Aerosol Hygroscopicity during the Haze Red-Alert Period in December 2016 at a Rural Site of the North China Plain

A humidification system was deployed to measure aerosol hygroscopicity at a rural site of the North China Plain during the haze red-alert period 17–22 December 2016. The aerosol scattering coefficients under dry [relative humidity (RH) < 30%] and wet (RH in the range of 40%–85%) conditions were simultaneously measured at wavelengths of 450, 550, and 700 nm. It is found that the aerosol scattering coefficient and backscattering coefficient increased by only 29% and 10%, respectively when RH went up from 40% to 80%, while the hemispheric backscatter fraction went down by 14%, implying that the aerosol hygroscopicity represented by the aerosol scattering enhancement factor f(RH) is relatively low and RH exerted little effects on the aerosol light scattering in this case. The scattering enhancement factors do not show significant differences at the three wavelengths, only with an approximate 2% variation, suggesting that the aerosol hygroscopicity is independent of the wavelength. Aerosol hygroscopicity is highly dependent on the aerosol chemical composition. When there is a large mass fraction of inorganics and a small mass fraction of organic matter, f(RH) reaches a high value. The fraction of NO₃^– was strongly correlated with the aerosol scattering coefficient at RH = 80%, which suggests that NO₃^– played an important role in aerosol hygroscopic growth during the heavy pollution period.     

武汉大气能见度与PM2.5浓度及相对湿度关系的非线性分析及能见度预报

武汉作为中部地区高湿度代表城市,大气污染严重,霾天气多发,但有关该地区大气能见度与PM2.5浓度及相对湿度(RH)的定量关系尚不明确。利用2014年9月—2015年3月武汉地区逐时能见度、相对湿度及颗粒物质量浓度观测数据,研究分析了武汉大气能见度与PM2.5浓度及相对湿度的关系,并进行能见度非线性预报初探,得到以下结论:武汉霾时数发生比例高,霾的发生和加重是能见度降低的主要原因;能见度降低伴随大量细粒子产生和累积,这是武汉大气能见度恶化的重要诱因。细颗粒物浓度与相对湿度共同影响和制约大气能见度变化,高湿高浓度时能见度显著下降,湿情景下(RH≥40%),能见度恶化主要是由湿度增高诱使细颗粒物粒径吸湿增长导致其散射效率增大造成的。当RH >90%时,能见度随湿度升高成线性递减,相对湿度每升高1%,武汉平均能见度降低0.568 km。而干情景下(RH2.5质量浓度升高。在城市大气细粒子污染背景下,能见度与相对湿度成非线性关系,这主要与PM2.5对能见度的影响及吸湿性颗粒物的散射效率变化有关。PM2.5浓度与能见度成幂函数非线性关系,80%≤RH2.5浓度对能见度的影响敏感阈值是随着湿度升高而减小的,干情景下能见度10 km对应的PM2.5浓度阈值为70 μg/m3,湿情景下该阈值为18—55 μg/m3。当PM2.5质量浓度低于约40 μg/m3时,继续降低PM2.5可显著提高武汉大气能见度。预报试验表明,基于神经网络方法建立大气能见度非线性预报模型是可行的,预报能见度相关系数为0.86,均方根误差为1.9 km,能见度≤10 km的TS评分为0.92。网络模型具有较高预报性能,对霾的判别有较高准确性,为衔接区域环境气象数值预报模式,建立大气能见度精细化动力统计模型提供参考依据。      

Using hourly measurements to explore the role of secondary inorganic aerosol in PM2.5 during haze and fog in Hangzhou,China

This paper explores the role of the secondary inorganic aerosol （SIA） species ammonium,NH4＋,nitrate,NO3-,and sulfate,SO24-,during haze and fog events using hourly mass concentrations of PM2.5 measured at a suburban site in Hangzhou,China.A total of 546 samples were collected between 1 April and 8 May 2012.The samples were analyzed and classified as clear,haze or fog depending on visibility and relative humidity （RH）.The contribution of SIA species to PM2.5 mass increased to ～50％ during haze and fog.The mass contribution of nitrate to PM2.5 increased from 11％ during clear to 20％ during haze episodes.Nitrate mass exceeded sulfate mass during haze,while near equal concentrations were observed during fog episodes.The role of RH on the correlation between concentrations of SIA and visibility was examined,with optimal correlation at 60％-70％ RH.The total acidity during clear,haze and fog periods was 42.38,48.38 and 45.51 nmol m-3,respectively,indicating that sulfate,nitrate and chloride were not neutralized by ammonium during any period.The nitrate to sulfate molar ratio,as a function of the ammonium to sulfate molar ratio,indicated that nitrate formation during fog started at a higher ammonium to sulfate molar ratio compared to clear and haze periods.During haze and fog,the nitrate oxidation ratio increased by a factor of 1.6-1.7,while the sulfur oxidation ratio increased by a factor of 1.2-1.5,indicating that both gaseous NO2 and SO2 were involved in the reduced visibility.     

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.     

基于能见度数据分析的气溶胶吸湿增长特征研究

基于相对湿度、能见度等气象数据,分析气溶胶吸湿增长特性,有助于了解气溶胶对大气环境和区域气候的影响。利用南京地区2016年1—12月、2017年2—12月、2018年1—8月和12月相对湿度和能见度等数据,通过非线性拟合研究气溶胶吸湿增长因子(f(RH))与相对湿度(RH)之间的关系。结果表明,吸湿增长因子在RH值较低(<80%)时,增长率较小;当RH值较高(>80%)时,增长率迅速增大。吸湿增长因子随着月RH值变化而表现出较大差异。此外,当南京地区盛行西风时,高能见度出现的时次较多。f(RH)与PM_2.5/PM₁₀成正比,PM_2.5/PM₁₀值越高,对应的气溶胶光学吸湿增长因子往往会越高。     

京津冀及周边地区冬季能见度与PM2.5浓度和环境湿度的多元回归分析

2013年至今,中国冬季与雾霾相伴的低能见度事件频发,京津冀及周边地区尤为严重。PM_2.5浓度与环境湿度是导致低能见度的最关键影响因素。为了深入研究PM_2.5浓度与环境湿度对大气能见度的影响,利用2017年1月京津冀及周边地区MICAPS气象数据与PM_2.5观测数据,运用天气学诊断分析方法讨论了不同相对湿度下PM_2.5浓度、环境湿度对冬季能见度变化的相对贡献,按照地理环境与污染程度差异将京津冀及周边地区划分为北京-天津地区与河北-山东地区,建立了PM_2.5浓度与环境湿度（由露点温度、温度代表）对能见度的多元回归方程,并对2015、2016、2018、2019年冬季能见度进行了回算检验。结果显示:相对湿度低于70%、PM_2.5浓度低于75 μg/m3时,北京-天津地区与河北-山东地区能见度多高于10 km,PM_2.5浓度升高是此时能见度迅速降低的主导因素;相对湿度从70%上升至85%和PM_2.5浓度从75 μg/m3升高200 μg/m3的共同作用导致了能见度降低到10 km至5 km;能见度进一步从5 km下降至2 km则更多依赖于相对湿度进一步从85%升高至95%,PM_2.5浓度与此时能见度相关减弱;能见度降低至2 km甚至更低主要是由于水汽近饱和状态下（相对湿度95%以上）的雾滴消光引起,与PM_2.5浓度的变化关系不大。与不分组直接拟合相比,以相对湿度85%为界线,分别拟合能见度能够很大程度优化多元回归模型,相对湿度高于85%时能见度拟合值的均方根误差从9.2和5.2 km下降至0.5和0.7 km,5 km以下拟合能见度的误差大幅度减小。按相对湿度85%将数据分组所得的拟合方程对2015、2016、2018、2019年1月能见度估算结果较好,观测值与拟合值相关系数均高于0.91,为雾-霾数值预报系统提供了新的能见度参数化算法。      

天津冬季大气能见度与空气污染的相互关系

为探求天津冬季大气能见度特征与空气污染的相互关系,于2008年12月至2009年1月连续观测大气能见度和空气污染物浓度(PM10、PM2.5质量浓度,O_3、NO_2和SO_2体积浓度),并结合相对湿度进行相关分析。结果表明:天津冬季大气能见度平均值为11.59 km,日变化呈明显的单峰特征,其变化特征受到空气污染物,尤其是气溶胶质量浓度及相对湿度变化共同影响;观测期内霾的发生频率接近50%;采用非线性回归方程拟合能见度与气溶胶质量浓度相互关系显示,PM2.5质量浓度对水平能见度的贡献大于PM10质量浓度,并且高湿情况下,能见度与气溶胶质量浓度相关性更好。     

Comparison of Two Air Pollution Episodes over Northeast China in Winter 2016/17 Using Ground-Based Lidar

This study analyzes and compares aerosol properties and meteorological conditions during two air pollution episodes in 19–22 (E1) and 25–26 (E2) December 2016 in Northeast China. The visibility, particulate matter (PM) mass concentration, and surface meteorological observations were examined, together with the planetary boundary layer (PBL) properties and vertical profiles of aerosol extinction coefficient and volume depolarization ratio that were measured by a ground-based lidar in Shenyang of Liaoning Province, China during December 2016–January 2017. Results suggest that the low PBL height led to poor pollution dilution in E1, while the high PBL accompanied by low visibility in E2 might have been due to cross-regional and vertical air transmission. The PM mass concentration decreased as the PBL height increased in E1 while these two variables were positively correlated in E2. The enhanced winds in E2 diffused the pollutants and contributed largely to the aerosol transport. Strong temperature inversion in E1 resulted in increased PM_2.5 and PM₁₀ concentrations, and the winds in E2 favoured the southwesterly transport of aerosols from the North China Plain into the region surrounding Shenyang. The large extinction coefficient was partially attributed to the local pollution under the low PBL with high ground-surface PM mass concentrations in E1, whereas the cross-regional transport of aerosols within a high PBL and the low PM mass concentration near the ground in E2 were associated with severe aerosol extinction at high altitudes. These results may facilitate better understanding of the vertical distribution of aerosol properties during winter pollution events in Northeast China.     

Characteristics of Chemical Speciation in PM_1 in Six Representative Regions in China

A better knowledge of aerosol properties is of great significance for elucidating the complex mechanisms behind frequently occurring haze pollution events. In this study, we examine the temporal and spatial variations in both PM_1 and its major chemical constituents using three-year field measurements that were collected in six representative regions in China between 2012 and 2014. Our results show that both PM_1 and its chemical compositions varied significantly in space and time, with high PM_1 loadings mainly observed in the winter. By comparing chemical constituents between clean and polluted episodes, we find that the elevated PM_1 mass concentration during pollution events should be largely attributable to significant increases in organic matter(OM) and inorganic aerosols like sulfate, nitrate, and ammonium(SNA),indicative of the critical role of primary emissions and secondary aerosols in elevating PM_1 pollution levels. The ratios of PM_1/PM_(2.5) are found to be generally high in Shanghai and Guangzhou, while relatively low ratios are seen in Xi'an and Chengdu, indicating anthropogenic emissions were more likely to accumulate in forms of finer particles. With respect to the relative importance of chemical components and meteorological factors quantified via statistical modeling practices, we find that primary emissions and secondary aerosols were the two leading factors contributing to PM_1 variations, though meteorological factors also played important roles in regulating the dispersion of atmospheric PM.     

丹东地区大气能见度变化及其主要影响因子分析

利用2007年6月至2009年11月丹东市区器测能见度资料和人工观测能见度资料,分析目测能见度与器测能见度的关系,探讨不同季节中,PM_10。和PM_2．5、PM_1.0。颗粒物浓度对大气能见度的影响,研究降水强度与PM的湿清除效率关系。结果表明：在低能见度情况下,目测能见度与器测能见度的比值相对较小;能见度较差时,PM_1.0粒子比重增大,PM_1.0／PM_1.0和PM2．s／PM_1.0比值与月平均风速为负相关;天气现象日数较多的月份,其对应的月平均能见度较小;PM日平均质量浓度的改变量以及降雨量对PM的湿清除效率与降水强度相关,器测目测能见度平均值随着降水强度的增加而降低。     

北京一次持续性雾霾过程的阶段性特征及影响因子分析

利用北京地区高时间分辨率观测资料对2009年11月3—8日一次持续性雾霾天气过程中的气象因素和气溶胶演变特征进行了分析。结果表明,该次雾霾过程具有明显的阶段性特征,前期以霾为主,中期发展为雾霾交替,后期随着相对湿度减小再次转换为霾并最终消散。边界层逆温是低能见度过程形成的必要条件,但并不最终决定雾霾低能见度强度。相对湿度和PM_2.5浓度是决定能见度大小的两个关键影响因子,对能见度的影响体现出阶段性特征。大部分时段PM_2.5浓度是影响能见度的主要因子,当能见度小于1 km时,能见度变化更多受相对湿度影响。不同的情景计算表明,控制PM_2.5浓度对于改善本次过程的能见度有重要作用。     

HAZE-TO-FOG TRANSFORMATION DURING A LONG LASTING, LOW VISIBILITY EPISODE IN NANJING

Haze-to-fog transformation during a long lasting, low visibility episode was examined using the observations from a comprehensive field campaign conducted in Nanjing, China during 4-9 December 2013. In this episode, haze was transformed into fog and the fog lasted for dozens of hours. The impacts of meteorological factors such as wind, temperature (T) and relative humidity (RH) on haze, transition and fog during this episode were investigated. Results revealed significant differences between haze and fog days, due to their different formation mechanisms. Comparison was made for boundary-layer conditions during hazy days, haze-to-fog days and foggy days. Distributions of wind speed and wind direction as well as synoptic weather conditions around Nanjing had determinative impacts on the occurrences and characteristics of haze and fog. Weakened southerly wind in southern Nanjing resulted in high concentration of pollutants, and haze events occurred frequently during the study period. The wind speed was less than 1 m s-1 in the haze event, which resulted in a stable atmospheric condition and weak dispersion of the pollutants. The height of the temperature inversion was about 400 m during the period. The inversion intensity was weak and the temperature-difference was 4°C km-1 or less in haze, while the inversion was stronger, and temperature-difference was about 6°C km-1, approaching the inversion layer intensity in the fog event. Haze event is strongly influenced by ambient RH. RH values increased, which resulted in haze days evidently increased, suggesting that an increasing fraction of haze events be caused by hygroscopic growth of aerosols, rather than simply by high aerosol loading. When RH was above 90%, haze aerosols started to be transformed from haze to fog. This study calls for more efforts to control emissions to prevent haze events in the region.     

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.     

广州市大气能见度影响因子的贡献研究

通过对广州市2004年10月1日~11月5日污染性气体NO2、气溶胶散射系数和吸收系数以及粒子化学成分组成等观测资料的分析,得到了影响广州市大气能见度因子的贡献比例:大气气溶胶散射bsa为75.26%、大气气溶胶吸收baa为12.89%、水汽bsw为5.78%、气体吸收bag为3.68%、大气分子散射bsg为2.38%;给出了不同粒径段气溶胶对散射的贡献比例;然后采用逐步多元线性回归得到了大气干气溶胶散射系数与PM2.5、PM10及化学成分的经验关系式;并给出了广州市区气溶胶的质量散射系数。     

Influence of High Relative Humidity on Secondary Organic Carbon: Observations at a Background Site in East China

To investigate the impacts of relative humidity(RH) on secondary organic aerosol(SOA) concentrations and chemical reactions, the carbonaceous aerosol components [i.e., organic carbon(OC) and element carbon(EC)] were quantified in daily PM2.5 samples collected at a background site in East China during summer 2015. Based on the method of EC-tracer, the concentration of secondary organic carbon(SOC) demonstrated an obvious negative relationship with RH higher than 60%. Moreover, the ratio of SOC/EC also exhibited obvious decreasing trends with increasing RH, indicating negative effects for chemical production of SOA under high RH conditions. Due to high RH,photochemistry was weakened, gaseous oxidant concentrations was lowered(e.g., significantly decreased O3 levels),and the production rates of SOA were relatively low. On the other hand, because of more water uptake under higher RH conditions, the aerosol droplet acidity was reduced and enhancement of SOA formation by acidity was accordingly absent. In addition, high RH also plays an important role in changing viscosity of pre-existing aerosol coatings,which can affect reactive uptake yield of SOA. Overall, the results from this study imply that SOA production may be more associated with photochemical processes, while aqueous-phase chemistry is not very important for some SOA formation in a moist ambient environment. In the ambient atmosphere, oxidant concentrations, reaction rates,airborne species, etc., are highly variable. How do these factors affect SOA yields under given ambient environment warrants further detailed investigations.     

Relative contributions of boundary-layer meteorological factors to the explosive growth of PM<Subscript>2.5</Subscript> during the red-alert heavy pollution episodes in Beijing in December 2016

Based on observations of urban mass concentration of fine particulate matter smaller than 2.5 μm in diameter (PM_2.5), ground meteorological data, vertical measurements of winds, temperature, and relative humidity (RH), and ECMWF reanalysis data, the major changes in the vertical structures of meteorological factors in the boundary layer (BL) during the heavy aerosol pollution episodes (HPEs) that occurred in winter 2016 in the urban Beijing area were analyzed. The HPEs are divided into two stages: the transport of pollutants under prevailing southerly winds, known as the transport stage (TS), and the PM_2.5 explosive growth and pollution accumulation period characterized by a temperature inversion with low winds and high RH in the lower BL, known as the cumulative stage (CS). During the TS, a surface high lies south of Beijing, and pollutants are transported northwards. During the CS, a stable BL forms and is characterized by weak winds, temperature inversion, and moisture accumulation. Stable atmospheric stratification featured with light/calm winds and accumulated moisture (RH > 80%) below 250 m at the beginning of the CS is closely associated with the inversion, which is strengthened by the considerable decrease in near-surface air temperature due to the interaction between aerosols and radiation after the aerosol pollution occurs. A significant increase in the PLAM (Parameter Linking Aerosol Pollution and Meteorological Elements) index is found, which is linearly related to PM mass change. During the first 10 h of the CS, the more stable BL contributes approximately 84% of the explosive growth of PM_2.5 mass. Additional accumulated near-surface moisture caused by the ground temperature decrease, weak turbulent diffusion, low BL height, and inhibited vertical mixing of water vapor is conducive to the secondary aerosol formation through chemical reactions, including liquid phase and heterogeneous reactions, which further increases the PM_2.5 concentration levels. The contribution of these reaction mechanisms to the explosive growth of PM_2.5 mass during the early CS and subsequent pollution accumulation requires further investigation.     

大气气溶胶散射吸湿增长特性研究进展

大气气溶胶的散射吸湿增长特性反映了相对湿度对气溶胶散射能力的影响。研究气溶胶散射吸湿增长对于评估气溶胶辐射强迫、了解气溶胶对大气能见度的影响具有重要意义。文中对气溶胶散射吸湿增长特性的研究意义、监测方法、中外研究进展、参数化拟合和模型模拟等方面进行了系统回顾和总结,并对未来发展方向做出展望。中国在该领域开展的研究数量偏少,广泛开展气溶胶散射吸湿增长特性研究十分必要。     

南京北郊2011年春季气溶胶粒子的散射特征

利用南京北郊2011年春季积分浊度仪的观测资料,结合PM_2.5质量浓度、能见度和常规气象资料,分析了南京北郊春季气溶胶散射系数的变化特征、散射系数与PM_2.5质量浓度和能见度的关系。结果表明,观测期间气溶胶散射系数平均值为311.5±173.3 Mm^-1,小时平均值出现频率最高的区间为100~200 Mm^-1;散射系数的日变化特征明显,总体为早晚大,中午及午后小。散射系数与PM_2.5质量浓度的变化趋势基本一致,但与能见度呈负相关关系。霾天气期间散射系数日平均值为700.5±341.4 Mm^-1,最高值达到近1 900 Mm^-1;结合地面观测资料、NCEP/NCAR再分析资料和后向轨迹模式分析显示,霾期间气块主要来自南京南部和东南方向。     

夏季厄尔尼诺-Modoki和东部型ENSO海表温度异常分布型特征及其与海洋性大陆区域气候异常的联系

2007年,Ashok等揭示了赤道太平洋区域存在一种三极型分布海表温度异常并称之为厄尔尼诺-Modoki,同时定义了相应的海表温度异常指数EMI（记为I_EM）。在此基础上,利用英国哈得来中心逐月海表温度资料、美国NCEP/NCAR月平均再分析数据集、美国国家海洋和大气管理局（NOAA）逐月降水资料（CMAP）,通过在太平洋海表温度异常中扣除厄尔尼诺-Modoki信号后,在Nino1+2区域上定义了东太平洋型海表温度异常指数EPNI（I_EPN）。据此,由I_EPN和I_EM可构成描述热带太平洋海表温度异常变化的一对指数。分析了两个指数相应的海气状态及对海洋性大陆区域气候异常的影响。结果表明,厄尔尼诺-Modoki和东太平洋型海表温度异常及其影响存在显著差异。在北半球夏季,当I_EM处于正位相时,热带太平洋海表温度异常呈现“负-正-负”的结构,海洋性大陆大部分区域海表温度异常为负,此时对流层低层太平洋地区辐合,海洋性大陆地区辐散,对流层高层太平洋地区辐散,海洋性大陆地区辐合。对应于辐合辐散中心,存在着自赤道中太平洋分别向赤道东太平洋和海洋性大陆中东部地区的异常垂直环流圈,同时也存在自海洋性大陆西部向印度洋西部的垂直环流。大气在海洋性大陆区域北部加热,南部冷却;在太平洋地区西部加热而东部冷却;在海洋性大陆区域10°N以南降水偏少,而10°N以北降水偏多。当I_EPN处于正位相时,热带太平洋海表温度异常呈现“西负东正”分布型,海洋性大陆区域海表温度异常呈现“西正东负”分布,对流层低层海洋性大陆地区辐散中心范围偏大、位置偏东、强度偏强,太平洋地区辐合中心范围偏小、位置偏东,热带环流异常在垂直方向上呈斜压结构,海洋性大陆区域北部大气加热而南部冷却,太平洋地区大气均呈加热正异常,海洋性大陆大部分区域降水均偏少,赤道太平洋降水偏多。以上这些结果有利于深刻理解热带太平洋海表温度异常的特征及其对海洋性大陆区域气候的影响。