Chemical characterization and source identification of submicron aerosols from a year-long real-time observation at a rural site of Shanghai using an Aerosol Chemical Speciation Monitor

Abstract

Real-time measurements of submicron aerosol species including sulfate, nitrate, ammonium, chloride, organics and black carbon were conducted from a regional site (Dianshan Lake) in the rural area of western Shanghai for 1 year from July 2015 to June 2016. An Aerosol Chemical Speciation Monitor (ACSM) and an Aethalometer were deployed to analyze the seasonal and diurnal variations of aerosol properties at 1-h time resolution. The ACSM and Aethalometer measurements of submicron particulate matter (PM1) concentration and composition agreed well with collocated measurements of PM2.5 using a Tapered Element Oscillating Microbalance, a Monitoring Instrument for Aerosols and Gases (MARGA) and a Sunset organic carbon and elemental carbon (OC/EC) analyzer. A rolling-window positive matrix factorization (RW-PMF) algorithm was applied to the ACSM mass spectra to examine organic aerosol (OA) sources and identified three factors representing hydrocarbon-like organic aerosol (HOA), more-oxidized oxygenated organic aerosol (MO-OOA) and less-oxidized oxygenated organic aerosol (LO-OOA), respectively.The annual average mass concentration of PM1 was 43.6 ± 29.9 μg m−3, with the seasonal average ranging from 32.3 ± 21.0 μg m−3 in summer to 60.3 ± 38.3 μg m−3 in winter. OA contributed the most to PM1 with a small seasonal difference (29–32%). More seasonal variations were observed in the mass fractions of nitrate (20–26%) and sulfate (18–26%) in PM1. Nitrate played an important role in PM pollution episodes since its fractional contribution to PM1 mass increased by approximately a factor of 3 (from ~10% to ~30%) from the lowest to the highest aerosol conditions. Photochemical production and gas-particle partitioning were the main drivers for secondary inorganic aerosol formation, leading to significant diurnal and seasonal cycles. The highest mass fractions of sulfate and nitrate occurred in summer and winter, respectively. Diurnal variation profiles of OA changed significantly between seasons, due to seasonal variations in dominant source contributions and meteorological conditions. HOA showed a predominant morning peak corresponding mainly to traffic emissions. Photochemical production of LO-OOA was observed in the afternoons of summer whereas a relatively flat diurnal cycle of MO-OOA suggested that it was mainly formed on a regional scale. Backtrajectory and potential source contribution function (PSCF) analyses indicated that the most important source region of LO-OOA was located in the coastal areas to the south of Shanghai while MO-OOA had a dominant transport pathway in the northwest direction.