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

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 PM2.5 concentrations, which were slightly underestimated. VOCs-sensitivity (VOCs-S) based on a PM2.5 -dependent model was used to investigate the variation between VOCs and PM2.5 concentrations in the morning rush hour. It was found that VOCs were more sensitive to PM2.5 on clean days than during periods of heavy particulate pollution. VOCs-sensitivity was significantly correlated with the ratio of specific PM2.5 to background PM2.5 , 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.