Impact of NOx reduction on long-term surface ozone pollution in roadside and suburban Hong Kong: Field measurements and model simulations.

Continuous measurements of ozone (O3) and nitrogen oxides (NOx = NO + NO2) were conducted from 2007 to 2019 in Hong Kong in order to evaluate the effectiveness of control strategies for NOx emission from diesel commercial vehicles (DCV). DCV control programs were periodically applied in three phases starting from 2007, 2010 and 2014. It was found that NO and NO2 levels decreased during the study period but more dramatically after the implementation of DCV Phase III than pre-DCV Phase III. Source apportionment analysis confirmed that the ambient NO and NO2 in Hong Kong attributed to the regulated DCV emissions in Phase III reduced at rates of 5.1-14.4 ppbv/yr in roadside environment and 1.6-3.1 ppbv/yr in suburban area. Despite overall NOx reduction, increased NO2/NOx ratios were recorded during the study period possibly due to the application of diesel particulate filter (DPF) in DCVs. However, after introducing DCV Phase III, observed O3 values experienced more dramatic increasing trends in most areas of Hong Kong than pre-DCV Phase III. Model simulations revealed that O3 production rate kept increasing and turned to be less sensitive to NOx from 2014 to 2019. On the roadside, net O3 production rate was more than doubled during 2014-2019 owing to NOx reduction. Moreover, the levels of oxidants (OH, HO2 and RO2) were 1.5-5 times those before 2014. In suburban environment, NOx reduction also facilitated O3 production and radical cycling, but made smaller contributions than those on the roadside during 2014-2019. This study unraveled that NOx reductions benefited from DCV regulations caused increase in surface O3 and fueled O3 photochemistry in various environments. More stringent control measures on emissions of VOCs, especially those with high OH reactivity, might help to better mitigate O3 pollution.

Species profiles, in-situ photochemistry and health risk of volatile organic compounds in the gasoline service station in China.

Accompanying with increases in vehicle population and gasoline consumption, gasoline evaporation accounted for an enlarged portion of total volatile organic compound (VOC) emissions in China, raising increasing environmental concerns especially in megacities. In this study, an intensive sampling campaign was performed in a gasoline service station, to reveal emission characteristics, environmental and health impacts of VOCs. It was strikingly found that 24 % of air samples exceeded the national standard of 4 mg/m3 for non-methane hydrocarbons (NMHCs) on the boundary of the station, with the equipment of Stage I and II controls. VOC groups and species profiles showed that alkanes dominated total VOCs. As typical markers of evaporative loss of gasoline, C4-5 species (i-pentane, n-pentane and n-butane) as well as methyl tert-butyl ether (MTBE) accounted for 49.6 % of VOCs. Species profile and diagnostic ratios indicated the prominent contribution of gasoline evaporative losses from refueling or breathing processes, as well as the interference of vehicle exhaust in the ambient air at the site. Intensive O3 production was reproduced by the photochemical box model, demonstrating that O3 formation was co-limited by both VOCs (especially trans-2-butene) and NOx. Inhalation health risk assessment proved that exposure to hazardous VOCs caused non-cancer risk (HQ = 3.08) and definitely posed cancer risks at a probability of 1.3 × 10-4 to workers. Remarkable health risks were mainly imposed by halocarbons, aromatics and alkenes, in which 1,2-dichloropropane caused the highest non-cancer risk (HQ = 1.3) and acted as the primary carcinogen (ICR = 5.1 × 10-5). This study elucidated the high unqualified rate in gasoline service stations after the implementation of latest standards in China, where new regulations targeted halocarbons and updates in existing vapor recovery systems were suggested for VOC mitigation.