Recent Progress of Aerosol Light-scattering Enhancement Factor Studies in China

Assessment of the radiative forcing of aerosols in models still lacks sufficient input data for aerosol hygroscopicity. The light scattering enhancement factor [ f(RH, λ)] is a crucial parameter for describing aerosol hygroscopic growth properties.In this paper, we provide a survey of f(RH, λ) studies in China for the past seven years, including instrument developments of humidified nephelometers, ambient f(RH, λ) measurements in China, f(RH, λ) parameterization schemes, and f(RH, λ)applications in aerosol measurements. Comparisons of different f(RH, λ) parameterizations are carried out to check their performance in China using field measurement datasets. We also summary the parameterization schemes for predicting f(RH, λ)with aerosol chemical compositions. The recently developed methods to observe other aerosol properties using f(RH, λ)measurements, such as calculating the aerosol hygroscopicity parameter, cloud condensation nuclei number concentration,aerosol liquid water content, and aerosol asymmetry factor, are introduced. Suggestions for further research on f(RH, λ) in China are given.     

Effect of model errors in ambient air humidity on the aerosol optical depth obtained via aerosol hygroscopicity in eastern China in the Atmospheric Chemistry and Climate Model Intercomparison Project datasets

当前全球气候模式普遍低估中国东部气溶胶光学厚度(AOD)。本文表明,模式低估中国东部地表大气相对湿度是AOD低估的原因之一。研究基于国际大气化学气候模式比较计划的干气溶胶质量浓度结果,用再分析湿度数据替换模拟湿度以计算AOD变化。结果表明,替换后我国大气相对湿度增加,多模式平均的华南年均AOD因此增长45%,华北6-8月AOD增长33%。不同模式采用相似的硫酸盐吸湿增长曲线。相对湿度低估对AOD模拟影响程度主要受吸湿颗粒质量浓度和相对湿度概率分布影响,该结论有助于解释我国AOD模拟误差。     

中国东部不同污染环境中气溶胶吸湿性的差异及其成因

气溶胶吸湿性与其光学性质、活化能力和生命周期等密切相关。本研究基于华北和长三角地区6个典型站点的吸湿性-串联差分电迁移率分析仪观测数据,开展气溶胶吸湿性的对比分析。研究发现华北地区气溶胶吸湿性整体强于长三角地区,不同污染环境下粒子混合状态存在明显差别。人为一次排放产生的大量疏水性物质能够削弱气溶胶吸湿性,增强粒子外混程度;而二次生成过程(如光化学反应)产生的强吸湿性物质能够增强气溶胶吸湿性,同时增强粒子内混程度,造成两个地区不同站点间同一模态气溶胶吸湿性参数存在显著差异。一般情况下,积聚模态粒子吸湿性强于核模态粒子。特别地,受频繁的新粒子生成事件影响,邢台站点的核模态气溶胶也具有较强的吸湿性。以上结果表明,不同污染环境中气溶胶吸湿性存在明显差异,在进行气溶胶吸湿性的模拟实验及研究其对雾霾形成的影响时应注意区分。     

Dependence of Mixed Aerosol Light Scattering Extinction on Relative Humidity in Beijing and Hong Kong

The hygroscopic properties of mixed aerosol particles are crucial for the application of remote sensing products of aerosol optical parameters in the study of air quality and climate at multiple scales. In this study, the authors investigated aerosol optical properties as a function of relative humidity (RH) for two representative metropolises: Beijing and Hong Kong. In addition to the RH data, mass concentrations of PM 10 (particulate matter up to 10 μm in diameter) and aerosol scattering extinction coefficient (σ ext ) data were used. The relationship between the mass scattering extinction efficiency (MEE, defined as σ ext /PM 10 ) and RH can be expressed by regression functions as f = 1.52x + 0.29 (r2 = 0.77), f = 1.42x + 1.53 (r2 = 0.58), f = 1.19x + 0.65 (r2 = 0.59), and f = 1.58x + 1.30 (r2 = 0.61) for spring, summer, autumn, and winter, respectively, in Beijing. Here, f represents MEE, x represents 1/(1 RH), and the coefficients of determination are pre- sented in parentheses. Conversely, in Hong Kong, the corresponding functions are f = 1.98x 1.40 (r2 = 0.55), f = 1.32x 0.36 (r2 = 0.26), f = 1.87x 0.65 (r2 = 0.64), and f = 2.39x 1.47 (r2 = 0.72), respectively. The MEE values for Hong Kong at high RHs (RH 70%) are higher than those for Beijing, except in summer; this suggests that aerosols in Hong Kong are more hygroscopic than those in Beijing for the other three seasons, but the aerosol hygroscopicity is similarly high in summer over both cities. This study describes the effects of moisture on aerosol scattering extinction coefficients and provides a potential method of studying atmospheric visibility and groundlevel air quality using some of the optical remote sensing products of satellites.     

中国超大城市综合实验:京、沪、穗气溶胶理化和吸湿特性

气溶胶对环境、气象和人体健康都有较大影响,这些影响与气溶胶理化特性(粒子尺度谱、化学组分、混合状态等)密切相关。为了深入研究气溶胶的环境和气候效应,发展了一套气溶胶在线综合观测系统。本文介绍了利用该系统在北京、上海、广州三个超大城市开展的综合观测实验结果。通过对比分析发现,广州气溶胶数浓度最高,其粒子尺度谱分布特征与北京特征相似,均以核模态为主,上海气溶胶数浓度则整体较低。对比三个超大城市的新粒子生成(New Particle Formation,NPF)特征发现,北京NPF的发生频率低于广州,主要由于北京地区大气中大粒径气溶胶更多,较高的碰并汇抑制了NPF的发生和发展。研究发现,观测期间北京和上海站点气溶胶的吸湿性强于广州,人为一次性排放气溶胶吸湿性较弱。气溶胶吸湿性日变化特征与人为活动、气溶胶老化程度密切相关。此外,三个超大城市中气溶胶光吸收系数的日变化特征存在明显差别,北京站点的气溶胶吸收系数呈现白天高、夜间低的特点,而广州站点气溶胶的吸收系数呈现相反的日变化趋势,这可能是由观测站周边的环境差异及大气边界层的变化特征差异造成的。     

The Great Dun Fell Experiment 1995: an overview

During March and April of 1995 a major international field project was conducted at the UMIST field station site on Great Dun Fell in Cumbria, Northern England. The hill cap cloud which frequently envelopes this site was used as a natural flow through reactor to examine the sensitivity of the cloud microphysics to the aerosol entering the cloud and also to investigate the effects of the cloud in changing the aerosol size distribution, chemical composition and associated optical properties. To investigate these processes, detailed measurements of the cloud water chemistry (including the chemistry of sulphur compounds, organic and inorganic oxidised nitrogen and ammonia), cloud microphysics and properties of the aerosol and trace gas concentrations upwind and downwind of the cap cloud were undertaken. It was found that the cloud droplet number was generally strongly correlated to aerosol number concentration, with up to 2000 activated droplets cm⁻³ being observed in the most polluted conditions. In such conditions it was inferred that hygroscopic organic compounds were important in the activation process. Often, the size distribution of the aerosol was substantially modified by the cloud processing, largely due to the aqueous phase oxidation of S(IV) to sulphate by hydrogen peroxide, but also through the uptake and fixing of gas phase nitric acid as nitrate, increasing the calculated optical scattering of the aerosol substantially (by up to 24%). New particle formation was also observed in the ultrafine aerosol mode (at about 5 nm) downwind of the cap cloud, particularly in conditions of low total aerosol surface area and in the presence of ammonia and HCl gases. This was seen to occur at night as well as during the day via a mechanism which is not yet understood. The implications of these results for parameterising aerosol growth in Global Climate Models are explored.     

Aerosol Chemistry, and Light-Scattering and Hygroscopicity Budgets during Outflow from East Asia

During the ACE-Asia field campaign, 41 aerosol filter samples were obtained by airborne sampling over the ocean to the north, south and west of Japan, generally under conditions of outflow from the Asian continent. These samples were analyzed for their water-soluble chemical components, particularly organic species. Suites of inorganic anions, carbohydrates, organic acids and metallic elements were identified and quantified (21 distinct species). Simultaneously, measurements were made of the aerosol hygroscopicity and light scattering. A factor analysis performed on the compositional data identified several sources for the aerosols sampled during the field campaign. Regression of the light-scattering data onto tracers for each of these factors suggests that the aerosol light scattering could be largely attributed to three of these factors: anthropogenic emissions, biomass burning, and soil dust. Similarly, the aerosol hygroscopicity was largely consistent with an empirical mixing model based on past measurements of the hygroscopicity of these individual aerosol types. Limited size-resolved , aerosol composition measurements were also obtained and suggested different sources for various chemical species.     

Changes in bark properties and hydrology following prescribed fire in Pinus taeda and Quercus montana

In the eastern United States, the use of prescribed fire as a silvicultural technique to manage for desirable upland tree species is increasing in popularity. Bark physical properties such as thickness, density, and porosity have known associations with fire tolerance among species. These physical properties simultaneously influence rainfall interception and canopy storage and thus are of interest across a range of disciplines. Furthermore, while these characteristics are innate to a species, it is unknown whether repeated exposure to fire facilitates physical change in bark structure and whether these changes are consistent among species. To answer these questions, bark samples were collected from mature pine (Pinus taeda L.) and oak (Quercus montana Willd.) trees from sites across the Bankhead National Forest in Alabama, USA under three different burn regimes: 3-year cycle, 9-year cycle, and no fire. Samples were analysed in the laboratory for bulk density, porosity, water storage capacity, and hygroscopicity (the amount of atmospheric water vapour absorbed by bark during non-rainfall conditions). Drying rates of saturated samples under simulated wetting conditions were also assessed. Oak bark had higher bulk density, lower porosity, and dried slower than pine bark. Interestingly, bark from both species had lower bulk density, higher porosity, greater water storage capacity, and dried faster in stands that were burned every 3 years compared to other fire regimes (p < 0.001). In summary, this study demonstrates that prescribed fire regimes in an eastern US forest alter bark structure and thus influence individual tree control on hydrological processes. The increase in bark water storage capacity, coupled with faster bark evaporation times may lead to less water inputs to the forest floor and drier overall conditions. Further investigation of this fire-bark-water feedback loop is necessary to understand the extent of these mechanisms controlling landscape-scale conditions.