Sources And Sinks Of Atmospheric Volatile Organic Compounds In The High-Elevated Forests In Nanling Mountains,China

Volatile organic compounds?VOCs?are key precursors of tropospheric ozone and secondary organic aerosols and play an extremely important role in atmospheric chemistry.The research on the sources and sinks of atmospheric VOCs can help to understand the formation mechanisms of the complex air pollution problems in China.At present,research on the concentration,source and chemical behavior of atmospheric VOCs in areas with both strong-biogenic and strong-anthropogenic sources are not understood well.This study takes the lead in the ecological sensitive area of Nanling Alpine Forest that around the atmospheric photochemical active area in South China.Five-year consecutive and comprehensive field observations were conducted during dry and wet seasons.By using online and offline techniques,such as online-GC-MS/FID,PTR-MS,DOAS and Tenax TA+TD-GC-MS,comprehensive systematic observation and analysis of various VOCs including NMHCs,halogenated hydrocarbons,OVOCs and BVOCs were conducted.The model analysis and parameterization method were used to estimate the concentration,composition,variation,reactivity and source of atmospheric VOCs in the Nanling alpine forest.The regional transport and photochemical aging characteristics of typical AVOCs were described.The atmospheric oxidation process and the formation and transformation mechanism of typical BVOCs and typical OVOCs were mainly explored.This study aims to deepen the understanding of the mechanism of source and sink of atmospheric VOCs in alpine forests in South China,and for the first time to explore the"amplification mechanism"of VOCs oxidation under the influence of the anthropogenic-biogenic interactions,which is of great significance for the future study of mutual feedback mechanism between forest and city.This study mainly got the following conclusions:1)Overall,the proportion of OVOCs in the atmospheric VOCs of the Nanling Alpine Forest is the highest?54.5%?,followed by alkanes?14.7%?,BVOCs?13.0%?,halogenated hydrocarbons?8.1%?,olefins?7.4%?and aromatic hydrocarbons?2.3%?.The concentrations of BVOCs and OVOCs in the wet season?July?September?are generally higher than that in the dry season?October?November?,while the concentrations of aromatic and halogenated hydrocarbons are higher in the dry season.In comparison with worldwide alpine forest regions,the concentrations of typical BVOC isoprene?189±130 pptv?,?-pinene?168±115 pptv?and typical aromatic hydrocarbon benzene?144±21 pptv?,toluene?205±36 pptv?is significantly lower,while the main oxidation products methanol?3763±788 pptv?,formaldehyde?1884±356 pptv?and acetone?1247±150 pptv?are higher,indicating that the ageing of VOCs in the Nanling atmosphere might be higher than other forested areas.2)The photochemical box model incorporating the master chemical mechanism?PBM-MCM?was used to simulate the oxidation capacity of the Nanling atmosphere in July?August 2016.The results show that Nanling has strong atmospheric oxidation capacity,.The concentration of O₃?53.5±1.3 ppbv?and atmospheric free radicals(daytime?OH and nighttime?NO₃ concentrations were 7.3±0.5×10⁶,6.0±0.5×10⁸ molecules cm^-3,respectively)were significantly higher than other forest areas around the world,while were comparable to the level in background area of the Pearl River Delta region in southern China.As a supplement to PBM-MCM,the regional atmospheric oxidation capacity during 9:00?15:00 was estimated by using a parameterization method that based on photochemical aging of aromatics.The results show that the regional scale?OH concentration(19.7±2.3×10⁶ molecules cm^-3)was significantly higher than the site-level?OH concentration(11.7±0.4×10⁶ molecules cm^-3)modelled by PBM-MCM.The above results indicate that the atmospheric oxidation capacity of Nanling Mountains may be significantly enhanced under the influence of the Pearl River Delta urban agglomeration.3)Analysis of isoprene and its oxidation intermediates methyl vinyl ketone?MVK?and methacrolein?MACR?showed that although the Nanling has a large number of sub-tropical evergreen broad-leaved forests which are high isoprene emitters However,the daytime isoprene concentration?377±46 pptv?observed in the highest temperature season was significantly lower than that of the same type of alpine forest region worldwide,and the?MVK+MACR?/isoprene ratio?1.9±0.5?was high.Further calculations of the atmospheric reaction time?0.27 h?and initial concentration?1213±108 pptv?of isoprene indicate that isoprene undergoes rapid and thorough oxidation under strong regional atmospheric oxidative capacities.The above results indicate that the atmospheric oxidation capacity of Nanling may be significantly enhanced by the influence of the Pearl River Delta urban agglomeration,which lead to rapid and fully oxidation of isoprene from forests and low-level observed isoprene in the atmosphere.4)To further verify the effects of anthropogenic emissions from urban areas on the oxidation of VOCs in the Nanling atmosphere,the characteristics of glyoxal?Gly?and methylglyoxal?Mgly?,which are important oxidation intermediates of VOCs,were analyzed for the observation conducted during July?November 2016.The results showed that the concentration of Gly?509±31pptv?and Mgly?340±32 pptv?in the Nanling atmosphere and their ratios?Gly/Mgly=1.8±0.2?were significantly higher than those in the forest regions worldwide.Theoretical calculations based on precursors show that local photochemical oxidation can explain?67%of observed Mgly and?9%of observed Gly,indicating that Gly in the Nanling atmosphere may be affected by strong regional transport.The analysis of air masses in different directions during the observation period shows that the natural source emissions from East China and the aromatic emissions from the Pearl River Delta have a greater contribution to the formation of Gly and Mgly in the Nanling atmosphere.The analysis of the air mass back trajectory combined with the biomass burning tracer methyl chloride and MODIS fire point map shows that the biomass burning emissions in Central and North China in the dry season were also have an important impact on the concentration of Gly and Mgly at Nanling.The above results indicate that strong regional atmospheric oxidation capacity and anthropogenic emissions from urban areas may have serious disturbance effects on the oxidations of atmospheric VOCs in Nanling Mountains.5)In addition,this study reported for the first time that the concentration of typical chlorofluorocarbon CFC-11?332±13 pptv?in the background atmosphere of South China is significantly higher than the East Asian background?235±1 pptv?and the global baseline?230±1pptv?.Further analysis found that CFC-11 was highly correlated with the anthropogenic tracers carbon monoxide and benzene during polluted periods.The analysis of potential sources based on PSCF shows that the high concentration of CFC-11 in the Nanling atmosphere mainly comes from underdeveloped areas such as the southwestern and central parts of China,Vietnam and Myanmar,while the contribution from developed areas such as the Pearl River Delta is small.The findings of this study suggest that unbalanced regional economic development and a lack of effective regulatory actions are likely the main factors for increased CFC-11 emissions in East Asia.This research has received great attention from the core project of World Climate Research Programme SPARC and is of great significance for accurately controlling China's ozone-depleting substances and improving our ability to fulfill international environmental responsibility.