First real-time size distribution measurements of aerosol particles in Ulaanbaatar, Mongolia

Mongolia in northeastern Asia encounters severe atmospheric particulate matter (PM) pollution during winter. Although many researchers have investigated PM haze in Mongolia using filter sample analyses to understand its formation mechanisms and sources, the formation and growth of atmospheric PM in Mongolia are currently unknown because of the deficiency of real-time measurements of particle size distributions. In this study, the number size distributions of atmospheric particles (PNSDs) in Ulaanbaatar, Mongolia, were measured over a year using real-time aerosol instruments to better understand the formation and growth of atmospheric PM at different timescales using meteorological data. The diurnal variations in PNSDs in different seasons had similar patterns, but different particle number concentrations (PNCs). A weak new particle formation (NPF) in the daytime was observed only in summer under the cleanest air quality of the year. In winter, the PNSD exhibited 4-11 times higher PNC than that in the other seasons. The highest PNCs in winter were strongly correlated with the primary particles produced by combustion such as that in heating and power generation. In addition, the PNCs at nighttime were generally higher than those during the daytime, with larger modal sizes throughout the year. In terms of particle size ranges, Aitken- and accumulation-mode particles (which are detrimental to human health) were dominant in the number and volume size distributions of particles, respectively. The diurnal variations in PNC displayed dual peaks in the morning and late evening. This matched the primary particle emissions from traffic and coal/biomass burning. The nitrate-to-sulfate ratios revealed that stationary sources mainly contributed to high PM concentrations throughout the year except in summer. The significant Pearson correlation coefficients between particles and gaseous pollutants also supported primary particle emissions through combustion in winter.