Roles of regional transport and heterogeneous reactions in the PM2.5 increase during winter haze episodes in Beijing.

Regional transport and chemical conversions are two major processes that lead to the severe haze pollution in China. Our observations during five haze episodes in Beijing between February 19 and March 12 of 2014 show that the two processes played different roles as PM2.5 increased from the clean (<75μgm-3) to the light-medium pollution level (75-150μg m-3) and to levels of heavy (150-250μgm-3) and severe (>250μgm-3) pollution. In the initial twelve hours of each episode, the PM2.5 reached the light-medium level with an increase of approximately 120μgm-3. At the same time, the particle (~10-700nm) number concentration also showed a distinct increase accompanied by a rapid increase in the mean diameter. A light-medium PM2.5 occurred in the south areas prior to the haze occurrence in Beijing and the southerly winds were predominant, indicating the rapid increase of PM2.5 in the initial stage was caused by the regional transport from the south. Subsequently, PM2.5 elevated to the heavy and severe levels when the wind was weak, relative humidity was high and ozone concentration was low. The increase of PM2.5 in the elevated stages was characterized by a high percentage (45% for the heavy level and 55% for the severe level) of secondary inorganic components, indicating the substantial contribution of the formation of secondary aerosols. In addition, the increases of the mean diameter (from 108nm to 120nm) and the total volume concentration (by 67%) are regarded as a consequence of heterogeneous reactions on the surfaces of aerosol particles because the particle number concentration remained nearly constant in these two stages. Our results indicate that, during the five winter haze episodes, the regional transport from the south was the major reason for the initial-stage PM2.5 increase, while heterogeneous reactions dominated the later elevation.