Temperature-Dependent Coalescence of Individual Nonpolar Gold Nanoparticles in Liquid

Air quality in China significantly improved after strict regulations were put in place. To assess the efficacy of volatile organic compound (VOC) control strategies, online measurements of wintertime ambient VOCs at a Beijing urban location were taken in December and January from 2014 to 2019. We assessed the changes in mixing ratios of 81 VOC species and cancer risks of 10 hazard species. The impact of meteorology on VOC mixing ratios was decoupled using a machine-learning approach. The highest and lowest VOC mixing ratios were observed in wintertime 2017 (December 2016 and January 2017) and 2019 (December 2018 and January 2019), respectively, with a decrease in 63.5%. Both emission control policies and meteorological conditions helped reduce VOC mixing ratios in the winter in Beijing. Alkenes and alkynes decreased significantly from 2015 to 2019. In contrast, the contribution of aromatics increased considerably from 2015 to 2017, and many aromatics had higher mixing ratios in 2019 than in 2015, indicating that aromatics may be a key group for future air quality improvement. The cancer risks due to VOC exposure in the wintertime in Beijing in 2019 were lower than in other years but were still much higher than the safety level.