Vertical Characterization of Aerosol Particle Composition in Beijing, China: Insights From 3-Month Measurements With Two Aerosol Mass Spectrometers

Despite extensive studies for characterization of aerosol chemistry near ground level, long-term vertical characterization of aerosol particle composition in Beijing is very limited. In this work, non-refractory submicron aerosol (NR-PM1) species were simultaneously measured at ground level and 260 m on a meteorological tower from 14 October 2014 to 18 January 2015 in Beijing using two aerosol mass spectrometers. Our results showed overall lower concentrations of NR-PM1 at 260mby 2%-36% than ground level in both heating and non-heating seasons, and also larger vertical gradients during clean periods than polluted episodes. The vertical ratios and correlations of aerosol species between ground level and 260 m presented higher values in daytime and lower ones at nighttime mainly due to the influences of planetary boundary layer and local source emissions, respectively. Nitrate showed ubiquitously higher ratio of 260 mto ground (ratio260m/ground) than sulfate due to a higher formation potential from gas-particle partitioning and heterogeneous reactions at higher heights. A detailed analysis of vertical evolution showed relatively stable ratios260m/ground for secondary aerosol species during the early formation stage of haze episodes, which decreased simultaneously along with mixing layer height during the later severely polluted periods with NR-PM1 concentration above 150 mu g/m(3). Our results demonstrated that the evolution of vertical differences in megacities is subject to the influences from both physical (e. g., regional transport, mountain-valley winds, inversions of temperature and relative humidity, and mixing layer height) and chemical processes (e. g., gas-particle partitioning and aqueous-phase processing).