相对湿度对吸收性气溶胶指数的影响

为提高风云三号气象卫星(FY-3)紫外臭氧总量探测仪(TOU)观测数据得到的吸收性气溶胶指数AAI(Absorbing Aerosol Index)或AI(Aerosol Index)的可靠性,需要了解AAI指数与相对湿度之间的内在关系。本研究利用大气辐射传输模型DAK(Doubling-Adding KNMI)分别模拟了在城市气溶胶和乡村气溶胶模式下AAI同相对湿度之间的关系,并将结果同已观测到的实际结果进行对比。结果发现,在相对湿度呈高值时AAI指数出现很大的变化,但相对湿度对两种气溶胶模型的影响具有相反的效应,分析显示当大气中含有吸收性气溶胶如含碳类气溶胶(在中国北部的污染过程中很常见)时,AAI结果对RH(Relative Humidity)有很强的依赖。在应用AAI指数产品检测污染过程中需要注意气溶胶的具体类型和相对湿度的影响,必要时在高相对湿度过程中进行数据校正或剔除。

Effects of humidity on absorbing aerosol index

Air pollution processes in Northern China have been analyzed using Absorbing Aerosol Index (AAI) products and ground-based data, including Relative Humidity (RH) and Particulate Matter (PM) 2.5. The AAI data are obtained from the total ozone unit load on FY-3B, which is the second-generation polar orbiter of China. The correlation between AAI, which has a fairly high value, and PM2.5 is weak when RH has a high value. In this research, the relationship between satellite-retrieved AAI and RH is simulated.The effects of humidity on AAI are investigated under different aerosol types, such as urban and rural aerosols, using radiative transfer models Doubling-Adding KNMI (DAK) with consideration of the characteristics of the aerosol models of the low atmosphere and mode radii for the aerosol model as a function of RH. Rural aerosol consists of 30% dust-like aerosol and 70% water-soluble materials, including ammonium, calcium sulfate, and organic compounds. Urban aerosol consists of 80% rural aerosol and 20% carbonaceous aerosol-like soot caused by industrial emissions.The AAI increases with Aerosol Optical Depth (AOD) for the two aerosol models, whereas the AAI is higher and changes faster with AOD than with urban aerosol for the rural aerosol model. For the two aerosol models, the AAI remains constant with RH when the air is dry but changes rapidly when the air becomes humid, and the effects of RH on the AAI are opposite for rural and urban aerosols. Analysis results show that the AAI strongly depends on RH when absorbing aerosols, such as carbonaceous aerosol-like soot, exist in the atmosphere during air pollution, which frequently occurs in Northern China. This finding can well explain the huge discrepancy of the comparison between the AAI and ground-based measurements under high RH condition using the microphysical properties of aerosols.Under a dry condition and a certain AOD value, the AAI is larger in rural aerosol than in urban aerosol. This condition is due to more dust-like aerosols exist in the rural aerosol model than in the urban aerosol model, and dust-like aerosols have stronger absorbing ability than carbonaceous aerosols. Under the humid condition, dust-like aerosols in the rural model exert a weaker hygroscopic effect than carbonaceous aerosols caused by industrial emission in the urban model, whereas sulfate aerosols demonstrate a strong hygroscopic effect but only produce negative or zero AAI. Thus, the AAI of rural aerosol slightly decreases with the increase in RH. Soot-like aerosol, which only exists in urban aerosol, can grow large by taking a large amount of water in high RH ambient atmosphere and change the chemical composition of particles. Thus, the absorption ability enormously increases with RH. This condition makes RH have a stronger effect on the AAI in the urban aerosol model than in the rural aerosol model. The aerosol type and humidity must be considered important impacting factors when monitoring the AAI data derived from spaceborne UV instruments.