Abstract:

Wet and dry deposition processes are the major removal mechanisms for atmospheric pollutants and are important sources of nutrients and toxins in ecosystems. Compared to wet deposition, dry deposition is more difficult to measure directly; thus, an inferential approach is often used to estimate dry deposition fluxes. In this study, size-resolved particle dry deposition velocities (V_d) were calculated using classical parameterization, which was then combined with measured size- and chemically resolved ambient concentrations to estimate the dry deposition fluxes of particulate matter, with a focus on heavy pollution days. The calculated size-resolved V_d of particulate matter varied from 0.037 to 0.50 cm·s⁻¹, with increased values at high wind speeds and low relative humidity. The maximum and minimum values of V_d occurred at 5.8‒9 μm and 0.65‒1.1 μm or 1.1‒2.1 μm, respectively. With a minimum V_d value at a particle size of 1.1 μm, V_d increased as the particle size decreased or increased, reflecting the different dominant microphysical mechanisms of dry deposition for different particle sizes. During the observation period, dry deposition fluxes of particulate matter ranged from 9.41 to 108.71 mg·m⁻²·d⁻¹, with a mean value of 36.65 mg·m⁻²·d⁻¹. The concentration and V_d had a greater influence on the dry deposition fluxes of the coarse and fine particles, respectively. During heavy polluted days, the dry deposition fluxes of particles at each size range showed an upward trend, among which the dry deposition of secondary aerosols of nitrate, sulfate and ammoniums showed an obvious “size distribution shift” phenomenon. Such a phenomenon occurred in both January 2013 and January 2022, indicating that persistent heavily polluted weather in the North China Plain may increase the input of particulate species to the surface ecosystem through dry deposition.

Key words: dry deposition, particulate matter, size distribution, heavy polluted weather, chemical composition

摘要：

干湿沉降是大气污染物的主要汇机制，也是生态系统重要的物质来源。相比于湿沉降，干沉降通量直接测量难度大，研究进展相对缓慢，稀缺的研究资料与其重要性不匹配。基于北京2013年1月大气颗粒物粒径分级采样观测资料，利用经典的大气阻力模型计算了不同粒径段颗粒物的干沉降速度（V_d），重点分析了重污染期间颗粒物的干沉降通量及其化学成分的粒径变化。结果显示，所研究的8个粒径段颗粒物V_d变化范围为0.037－0.50 cm·s⁻¹，高风速和低相对湿度的气象条件有利于V_d的增加；不同粒径段的颗粒物V_d最大值出现在5.8－9 μm，最小值出现在0.65－1.1 μm或1.1－2.1 μm；以最小值出现的1.1 μm为界，随着颗粒物粒径变小或增大，V_d均呈现增加的趋势，反映了不同粒径段颗粒物干沉降微物理机制的差异。观测期间，大气颗粒物总的干沉降通量变化范围为9.41－108.71 mg·m⁻²·d⁻¹，平均值为 (36.65±25.61) mg·m⁻²·d⁻¹，粗粒子干沉降通量的变化更多受颗粒物浓度的影响而细粒子干沉降通量的变化更多受V_d影响；在重污染天气时，所有粒径段的颗粒物浓度和干沉降通量均有所上升，其中硝酸盐、铵盐和硫酸盐等二次化学组分的浓度和干沉降通量峰值所在的粒径出现了由小变大的“粒径转移”现象。2022年1月大气重污染期间依然能观测到类似的“粒径转移”特征，说明这种现象具有一定的持续性和普遍性。迄今为止，北京及周边地区尚未彻底消除重污染天气，这种天气很可能会增加大气颗粒物及化学成分的干沉降通量，进而通过跨介质迁移间接影响水体、土壤和植被等生态环境质量。

关键词: 干沉降, 大气颗粒物, 粒径分布, 重污染天气, 化学成分