Source Apportionment Of Surface Ozone Production Based On Field Observation

Tropospheric ozone(O3),the main component of photochemical smog,is generated by volatile organic compounds(VOCs)and nitrogen oxides(NOx)in the presence of sunlight.It has become a pollutant of much concern,given its increasing concentration and negative effects on air quality during summertime.O3 pollution is caused by the cooperation of photochemical reaction and regional transport.Persistent O3 pollution is extremely harmful to human health and hampers ecologically sustainable development.Source apportionment of O3 is of great significance for prevention and control decision to alleviate O3 pollution.The contributions of the O3 production and transportation can be simulated by complex air quality model with the emission inventory data of precursors.However,the timeliness and accuracy of emission inventory cannot be ignored.The precursors contribute to O3 formation by participating in a series of chemical reactions,and this process have been simulated well by the observation-based model.Then the O3 source analysis method based on observed data is recommended with the gradual improvement of the O3 and its precursors monitoring system.The framework for 0-D atmosphere modeling(FOAM)with the specific Master Chemical Mechanism(MCM v3.3.1)was coupled with the positive matrix factorization model(PMF)to assess the effects of VOCs on O3 formation in this study.It attempts to provide guidance for future research and assist the local government to make strategic decisions on VOCs and O3 reduction.Taking the observation data of O3 and its precursors in Changzhi as an example,the new method for source apportionment of O3 based on observed data could be applied to help systematically analyze the cause of O3 pollution,and the main conclusions are as follows:1、The pollution characteristics and sources of O3 and its precursors were analyzed.The results showed that daily maximum 8-hour O3 mixing ratio ranges from 162 to 261 ppbv and the daily average O3 hourly mixing ratio was 95 ppbv,both of which show O3 pollution during the observation period.It was favorable for photochemical reactions with the average temperature for 30℃ and relative humidity for 47%in daytime(06:00-20:00)during the whole observation period.For the precursors,the averaged mixing ratio of VOCs,CO,NO,NO2 was 20.34 ppbv,1 ppmv,3 ppbv,and 16 ppbv,respectively.Taking into consideration that the chemical losses of many active VOCs specie from their emitters to the atmosphere,the initial mixing ratio of VOCs were be calculated to be 30.68 ppbv in Changzhi.2、The O3 mixing ratios simulated by FOAM model based on observed data had a good correlation with the actual observed O3 mixing ratio(the correlation coefficient was greater than 0.96),which reflected that this model can truly simulate the environmental O3 production process.The gradient of O3 production caused by the variation of VOCs and NOx can be simulated by FOAM,and then the sensitivity factors of O3 production can be analyzed.Output results from FOAM simulation revealed that O3 production was controlled by VOCs in Changzhi.3、O3 production was affected by the concentration of its precursors(NOx and VOCs).Identifying the sources of sensitivity precursor was the precondition for source apportionment of O3 based on observation.Methods for identifying the source of O3 analysis method based on the FOAM coupled PMF receptor model was established in this study,which could simulate the net O3 production rate of emission sources,and then quantitatively analyzed the contribution of emission sources to O3 production.Source analysis conducted using the PMF model at Changzhi based on observation and initial VOCs showed that coal combustion and vehicle emissions made the two largest contributions(31.41%and 25.71%,27.61%and 28.98%,respectively)to atmospheric VOCs.Considering the chemical losses of VOCs,the contributions of vehicular emissions,solvent usage and industrial processes to ambient VOCs were relatively underestimated by 1.37%,2.23%,and 5.72,respectively.LPG/NG usage and coal combustion in neighboring regions could influence local VOCs via regional transportation processes.4、The change of O3 was mainly due to the joint action of photochemical generation and regional output or diffusion in Changzhi.Photochemical production of O3 in the northwest and central east areas contributes significantly to the local O3,and these areas are also the key potential sources of the O3 output of Changzhi.The Outward transport of O3 may lead to the O3 increase in the surrounding areas(including Shanxi-Jinzhong,Hebei-southern,Henan-northeastern,etc.).The application of FOAM-PMF-FOAM method showed that industrial processes,vehicles exhaust,and coal combustion made the three largest contribution(29.14%,26.68%,and 23.82%,respectively)to O3 production,followed by LPG/NG usage(11.33%),and solvent usage(9.04%).Sensitivity analysis indicated that the abatement ratio of VOC vs.NOx should be higher than 1.1 for preventing the increment of O3 mixing ratio when the reduction rate of environmental NOx mixing ratio was lower than 80%.In the short term,reducing VOCs emissions,especially controlling the VOCs from vehicles emissions,can effectively reduce the O3 production.Multi-sources and multi-pollutants joint prevention and control,especially controlling the emission of NOx,may be a long-term strategy to alleviate O3 pollution in Changzhi.5、FOAM model can factually simulate the O3 production process based on observation data.Development this model and establishment the method for identifying sources of O3 production based on observation can not only truly reflect the main sources of O3 photochemical production,but also have important promotion and application value.It will provide a new method and idea for exploring the causes of O3 pollution.