A pervasive and persistent Asian dust event over North America during spring 2010: lidar and sunphotometer observations

Abstract

Abstract. Among the many well-documented cases of springtime trans-Pacific transport of crustal dust from Asia to North America (significant events include those of 1998, 2001, and 2005), the events of March and April 2010 were extraordinary both in the extent of the dust distribution and in the unique meteorological conditions that caused the dust layers in the free troposphere to linger and be detectable across Canada and the Northern United States for over a month. This study focuses on extending previous research by combining data from CORALNet lidars in Vancouver, BC and Egbert, ON with AERONET sunphotometer retrievals and model results from HYSPLIT and NAAPS to monitor the arrival and distribution of dust layers across North America. This is the first documented instance of lidar detection of Asian dust from the Egbert CORALNet installation, where layers identified as dust using depolarization ratios corresponded with retrievals of coarse mode optical depth at the co-located AEROCAN/AERONET site. In Vancouver dust layer depolarization ratios varied from 0.27 for dust above 6 km to less than 0.10 for the first 1.5–2 km above the surface. Similar layers of elevated dust exhibited much lower bulk depolarization ratios for all altitudes in Egbert, ON where maximum depolarization ratios stayed below 0.15 for all layers from 2–8 km with no clear variation with altitude, or over time. The relative lack of variation is an indication that as the dust particles aged the rate of change in chemical composition and optical properties slowed. HYSPLIT back trajectories performed throughout the free troposphere above these sites showed a majority of air parcels originating from Central Asia on the days in question. Using these techniques, it was shown that elevated layers of aerosol reaching the west coast of North America as early as 17 March also included dust from the same Central Asian sources, extending the known duration of the 2010 event by almost a full month.