| The data of the hourly concentrations of O3were obtained for each station from the National Urban Air Quality Real-time Publishing Platform.The North China?NC?provinces of Beijing,Tianjin,Hebei,Henan,Shanxi and Shandong are considered in this research.For each city,the maximum daily 8h average concentration?MDA8?of O3is calculated by utilizing an 8 h moving average window for each day.We explored the regional pollution characteristics and meteorological formation mechanism of surface O3in North China during 2013-2018.The results are as follows:?1?The surface ozone pollution in NC showed an overall worsening trend from 2013 to 2018.The concentration of MDA8 O3reached a peak in June,among which southern Hebei,northern Henan and western Shandong were the most polluted regions.O3pollution showed obvious annual variation characteristics,and the number of days of large-regional-moderate pollution and large-regional-severe pollution type increased significantly from 2014 to 2018.Moreover,O3pollution event first occurred in September in 2017,indicating the duration of O3showed a growing trend on season scale.?2?When regional ozone pollution event occurs,the North China region is under control of strong solar radiation,high temperature and low humidity,and the south wind anomaly prevails in the lower troposphere.By constructing an regional ozone pollution index?I?ROP?to comprehensively characterize such meteorological element fields,we found that the correlation coefficient between I?ROP and MDA8 O3is 0.77,which is higher than that between each individual index and MDA8 O3,indicating that MDA8 O3concentrations in NC are influenced by multiple meteorological factors rather than a single factor.The good correlation between I?ROP and MDA8 O3indicates that it can be used as a meteorological forecast indicator for ozone pollution in North China.?3?Based on the automatic identification algorithm for daily ozone pollution type and definition of regional pollution evens,19 regional ozone pollution events were identified over NC during 2014 to 2018.The regional ozone pollution can be classified into two types:zonal westerly airflow and high pressure ridge type according to the sea surface field and the circulation patterns in 500 and 200h Pa.It should be noted there was a special pollution event,in which the ozone pollution occurred under local high-pressure center,and the pollution process was classified as high pressure center type.?4?By analyzing the high pressure ridge and high pressure center pollution types,we found that when local high pressure system dominated over NC,the high temperature and stable weather conditions are conducive to the enhancement of ozone photochemical reactions,resulting in the increased local ozone production.The southerly winds prevailing in the lower troposphere are conducive to the horizontal transport of pollutants.And the deep downdraft over the North China further inhibits the vertical diffusion of pollutants,leading to aggravated pollution.Local photochemical generation,horizontal transportation,and unfavorable vertical diffusion conditions are significant causes for the development of large-scale regional ozone pollution events in NC.In addition to the synoptic systems that directly affect regional meteorological conditions,the modulation of background circulation and mesoscale systems activities,including the Rossby wave propagation and the typhoon-related secondary circulation,also have an important impact on the ozone pollution characteristics in North China. |
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