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大气气溶胶在广义上是指大气与悬浮在其中的固体和液体微粒共同组成的多相体系[1],但在通常情况下是指大气中悬浮的固态和液态颗粒物[2]. 气溶胶能够吸收和散射太阳辐射,从而影响整个地球的辐射收支[3],直接或间接地影响着全球环境和气候变化[4],也成为了人类健康的影响因子 [5]. 散射系数是气溶胶光学性质的重要参数之一,它反映了气溶胶对辐射传输过程的影响. 在国际上,20世纪90年代以来进行了多次气溶胶观测实验来了解其散射特性,如澳大利亚ACE-1、大西洋ACE-2、亚洲ACE-Asia和印度洋INDOEX等[6-9],获取了很多值得研究的气溶胶资料. 国内也有许多研究气溶胶散射系数的基本变化特征. 韩素芹等[10]根据GRIMM气溶胶粒谱分析仪得到的在线观测资料,计算分析天津地区春季的气溶胶消光特征. 袁亮等[11]结合Hysplit后向轨迹模式,探讨了黄山夏季在不同气团影响下气溶胶光学参数的变化特征. 于大江等[12]使用美国TSI公司生产的三波段积分浊度仪观测得到气溶胶散射系数,同时计算得到PM10气溶胶的散射Angstrom指数(SAE),反映了在观测期间气溶胶散射特征的季节变化和日变化. 任丹阳等[13]利用高时间分辨率大气气溶胶消光系数以及PM2.5化学组分的观测数据重建本地化消光系数与颗粒物化学组分浓度的经验关系式——IMPROVE公式,并发现观测期间硫酸盐对散射系数的贡献最高. 兰剑等[14]通过Hysplit后向轨迹分析表明,位于上海市西南中心城区的环境科学研究院的西向城市对上海市大气污染贡献大,潜在源分布较广.
然而针对前人的研究,在不同的气象要素下,关于较长时间序列的气溶胶散射特性的研究开展得不多. 本文主要结合天气要素,阐述杭州市主城区大气气溶胶散射系数的变化特征,并主要深入探讨2018年11月27日—12月3日一次典型的空气污染过程.
2018年杭州市主城区气溶胶散射特性的观测
Observation of aerosol scattering characteristics in Hangzhou City in 2018
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摘要: 本文利用2018年杭州国家基准气候站Aurora-3000浊度仪和相关气象要素观测资料,研究了杭州市主城区大气环境中气溶胶散射系数的变化特征. 结果表明,2018年杭州市主城区年平均散射系数为(217.4±161.9)Mm-1. 受大气环流形势,季节性气象条件变化以及人为源的影响,气溶胶散射系数表现为冬季和春季高于秋季和夏季. 在逆温层,交通排放,人为活动的共同作用下,散射系数呈“一峰一谷型”的日变化特征,峰值出现在8:00(北京时,下同),谷值出现在15:00. 随着PM2.5质量浓度的增加,散射系数和PM2.5质量浓度表现出越来越明显的正相关关系. 散射系数随地面风的增大而减小. 并且由于春季地面风速达全年最大值,具有较好的扩散条件,因此导致散射系数较冬季小;秋冬季,由于人为排放的气溶胶局地性很强,因此不同风向的散射系数分布差异不大. 对2018年11月27日—12月3日一次典型的空气污染过程分析表明,混合层高度与散射系数,PM2.5浓度呈现负相关关系. 该过程中散射系数的高值区一般都保持高湿低温的状态. 结合天气形势和后向轨迹模式分析可知,该时段由于受到西南低空急流的影响,污染气团主要是来自我国西南及周边地区.Abstract: In this paper, the variation characteristics of aerosol scattering coefficient in the atmospheric environment in the main urban area of Hangzhou were studied by using the Aurora-3000 turbidimeter of Hangzhou National Reference Climatological Station in 2018 and the observation data of related meteorological elements. The results show that the average annual scattering coefficient is (217.4±161.9) Mm-1 in the main urban area of Hangzhou in 2018. The aerosol scattering coefficient in winter and spring is higher than that in autumn and summer due to the influence of atmospheric circulation, seasonal meteorological conditions and anthropogenic sources. Under the combined effect of inversion layer, traffic emission and human activities, the scattering coefficient presents a diurnal variation characteristic of “one peak and one valley”, with the peak value appearing at 8:00 (Beijing time, the same below) and the valley value appearing at 15:00. With the increase of PM2.5 mass concentration, the scattering coefficient and PM2.5 mass concentration show more and more obvious positive correlation. The scattering coefficient decreases with the increase of the surface wind. Moreover, the surface wind speed in spring reaches the maximum of the whole year and has better diffusion conditions, so the scattering coefficient is smaller than that in winter.In autumn and winter, because of the strong local nature of anthropogenic aerosol emission, the scattering coefficient distribution of different wind direction has little difference. The analysis of a typical air pollution process from November 27 to December 3, 2018 shows that the mixing layer height is negatively correlated with the scattering coefficient and PM2.5 concentration. In this process, the high value region of scattering coefficient generally keeps the state of high humidity and low temperature. Combined with the analysis of the weather situation and the backward trajectory model, it can be seen that due to the influence of the southwest low-level jet during this period, the polluted air mass mainly came from southwest China and its surrounding areas.
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