meteorological index that captures 40–60% of the variance of Asian transport events to the Arctic in winter and spring. Simulations with the GEOS-Chem model show that 6 major export events from Asia to the Arctic occur each year, on average. The maximum probability for such events is during March–June, with a secondary maximum in October–November. During these events, Asian pollution and dust aerosols account for 50–70% of the aerosol optical depth over the Siberian sector of the Arctic, compared to a mean background contribution of 33%.