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

Abstract

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. We quantified 53 NMHCs, 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 in the NMHC concentrations were basically identical and exhibited remarkable fluctuation, which was mainly ascribed to the variation in meteorological conditions, especially wind speed. The diurnal variations in NMHCs on clear days exhibited two peaks during the morning and evening rush hours, whereas the rush hours' peaks diminished or even disappeared on the haze days, implying that the relative contribution of the vehicular emissions to atmospheric NMHCs depended on the pollution status. Two evident peaks of the propane / propene ratios appeared in the early morning before sun rise and at noontime on 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, NO 3 , and O 3 . Based on the chemical kinetic equations, the daytime OH concentrations were calculated to be in the range of 3.47 × 10 5 -1.04 × 10 6 molecules cm −3 on clear days and 6.42×10 5 -2.35×10 6 molecules cm −3 on haze days. The nighttime NO 3 concentrations were calculated to be in the range of 2.82 × 10 9 -4.86 × 10 9 molecules cm −3 on clear days. The correlation coefficients of typical hydrocarbon pairs (benzene / toluene, o-xylene / m,p-xylene, isopentane / n-pentane, etc.) revealed that vehicular emissions 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 gasoline-related emissions (gasoline exhaust and evaporation), coal combustion, diesel exhaust, acetylene-related emissions, and consumer and household products. Coal combustion (probably domestic coal combustion) was found to make the greatest contribution (29.6-33.4 %) to atmospheric NMHCs during haze days.

Source Apportionment and Secondary Transformation of Atmospheric Nonmethane Hydrocarbons in Chengdu, Southwest China

“…NMHCs originate entirely from direct emissions, while PM 2.5 originates from both direct sources and secondary formation processes. The similarity of the trends shows that the meteorological conditions, especially WS, play an important role in the accumulation and dispersion of NMHCs and PM 2.5 (C. Liu et al, ). By contrast, the trend of TNMHCs is inversely related to that of O 3 .…”

“…After this period, with the increase in temperature, photochemical reactions were enhanced, and NMHCs reacted to form secondary pollutants; consequently, NMHCs gradually decreased. During this period, relatively high boundary layer and WS also accelerated the spread of pollutants (X. Liu et al, ; C. Liu et al, ) and NMHCs decreased to their lowest value at 18:00 CST. After this time, the temperature gradually decreased, photochemical reactions weakened, and TNMHCs gradually increased until approximately midnight (23:00–00:00 CST), when they reached a peak.…”

“…The specific ratios of NMHCs are often used to make preliminary determinations regarding the sources of atmospheric NMHCs and the aging of air masses (An, Zhu, Wang, Li, et al, ; C. Liu et al, ; Yurdakul et al, ). Benzene is an important component of NMHCs, and the ratio of toluene‐to‐benzene (T/B) is often used to identify sources of pollution.…”

“…Source 5 is identified as vehicle exhaust 2 sources and is characterized by a high content of 2‐methylpentane, methylcyclohexane, and n‐decane. Studies have indicated that 2‐methylpentane is a tracer of gasoline vehicle exhaust (C. Liu et al, ; J. Sun et al, ) and that methylcyclohexane and n‐decane are tracers of diesel vehicle exhaust (Liu et al, ).…”

“…Studies have indicated that 2‐methylpentane is a tracer of gasoline vehicle exhaust (C. Liu et al, ; J. Sun et al, ) and that methylcyclohexane and n‐decane are tracers of diesel vehicle exhaust (Liu et al, ). As illustrated in Figure , the proportions of 2‐methylpentane, methylcyclohexane, and n‐decane in factor 5 are relatively large, and factor 5 is therefore characterized by vehicle exhaust sources.…”