Influence of photochemical loss of volatile organic compounds on understanding ozone formation mechanism

Volatile organic compounds (VOCs) tend to be consumed by atmospheric oxidants, resulting in substantial photochemical loss during transport. An observation-based model was used to evaluate the influence of photochemical loss of VOCs on the sensitivity regime and mechanisms of ozone formation. Our results showed that a VOC-limited regime based on observed VOC concentrations shifted to a transition regime with a photochemical initial concentration of VOCs (PIC-VOCs) in the morning. The net ozone formation rate was underestimated by 3 ppbh(-1) (similar to 36 ppb d(-1)) based on the measured VOCs when compared with the PIC-VOCs. The relative contribution of the RO2 path to ozone production based on the PIC-VOCs accordingly increased by 13.4 %; in particular, the contribution of alkene-derived RO(2 )increased by approximately 10.2 %. In addition, the OH-HO2 radical cycle was obviously accelerated by highly reactive alkenes after accounting for photochemical loss of VOCs. The contribution of local photochemistry might be underestimated for both local and regional ozone pollution if consumed VOCs are not accounted for, and policymaking on ozone pollution prevention should focus on VOCs with a high reactivity.Peer reviewe

Photochemical production of carbonyl sulfide, carbon disulfide and dimethyl sulfide in a lake water

Photochemical production of carbonyl sulfide (COS), carbon disulfide (CS2) and dimethyl sulfide (DMS) was intensively studied in the water from the Aohai Lake of Beijing city. The lake water was found to be highly supersaturated with COS, CS2 and DMS, with their initial concentrations of 0.91 +/- 0.073 nmol/L, 0.55 +/- 0.071 nmol/L and 0.37 +/- 0.062 nmol/L, respectively. The evident photochemical production of COS and CS2 in the lake water under irradiation of 365 nm and 302 nm indicated that photochemical production of them might be the reason for their supersaturation. The similar dependence of wavelength and oxygen for photochemical production of COS, CS2 and DMS implied that they might be from the same precursors. The water cage effect was found to favor COS production but inhibit CS2 and DMS formation, indicating that COS photochemical production was mainly from direct degradation of the precursors and the formation of CS2 and DMS needed intermediates via combination of carbon-centered radicals and sulfur-centered radicals. The above assumptions were further confirmed by simulation experiments with addition of carbonyls and amino acids (cysteine and methionine), and the photochemical formation mechanisms for COS, CS2 and DMS in water were derived from the investigations. (C) 2016 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V

The levels, variation characteristics and sources of atmospheric non-methane hydrocarbon compounds during wintertime in Beijing, China

Abstract. Atmospheric non-methane hydrocarbon compounds (NMHCs) were measured at a sampling site in Beijing city from 15 December 2015 to 14 January 2016 to recognize their pollution levels, variation characteristics and sources. Fifty-three NMHCs were quantified and the proportions of alkanes, alkenes, acetylene and aromatics to the total NMHCs were 49.8 % ~ 55.8 %, 21.5 % ~ 24.7 %, 13.5 % ~ 15.9 % and 9.3 % ~ 10.7 %, respectively. The variation trends of the NMHCs concentrations were basically identical and exhibited remarkable fluctuation, which were mainly ascribed to the variation of meteorological conditions, especially wind speed. The diurnal variations of NMHCs in clear days exhibited two peaks during the morning and evening rush hours, whereas the rush hours' peaks diminished or even disappeared in the haze days, implying that the relative contribution of the vehicular emission to atmospheric NMHCs depended on the pollution status. Two evident peaks of the propane/propene ratios respectively appeared in the early morning before sun rise and at noontime in clear days, whereas only one peak occurred in the afternoon during the haze days, which were attributed to the relatively fast reactions of propene with OH, NO3 and O3. Based on the chemical kinetic equations, the daytime OH concentrations were calculated to be in the range of 3.47 × 105–1.04 × 106 molecules cm−3 in clear days and 6.42 × 105–2.35 × 106 molecules cm−3 in haze days, and the nighttime NO3 concentrations were calculated to be in the range of 2.82 × 109–4.86 × 109 molecules cm−3 in clear days. The correlation coefficients of typical hydrocarbons pairs (benzene/toluene, o-xylene/m,p-xylene, isopentane/n-pentane, etc.) revealed that vehicular emission and coal combustion were important sources for atmospheric NMHCs in Beijing during the wintertime. Five major emission sources for atmospheric NMHCs in Beijing during the wintertime were further identified by positive matrix factorization (PMF), including vehicular emission and gasoline evaporation, coal combustion, solvent usage, acetylene-related emission and consumer and household products. Coal combustion (probably domestic coal combustion) were found to make the greatest contribution (29.4 ~ 33.4 %) to atmospheric NMHCs during haze days. </jats:p