VOC characteristics, emissions and contributions to SOA formation during hazy episodes

摘要

Volatile organic compounds (VOC) are important precursors of secondary organic aerosols (SOA). The pollution processes in Beijing were investigated from 18th October to 6th November 2013 to study the characteristics, SOA formation potential and contributing factors of VOC during hazy episodes. The mean concentrations of VOC were 67.4 +/- 33.3 mu g m(-3) on clear days and have 5-7-fold increase in polluted periods. VOC concentrations rapidly increased at a visibility range of 4-5 km with the rate of 25%/km in alkanes, alkenes and halocarbons and the rate of 45%/km in aromatics. Analysis of the mixing layer height (MLH); wind speed and ratios of benzene/toluene (B/T), ethylbenzene/m,p-xylene (E/X), and isopentane-pentane (i) under different visibility conditions revealed that the MLH and wind speed were the 2 major factors affecting the variability of VOC during clear days and that local emissions and photochemical reactions were main causes of VOC variation on polluted days. Combined with the fractional aerosol coefficient (FAC) method, the SOA formation potentials of alkanes, alkenes and aromatics were 0.3 +/- 0.2 mu g m(-3), 1.1 +/- 1.0 mu g m(-3) and 6.5 +/- 6.4 mu g m(-3), respectively. As the visibility deteriorated, the SOA formation potential increased from 2.1 mu g m(-3) to 13.2 mu g m(-3), and the fraction of SOA-forming aromatics rapidly increased from 56.3% to 90.1%. Initial sources were resolved by a positive matrix factorization (PMF) model. Vehicle-related emissions were an important source of VOC at all visibility ranges, accounting for 23%-32%. As visibility declined, emissions from solvents and the chemical industry increased from 13.2% and 63% to 34.2% and 23.0%, respectively. Solvents had the greatest SOA formation ability, accounting for 52.5% on average on hazy days, followed by vehicle -related emissions(20.7%). (C) 2016 Elsevier Ltd. All rights reserved.