Tracking sources of severe haze episodes and their physicochemical and hygroscopic properties under Asian continental outflow: Long-range transport pollution, postharvest biomass burning, and Asian dust

摘要

Aerosol physicochemical and hygroscopic properties were measured from 12 October to 21 November 2005 at a downwind area of the Asian continental outflow (Gwangju, Korea) to characterize severe haze episodes. Using optically measured elemental carbon (EC) at 660 nm (Opt.EC) and 880 nm (BC) wavelengths and Mie theory, it was estimated that the higher BC/ Opt.EC ratio during the cloudy day of the long-range transport (LTP) period was mainly due to EC particle growth from in-cloud processing with secondary aerosols such as sulfate and organic aerosols. Single scattering albedo (SSA) of biomass burning (BB) aerosol increased sharply from 0.89 to 0.94 under a relative humidity >70%, suggesting that organic aerosols emitted from rice straw burning contained high amounts of hydrophilic compounds. The contribution of aerosol water content to the total light extinction coefficient (b_(ext)) was determined as 51.4% and 68.4% during the BB and BB + LTP periods, respectively, indicating that the haze episodes were highly enhanced by an increase in aerosol water content. The Asian dust event was characterized by the highest SSA (0.92 ± 0.02), the lowest mass scattering efficiency of fine particles (2.5 + 1.0 m~2 g~(-1)), and the lowest hygroscopic nature (humidity-dependent light scattering enhancement factor,f(80%), which is defined by the ratio of light scattering coefficient at 80% relative humidity to that at dry condition, = ～1.37). Based on the Angstrom exponent (α) values observed at the source region of the Asian continent and the downwind area of South Korea during the BB + LTP period, it was found that the α value of urban aerosols decreased ～11% for 1-2 days of the transport, probably due to the increase in particle size through water uptake. Increasing rates of surface PM_(10) mass concentrations at western coastal areas of the South Korean peninsula were in the range 2.4-14.4 μgm~(-3) h~(-1) at the beginning of the BB + LTP period (24 October 2005, 0700-2300 LT). Based on in situ and remote measurement techniques, Asian aerosol outflow over the South Korean peninsula is of the order of 388-3789 tons h~(-1) at the beginning of the LTP event.