Studies On Concentration Variation Characteristics Of Ozone And Its Precursors In Xi’an High Technology District

Xi’an high technology district had the highest concentration of ozone in the city in2014 and 2015.With the continuous improvement of particulate matter concentration,the impact of ozone pollution on the environmental air quality in high technology district became more and more prominent.In order to find out the variation characteristics of ozone concentration in Xi’an high technology district and explore the key factors affecting ozone pollution,this paper observed volatile organic compounds（VOCs）in key industries in Xi’an high technology district in 2018 on the basis of simultaneous observation of ozone and its precursor nitrogen oxides and volatile organic compounds in 2015.The variation characteristics of ozone（O₃）and nitrogen oxide（NOx）concentrations in Xi’an high technology district from 2014 to 2018 were statistically analyzed.The main controlling factors and sources of ozone generation were identified,and feasible measures were put forward,which provided scientific basis for pollution control of ozone and its precursors in Xi’an high technology district.The main results are as follows.（1）According to the data from the automatic monitoring station of ambient air quality in high technology district from 2014 to 2018,the ozone concentration showed a fluctuating upward trend.The phenomenon of high concentration ozone pollution mostly concentrates in summer.The diurnal variation pattern is unimodal curve.From2014 to 2018,the concentration of NO₂ and NOx increased first and then decreased.The concentration values of five years all exceeded the national annual second-level standard limit value.Seasonal variation is basically opposite to ozone,and the diurnal variation pattern is bimodal curve.It is consistent with the morning and evening rush hours in cities,reflecting that vehicle exhaust pollution is the main factor affecting NOx concentration.（2）The concentration changes of ozone and NOx have significant correlation with meteorological factors.Ozone concentration increases when temperature increases and relative humidity decreases.The concentration of NO and NO₂ is negatively correlated with temperature and wind speed.With the increase of relative humidity,NOx concentration increases first and then decreases.Under the air flow of southeast,east,south and southwest,high technology district is prone to high concentration ozone phenomenon.（3）The VOCs in the atmosphere and emissions from key industries were observed and analyzed with the Hapsite ER portable gas chromatographic mass spectrometer.The diurnal variation trend of VOCs in ambient air shows a bimodal pattern,which is similar to that of NOx,The rising tendency of the concentration after 20 o’clock is mainly caused by the weakening of atmospheric diffusion capacity at night.The components with higher concentration are oxygen-containing organics and halogenated hydrocarbons.The concentration of VOCs in traffic environment is high,mainly from mobile source exhaust pollution and gasoline evaporative emissions.Among them,oxygen-containing organic matter,alkane and olefin are relatively large.Key industries,automotive maintenance and painting process due to the collection system are not perfect.A large number of unorganized emissions lead to the concentration of VOCs in the factory environment is significantly higher than that in other industries such as electronics,pharmaceuticals,printing,painting and so on.（4）Ozone formation potential（OFP）analysis showed that the OFP contribution of olefin,oxygen-containing organic compounds and aromatic hydrocarbons in the atmospheric environment of the high technology district was relatively high,accounting for 91.3%of the total volatile organic compounds（TVOC）.The OFP contribution and characteristic substances of each component in traffic environment are very similar to those in atmospheric environment.Naphthalene is the characteristic substance with high OFP in key industry environment.The concentration and OFP of naphthalene in ambient air are very low.The substances with the same OFP contribution ranking high as those in atmospheric environment mainly include butene,vinyl acetate and toluene.（5）The VOCs/NOx ratio was used to analyze that Xi’an high technology district was in the VOCs sensitive area,that is,O₃ pollution in summer was controlled by VOCs.E/X（ethylbenzene/m-xylene,p-xylene）ratio was used to analyze the existence of atmospheric degeneration and aging in the high-tech zone.The scope of pollutant expansion is not large.It tends to accumulate and react.The ratio of toluene/benzene（T/B）is 1.477,less than 2.0,indicating that VOCs pollution in high-tech zones is greatly affected by vehicle exhaust pollution.