Optimization And Application Of Volatile Organic Compounds(VOCs) Source Apportionment Based On Correction Of Photochemical Loss

The average annual concentrations of air pollutants represented by particulate matters had continued to decline in recent years,while the ground-level ozone pollution was gradually emerging.O₃ has become one of the most prominent pollutants affecting ambient air quality of China.Volatile organic compounds?VOCs?are a group of essential precursors of O₃.The component and sources of VOCs are complex,therefore effective control of VOCs lies in accurate identification of their sources and estimation of their ozone formation potential.As a maturated and convenient technique,PMF?Positive Matrix Factorization?model has been widely applied for VOCs source apportionment.However,the hypothesis of PMF model violated the characteristic of VOCs,which supposed the profile of pollutants remained unchanged during atmospheric transportation.Aiming at this problem,recent studies have used the photochemical aging method,to obtained the initial VOCs concentrations from the concentration differences of a pair of hydrocarbons,so as to make the PMF source apportionment free from the impact of VOCs photochemical loss.One of the basic assumptions of the photochemical aging method is the selected VOCs species pair are emitted from the same source,which makes the subsequent PMF source apportionment meaningless and neglects the differences of different VOCs sources.Therefore,a more comprehensive and precise VOCs source apportionment is needed to provide a scientific reference for assessing the impact of VOCs pollution and formulating priority prevention and control measures.In this study,Hong Kong,Shanghai and Wuhan,the representative cities of South China,East China and Central China,are taken as the research areas,where the VOCs field measurements were conducted off-line or on-line and the method of VOCs source apportionment was optimized.The optimization mainly includes two aspects:first,based on the photochemical characteristics of different VOCs,the Maximum Incremental Reactivity?MIR?of each VOCs is integrated into the PMF uncertainty calculation formula to create a reasonable range for uncertainty of respective VOCs species in sampling and analysis,instead of the traditional empirical coefficient-based calculation.Second,distinguishing the photochemical aging degree of PMF factors through correlation analysis,and combining the factors with local source profile to trace the whole process of a VOCs source from emission to final aging.According to this process,the initial concentrations of emissions,transportation loss and photochemical aging potential can be estimated,so that the source apportionment can better describe relative contributions to concentrations of VOCs and ozone.In Hong Kong,the mixture PMF factor profiles were differentiated and the factors were classified into two photochemical aging degrees based on photochemical reactivity of VOCs species.In Shanghai,over 70%concentration and 90%Ozone formation potential?OFP?values of VOCs emissions was consumed due to photochemical aging and physical dilution during atmospheric transportation.In Wuhan,over 40%OFP values were consumed due to photochemical aging during atmospheric transportation.As the essential precursors of ambient ozone,the transport loss of VOCs concentration is directly related to ozone generation.Hence,based on the activity of VOCs species,correcting the influence of photochemical reaction on the concentration of VOCs species in different sources is the key to achieve more accurate VOCs source apportionment,which can provide scientific references for evaluating the effect of VOCs emission control on prevention and control of secondary pollutants.