Chemical characterization of long-range transport biomass burning emissions to the Himalayas: insights from high-resolution aerosol mass spectrometry

摘要

An intensive field measurement was conducted at a remote, background, high-altitude site (Qomolangma Station, QOMS, 4276ma.s.l.) in the northern Himalayas, using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer?(HR-ToF-AMS) along with other collocated instruments. The field measurement was performed from 12?April to 12?May?2016 to chemically characterize the high time-resolved submicron particulate matter?(PM1) and obtain the dynamic processes (emissions, transport, and chemical evolution) of biomass burning?(BB), frequently transported from South Asia to the Himalayas during pre-monsoon season. Overall, the average (±1σ) PM1 mass concentration was 4.44?(±4.54)μgm?3 for the entire study, which is comparable with those observed at other remote sites worldwide. Organic aerosol?(OA) was the dominant PM1 species (accounting for 54.3% of total PM1 on average) followed by black carbon?(BC)?(25.0%), sulfate?(9.3%), ammonium?(5.8%), nitrate?(5.1%), and chloride?(0.4%). The average size distributions of PM1 species all peaked at an overlapping accumulation mode (～500nm), suggesting that aerosol particles were internally well-mixed and aged during long-range transport. Positive matrix factorization?(PMF) analysis on the high-resolution organic mass spectra identified three distinct OA factors, including a BB-related OA (BBOA, 43.7%), a nitrogen-containing OA (NOA, 13.9%) and a more-oxidized oxygenated OA (MO-OOA, 42.4%). Two polluted episodes with enhanced PM1 mass loadings and elevated BBOA contributions from the west and southwest of QOMS during the study were observed. A typical BB plume was investigated in detail to illustrate the chemical evolution of aerosol characteristics under distinct air mass origins, meteorological conditions, and atmospheric oxidation processes.