Characteristics, formation mechanisms, and sources of non-refractory submicron aerosols in Guangzhou, China

Abstract

Abstract Submicron aerosols (PM1) have more serious effect on the visibility, air quality, human health and climate change compared to coarse particles (particle size between 2.5 and 10\xa0μm). There is practical significance in understanding the formation mechanisms and sources of these particles. In this study, we conducted real-time measurements on the composition of non-refractory submicron aerosols (NR-PM1) in Guangzhou from 20 October to 1 December, 2016, using a quadripole aerosol chemical speciation monitor (Q-ACSM). Overall, the composition of NR-PM1 was dominated by organics (48.2%), followed by nitrate (NO3−, 20.3%), sulfate (SO42−, 17.4%), ammonium (NH4+, 11.4%), and chloride (Cl−, 2.7%). The high contribution of NO3− and NO3−/SO42− mass ratios were observed in high-PM episodes, suggesting that NOx emission control should be a priority in mitigating PM-pollution in South China. Primary organic aerosols (POA) accounted for 51% of organics as the campaign average, and POA contribution increased with PM concentrations especially during high PM episodes, indicating that local emissions played a very important role in high-PM pollution. The conversion of SO2 to SO42− and aqueous, heterogeneous production may not be the dominant source of SO42− in NR-PM1, while regional transport is likely to be the main source during high-sulfate episodes. The formation of NO3− in this study was related to gas-phase reactions of NO2 (photochemical reaction) during daytime, and heterogeneous reactions (hydrolysis reaction of N2O5) during the nighttime. Gas-phase reactions would be less important for nitrate formation during severe polluted events. Particle acidity analyses showed that particles were generally acidic and the NH4+ was not enough to fully neutralise the NO3− and SO42−, the existing forms of SO42− could be (NH4)2SO4, NH4HSO4 or (NH4)3H(SO4)2. Controlling the emissions of NH3 to reduce the NH4+ concentration can probably be a feasible direction in the future in Guangzhou if the problem of agriculture and aerosol acidity can be solved. Wind rose plots and back-trajectory analyses also demonstrated that local emission was more important to the formation of NO3− and regional transport was more conducive to the increase of SO42− concentration.