气溶胶直接辐射强迫研究进展

Progress of aerosol direct radiative forcing

近年来,气溶胶直接辐射强迫(aerosol direct radiative forcing,ADRF)的不确定性逐渐减少,但由于缺乏大范围、高精度气溶胶光学特征的观测,并且模式对气溶胶光学特性和物理化学过程的描述不够准确,ADRF的不确定性相比温室气体依然很大.本文首先回顾了气溶胶光学厚度(aerosol optical depth,AOD)和单次散射反照率(single scattering albedo,SSA)两种气溶胶光学特性及其相应ADRF时空分布的相关研究.AOD和SSA在不同的时空尺度上差异明显,在经济快速发展的地区(如印度),AOD呈现增长的趋势且平均值较高,而受环境保护政策影响的地区如北美和欧洲呈现下降的趋势且平均值较低.根据站点观测,大多数欧洲、北美洲、非洲和亚洲站点的SSA呈显著增长的趋势,而在生物质燃烧频繁或沙尘爆发的季节,部分地区如深秋和初春的华南和西南地区、春季的华北和西北地区,SSA会下降.未来在全球及大部分区域,随着气溶胶及其前体物排放的下降,ADRF也随之减弱,但减弱的趋势取决于各个区域的发展水平和发展路径.随后,本文系统总结了气溶胶光学特征如AOD、SSA、非对称因子(asymmetry factor,ASY)以及环境因素如地表反照率(surface albedo,SA)、气溶胶高度、气溶胶与云之间的相对位置、不同类型气溶胶之间的相对位置、太阳天顶角(solar zenith angle,SZA)等对ADRF的影响,并梳理了ADRF对不同影响因素的敏感性及不同影响因素对ADRF评估不确定性贡献的相关研究.研究发现,多数情况下SSA是ADRF不确定性最大的来源;而在污染严重的地区,SA和ASY造成的误差也不容忽视.最后,本文从观测和模式两个角度对提升ADRF评估的精确性作了简要展望.未来需要充分结合各种先进的观测和模式,如多角度、多(高)光谱、偏振的遥感观测、精细的原位测量和地球系统模式,获取更精确的气溶胶和环境信息,改进气溶胶及其前体物的模拟.随着对气溶胶与辐射相互作用机制理解的加深,以及相应观测技术和模式模拟能力的进步,ADRF的评估将更加精确.

Aerosols are liquid and solid particles suspended in the atmosphere and have a broad size range;they can cool the Earth by scattering radiation back to space or warm the Earth by absorbing radiation directly.Since the industrial revolution,the loading of aerosols in the Earth's atmosphere has increased significantly,yielding modifications to the Earth's energy budget and further affecting the climate state.Aerosol direct radiative forcing(ADRF),defined as the difference in radiation with and without total or specific aerosols,is an important concept used to describe the direct impact of aerosols on radiation.Accurate quantification of ADRF is the premise for understanding and predicting the Earth's climate state.To improve the estimation and evaluation of ADRF,numerous researchers have dedicated their efforts to developing a series of observations and models in recent decades.However,due to the limited availability of wide spatial and high-precision observations of aerosol optical characteristics,as well as an insufficient model description of aerosol properties and physical and chemical processes,the ADRF uncertainty is still high.This paper first reviews the spatio-temporal distribution of aerosol optical depth(AOD),single scattering albedo(SSA) and corresponding ADRF by using observations and models.The aerosol optical properties and ADRF show distinct discrepancies among various regions due to the impact of anthropogenic emissions and meteorological and climate conditions.In regions with rapid economic development,such as India,AOD demonstrates a long-term increasing trend with higher average values.However,regions influenced by environmental protection policies,such as North America and Europe,show a long-term decreasing trend in AOD,accompanied by lower average values.Based on site observations,most of Europe,North America,Africa,and Asia exhibit a significant long-term increasing trend in SSA.However,in seasons with biomass burning or dust outbreaks,specific regions,such as southern and southwestern China in late autumn and early spring,and northern and northwestern China in spring,exhibit a reduction in SSA.In the future,with the global and regional emissions of aerosols and precursors declining,ADRF is expected to weaken,highlighting the warming effect of greenhouse gases.However,the ADRF trend is closely linked to the present development level and trajectory of each region.Second,we systematically summarize the impacts of the influential factors on the ADRF,considering the AOD,SSA,surface albedo(SA),solar zenith angle(SZA),asymmetry factor(ASY),relative altitude between aerosols and clouds,and relative altitude between different types of aerosols.Subsequently,we proceed to review the sensitivities of ADRF to different influential factors,as well as the contributions of these factors to the overall uncertainty of ADRF,which indicate that ADRF is more sensitive to AOD and SSA while SSA emerges as the most significant source of uncertainty in ADRF due to the larger errors associated with its measurement.It should be noted that the uncertainty caused by SA and ASY cannot be ignored in polluted regions.Finally,from the perspective of observations and models,a brief outlook on improving the accuracy of ADRF evaluation is provided.In the future,advanced observation technologies,such as multi-angle,hyperspectral,polarized remote sensing observations,and precise in-situ measurements,should be developed to obtain more accurate information about the aerosols and environment.Furthermore,we need to properly combine various observations and models,including Earth system models,to improve the simulation of aerosols and their precursors.With improved understanding of aerosol-radiation interactions and refining techniques in observations and model simulations,the evaluation of ADRF will be more accurate.