输送、滞留叠加海上回流的长时间沙尘天气影响判断及贡献分析

Time-Determination and Contribution Analysis of Transport,Retention,and Offshore Backflow to Long-Term Sand-Dust Coupling

于2019年10月15日~11月7日对上海大气中颗粒物的质量浓度和PM_(2.5)的化学组分进行了在线连续观测,期间,华东地区遭遇了一次大范围的沙尘过程.根据相关规定,结合沙尘示踪组分和WRF-CMAQ数值模拟,将观测过程分为4个阶段:沙尘前、沙尘Ⅰ(输送和滞留过程)、沙尘Ⅱ(海上回流和清除过程)和沙尘后.基于相关规定、沙尘示踪物和空气质量模型这3种方法判定的沙尘开始时间为10月29日08:00~09:00,但结束时间存在明显分歧:相关规定的判定方法无法对海上回流的沙尘气团进行识别;不同示踪物判断的沙尘结束时间有明显差异;WRF-CMAQ模型虽然能够较好地模拟沙尘的时间变化趋势,但对于短期滞留的沙尘和海上回流沙尘存在高估.沙尘天气中PM10、PM_(2.5)和无机元素的质量浓度显著高于非沙尘天,最高日均浓度出现在10月29日,分别为(234.8±125.5)、(76.8±22.5)和(17.54±10.5)μg·m^(-3).沙尘期间地壳元素对PM_(2.5)的浓度贡献显著升高,二次离子(SO_(4)^(2-)、NO_(3)^(-)和NH_(4)^(+))对PM_(2.5)的浓度贡献明显降低,沙尘Ⅰ和沙尘Ⅱ过程Al、Si、Ca和Fe这4种地壳元素占PM_(2.5)的质量分数分别为23.5%和13.7%,二次离子分别占PM_(2.5)的质量分数为24.3%和41.9%.PMF源解析法、Ca含量丰度法、沙尘源区PM_(2.5)/PM10比值法和地壳物质重构法表明,沙尘颗粒对PM_(2.5)的直接浓度贡献为43.4%~50.0%,回流沙尘对PM_(2.5)的浓度贡献为19.2%~24.7%.

Continuous on-line observation of particulate matter and PM_(2.5) chemical composition was conducted from October 15th to November 7th 2019 in East China.During the observation period,a wide range of dust-related processes took place.According to supplementary urban air quality assessment affected by dust(hereafter referred to as supplementary provisions),the observations were divided into four stages including pre-dust event,dustⅠ,dustⅡ,and post-dust event.The dustⅠstage represented the processes of transportation and retention,while the dustⅡstage represented processes of backflow from the sea and scavenging.The start time of the studied dust event was October 29th 08:00-09:00 based on the supplementary provisions,dust tracers,and air quality models;however,disagreements existed between these data sources with respect to the finishing time.The supplementary provisions could not effectively distinguish backflow dust from sea,and results from different dust tracers were variable.The WRF-CMAQ model simulated dust variation trends well but overestimated short-term suspended dust and backflow dust.PM10,PM_(2.5),and trace element concentrations were much higher during dust events than during non-dust periods,with highest daily concentrations of(234.8±125.5),(76.8±22.5),and(17.54±10.5)μg·m^(-3),respectively,which occurred on October 29 th.During the dust event,concentration of crustal elements were remarkably high in PM_(2.5).At the same time,secondary ions(SO_(4)^(2-),NO_(3)^(-),and NH_(4)^(+))contributed less to PM_(2.5) mass concentrations.Four major crustal elements(Al,Si,Ca,and Fe)accounted for 23.5%and 13.7%of the mass concentration of PM_(2.5) and secondary ions accounted for 24.3%and 41.9%during dustⅠand dustⅡstages,respectively.Based on PMF source apportionment,Ca abundance,PM_(2.5)/PM10 in dust sources,and the reconstruction of crustal material,dust particulates accounted for 43.4%-50.0%of PM_(2.5) and backflow dust accounted for 19.2%-24.7%of PM_(2.5).