Spatiotemporal Patterns Of Ambient Non-methane Hydrocarbons In China

As an important class of volatile organic compounds(VOCs), non-methane hydrocarbons(NMHCs) play an important role in atmospheric chemistry and could significantly affect several regional or global environmental and climatic issues. NMHCs are important precursors of tropospheric ozone and secondary organic aerosols(SOA), which have adverse effects on air quality and human health as well as indirect effect on climate change. In addition, some NMHCs, such as benzene, are classified as hazardous air pollutants with adverse health effects. In recent several decades, China has experienced rapid economic growth and urbanization, the energy consumption and vehicle number also increases largely. This induces a large amount of primary air pollutants, including NMHCs, being emitted into the atmosphere. Consequentially, China faces serious air quality issues of photochemical smog and haze, as well as severe health risk problems of air toxics. It is of great importance to investigate the spatiotemporal patterns of levels, compositions and sources of NMHCs across China for current and future VOCs emission reduction, air pollution control and human health protection in China. In this study, we measured ambient NMHCs from March 2012 to February 2014 at twelve sites in different regions of China, including six rural sites in China’s developed coastal regions, five urban sites in central and western China, and one rural site in Qinghai-Tibet Plateau. The spatial distributions and temporal variations of NMHCs levels and compositions were studied, the contributions of NMHCs to ozone formation were explored, and the spatiotemporal patterns, sources and health risk of aromatic hydrocarbons were further investigated. The main results of this work were listed as follows:1. The annual average mixing ratios of total NMHCs among the twelve sites were in the range of 6795 ± 845- 29013 ± 3185 pptv, and the lowest value was observed at rural site in Qinghai-Tibet Plateau(Lhasa site) while the highest value was observed at urban site in central and western China(Chongqing site). Total NMHCs levels at rural sites in China’s developed coastal regions(13821- 22631 pptv) were comparable with those at urban sites in central and western China(15569- 29013 pptv). At all sites, alkanes were the most abundant among NMHCs and contributed 48.07- 65.94% in total NMHCs; while biogenic VOCs(BVOCs) contributed least to total NMHCs with percentages of 1.00- 7.49%. Ethane, ethylene, acetylene, propane, isobutane, n-butane and isopentane were found among TOP 10 NMHCs compounds at all the twelve sites. The TOP 10 compounds accounted for 69.98- 87.76% of total NMHCs mixing ratios.2. At the northern sites(Hailun, Shenyang, Yucheng and Taiyuan), total NMHCs levels in January, February and December were significantly higher than that in March- November, which would be primarily caused by additional source emissions, e.g. coal/biofuel combustion for heating, and/or low boundary layer heights. In the other sites, NMHCs mixing ratios showed different temporal variations due to the different influences of meteorological condition, emission strength and atmosphere reaction. The temporal variations of alkanes, alkenes, alkynes and aromatics, which were mainly emitted from anthropogenic sources, were similar with the temporal variations of total NMHCs. However, the variations of BVOCs, mainly from natural sources, were different: higher BVOCs levels and percentages were observed in June- August at each sites due to the impacts of ambient temperature and light levels.3. Among the twelve sites, total ozone formation potentials(OFPs) ranged from 21.16 ± 2.62(Lhasa site) to 104.18 ± 11.00 ppbv(Chongqing site). Different with the compositions of NMHCs mixing ratios, alkenes and aromatics were the largest contributors to total OFPs, together they contributed 58.98- 80.30% in total OFPs although they only accounted for 22.54- 38.21% of total NMHCs mixing ratios. The contributions of BVOCs to total OFPs also increased to 3.59- 27.90%. Ethylene, propylene, 1-butene, toluene and m,p-xylenes were found among TOP 10 OFPs contributors at all the twelve sites. The TOP 10 compounds contributed 63.09- 74.73% in total OFPs values.4. At rural sites in China’s developed coastal regions, benzene shared over 50% in total BTEX at sites in the Northeast China Plain(NECP) or in the North China Plain(NCP), while toluene, ethylbenzene and xylenes altogether accounted for over 77% of total BTEX at sites in the Yangtze River Delta(YRD) and the Pearl River Delta(PRD) in the south. BTEX at the northern sites showed significant correlations(p < 0.01) with combustion tracer- carbon monoxide(CO) but weak correlations with traffic marker-methyl tert-butyl ether(MTBE), suggesting coal and biofuel/biomass burning as their major sources with substantially elevated benzene levels during wintertime heating period. Contrarily, BTEX at the southern sites were mainly from traffic-related and/or industrial emission sources, as indicated by the poor correlations with CO but highly significant(p < 0.01) correlations with MTBE or tetrachloroethylene, an industrial emission tracers. The emission ratios of benzene/CO from measurement agreed well with that in the previous emission inventory within a factor of 2, but toluene/CO ratio at the NECP site and o-xylene/CO at the NCP site were 29% and 38% of that in the inventory, respectively; ethylbenzene/CO and m,p-xylenes/CO ratios at the YRD site were 3.2- 3.5 times of that in the emission inventory.5. At urban sites in central and western China, the total BTEX levels ranged from 2.42 ± 0.35(Kunming site) to 3.65 ± 0.51 ppbv(Chongqing site), lower than those previously observed in developed cities of China. However, the levels of leukemiacausing benzene, in the range of 0.72 ± 0.12(Kunming site)- 1.88 ± 0.46 ppbv(Taiyuan site), were comparable with those in developed cities of China and even higher than those in cities of the US or Europe. At all urban sites, good correlations between benzene(or toluene) and carbon monoxide or methyl tert-butyl ether, together with the toluene/benzene ratios within that for coal/biofuel burning and/or vehicle emissions, suggested that BTEX was largely originating from combustion sources. Industrial and solvent emissions may also occasionally influence the sampling sites, as indicated by several samples with higher ratios of toluene and C8-aromatics to benzene and CO.6. Health risk assessment of BTEX exposure showed that the lifetime cancer risks of benzene exposure among the twelve sites(1.02E-05- 7.31E-05) were one order of magnitude higher than 1E-06 set by USEPA as acceptable level for adults, suggesting that potential cancer risks due to ambient benzene exposure should be a health concern over China. The non-cancer risks of BTEX were in the range of 5.56E-02- 3.34E-01, lower than 1 set by USEPA, indicating that the exposure to ambient BTEX has no obvious non-cancer risk in China.