基于WRF/Chem模式天津地区重污染天气成因分析

Research on Causes of Severely Polluted Weather in Tianjin based on WRF/Chem

基于WRF/Chem数值模式,以CO为示踪物分解水平输送、湍流混合和垂直运动对近地面大气污染的影响,研究2014-2017年天津地区重污染天气成因。研究表明:基于上述方法可实现重污染天气水平输送、湍流混合和垂直运动影响的定量描述,完成重污染天气成因数值归因分析。如2017年1月26日的重污染天气湍流混合能力下降是其重要的影响因素,2017年2月12日重污染天气混合层厚度下降对其影响显著,2017年2月16日重污染天气水平输送对其有重要影响,2015年12月21日重污染天气是下沉气流、湍流混合能力下降和混合层降低共同导致。在分析中,可通过湍流混合导致地面CO质量浓度每小时下降速率小于40%,垂直运动使得CO质量浓度每小时上升速率大于等于1.4%,混层层厚度小于250 m,水平扩散导致地面CO质量浓度上升等指标表征气象条件易于重污染天气形成。2014-2017年99次重污染天气符合上述条件之一或者多个条件,覆盖所有重污染过程的85%,即使未满足上述条件,99%的过程也可以通过重污染天气成因分析标准予以解析。分析显示2014-2017年天津116次重污染过程,58%的过程由两个或者三个气象因素共同影响所致,且影响因素与天气类型密切相关,如高压后部型与水平输送、北部弱高压型与下沉运动影响等。相比水平输送和湍流混合能力下降,下沉运动带来的近地面大气污染物质量浓度上升往往会被忽略,但在部分过程中,下沉运动也会导致近地面质量浓度快速增加,成为重污染天气形成的重要影响因素,如2014年1月1011日。湍流扩散系数KZ和湍流混合导致地面CO质量浓度每小时下降速率β与近地面PM2.5质量浓度呈现较好的幂指数关系,其相关系数分别为0.57和0.73,可以在重污染成因分析和预报中发挥积极作用。

The causes of severely polluted weather in Tianjin from 2014 to 2017 were studied based on WRF/Chem model by decomposing the impact of horizontal transport,turbulent mixing and vertical motion on near-surface air pollution with CO as a tracer.The findings showed that horizontal transport,turbulent mixing and vertical motion of severely polluted weather can be quantitatively described,and attribution analysis was conducted on the values of causes of severely polluted weather based on the above method.For example,the severely polluted weather on January 26,2017 was mainly caused by decreasing turbulent mixing ability;That on February 12,2017 by decreasing mixing layer thickness;That on February 16,2017 by horizontal transmission;That on February 16,2017 by decreasing downdraft and turbulent mixing capacity and mixing layer thickness.In the analysis,the meteorological conditions that are prone to severely polluted weather are formed as follows:rate of decrease of CO mass concentration per hour caused by turbulent mixing is less than 40%,the rate of increase of CO mass concentration per hour caused by vertical movement more than 1.4%,the mixed layer thickness less than 250 m,and the rate of decrease of surface CO mass concentration per hour caused by horizontal diffusion less than zero.Up to 99 of 116 severely polluted days from 2014 to 2017 met one or more of the above conditions,covering 85%of all the severe pollution processes.Although some failed to meet any of the above conditions,99%of the processes can be interpreted as per the analysis standard for severely polluted weather.As shown by the analysis,58%of the processes were caused by two or three meteorological factors which are closely related to weather types,such as the pollution in the rear of high pressure and horizontal transport,the pollution caused by weak high pressure from north and sinking motion.Compared with horizontal transport and the decreasing turbulent mixing capacity,the rise of near-surface air pollutant mass concentration caused by sinking motion is often neglected.However,in some processes,the rise of near-surface air pollutant mass concentration may also be caused by sinking motion,making it an important influencing factor for the formation of severely polluted weather,such as January 1011,2014.The turbulent diffusion coefficient KZ and the hourly decrease rate of surface CO mass concentration caused by turbulent mixingβshow a good power exponential relationship with the near-surface PM2.5 mass concentration,with the correlation coefficients of 0.57 and 0.73 respectively,which can play an active role in the cause analysis and prediction of severe pollution.