Volatile organic compounds (VOCs) during non-haze and haze days in Shanghai: characterization and secondary organic aerosol (SOA) formation

Abstract

To better understand the characterization and secondary organic aerosol (SOA) formation of volatile organic compounds (VOCs) during non-haze and haze days, ambient VOCs were continuously measured by a vehicle-mounted online thermal desorption system coupled with a gas chromatography–mass spectrometry (TD–GC/MS) system in Shanghai, China. The average concentrations of VOCs in haze episodes (193.2 μg m⁻³) were almost 50% higher than in non-haze periods (130.8 μg m⁻³). VOC concentrations exhibited a bi-modal pattern in the morning and evening rush hour periods on both non-haze and haze days. The ratios of toluene to benzene (T/B) and m,p-xylene to ethylbenzene (X/E) indicated that VOCs were aged air mass transported from nearby areas. The estimated SOA yields were 12.6 ± 5.3 and 16.7 ± 6.7 μg m⁻³ for non-haze and haze days, respectively, accounting for 9.6 and 8.7% of the corresponding PM_2.5 concentrations, which were slightly underestimated. VOCs–sensitivity (VOCs–S) based on a PM_2.5-dependent model was used to investigate the variation between VOCs and PM_2.5 concentrations in the morning rush hour. It was found that VOCs were more sensitive to PM_2.5 on clean days than during periods of heavy particulate pollution. VOCs–sensitivity was significantly correlated with the ratio of specific PM_2.5 to background PM_2.5, with a simulated equation of y = 0.84x^−0.62 (r ² = 0.93, p < 0.001). Our findings suggest that strategies to mitigate VOC emissions and further alleviate haze episodes in Shanghai based on reducing gasoline vehicle-related sources would be very efficient.

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