Search

Seasonal and diurnal characteristics of the vertical profiles of aerosol properties are essential for detecting the regional transport and the climatic radiative effects of aerosol particles. We have studied the seasonal and diurnal characteristics of the vertical distribution of aerosols in urban Beijing from 2017 to 2021 based on long-term Raman–Mie LiDAR observations. The influence of the vertical distribution of aerosols, the meteorological conditions within the boundary layer, the optical–radiometric properties of aerosols, and their interconnections, were investigated during a heavy haze pollution event in Beijing from 8 to 15 February 2020 using both meteorological and sun photometer data. The aerosol extinction coefficient was highest in summer (0.4 km−1), followed by winter (0.35 km−1), and roughly equal in spring and autumn (0.3 km−1). The aerosol extinction coefficient showed clear daily variations and was different in different seasons as a result of the variation in the height of the boundary layer. During the haze pollution event, the particulate matter mainly consisted of scattered spherical fine particles and the accumulation time of pollutants measured via the AOD440nm and PM2.5 mass concentration was different as a result of the hygroscopic growth of the aerosol particles. This growth increased scattering and led to an increase in the aerosol optical depth. The vertical transport of particulate matter also contributed to the increase in the aerosol optical depth. [ABSTRACT FROM AUTHOR]

An assessment and verification of the Langley calibration method of the Sun photometer at Mt Foyeding (MFYD) Observatory in Beijing was performed. We explored whether the Langley plot calibration is practicable for this mountainous site by analyzing the aerosol climatology and carrying out a case study. Then, the aerosol optical depth (AOD) results were verified under the reference of AERONET AOD. The results showed that satisfactory atmospheric conditions are present on winter mornings, characterized by a smaller average AOD (~0.09–0.14) and a lower range ratio (~36.97–63.38%) than in the afternoons and over a whole day. The six days selected as the case study all showed stable atmospheric conditions characterized by daily average triplets of <2% for all wavelengths. The residual sum of squares for V 0 λ at all wavelengths was <0.0002 and the residual standard deviation was <0.2%. A large improvement was found in the linear regression at morning relative to the statistics obtained over the whole day, when the coefficient of determination and residual standard deviation were promoted by 0.22–2.90% and ~2.76–23.32, respectively. The final V 0 λ value was derived from 31 days of observation and the deviations from the reference V 0 λ were about −1.69, −1.29, −0.81, −0.42, −0.34, −0.22, −0.63 and −0.36% at 340, 380, 440, 500, 675, 870, 1020 and 1640 nm, respectively. The regression analysis of the AOD validation showed a perfect AOD performance, with 100% of the retrievals lying within the expected error (0.05 ± 10%) from 380 to 1640 nm and 99.99% for the 340 nm band. Good AOD agreement (correlation coefficients > 0.998) and residual standard deviation values ranging from ~0.006 to 0.011 were observed, with the relative mean bias varying from 0.999 to 1.066. The mean biases were concentrated within ±0.02 for the ultraviolet bands and within ±0.01 for the other bands; therefore, the results of this preliminary assessment and verification indicated that the Langley plots method is suitable for photometer calibration at the MFYD Observatory. [ABSTRACT FROM AUTHOR]

Aerosols can affect vertical thermal structure during heavily polluted episodes (HPEs). Here, we selected four typical HPEs in 2018, which were further subdivided into dust and haze events. The vertical distribution of aerosols extinction coefficient (EC) and variations in columnar optical properties were investigated based on sun-photometer and Lidar observation at an urban site in Beijing. The vertical characteristics in shortwave radiative heating rate (HR) of aerosols were studied using NASA/Goddard radiative transfer model along with observational data. In the haze episode, EC layer is less than 1.5 km and shows strong scattering, with single-scattering albedo (SSA440nm) of ~0.97. The heating effects are observed at the middle and upper atmosphere, and slight heating effects are found at the lower layer. The mean HR within 1.5 km can be up to 16.3 K day−1 with EC of 1.27 km−1, whereas the HR within 0.5 km is only 1.3 K day−1. In the dust episode, dust aerosols present the absorption with SSA440nm of ~0.88, which would heat the lower atmosphere to promote vertical turbulence, and the height of EC layer can be up to 2.0–3.5 km. In addition, the strong heating effects of dust layer produced cooling effects near the surface. Therefore, the accurate measurement of aerosols optical properties in HPEs is of great significance for modeling aerosols direct radiative effects. [ABSTRACT FROM AUTHOR]

The aerosol microphysical, optical and radiative properties of the whole column and upper planetary boundary layer (PBL) were investigated during 2013 to 2018 based on long-term sun-photometer observations at a surface site (~106 m a.s.l.) and a mountainous site (~1225 m a.s.l.) in Beijing. Raman-Mie lidar data combined with radiosonde data were used to explore the aerosol radiative effects to PBL during dust and haze episodes. The results showed size distribution exhibited mostly bimodal pattern for the whole column and the upper PBL throughout the year, except in July for the upper PBL, when a trimodal distribution occurred due to the coagulation and hygroscopic growth of fine particles. The seasonal mean values of aerosol optical depth at 440 nm for the upper PBL were 0.31 ± 0.34, 0.30 ± 0.37, 0.17 ± 0.30 and 0.14 ± 0.09 in spring, summer, autumn and winter, respectively. The single-scattering albedo at 440 nm of the upper PBL varied oppositely to that of the whole column, with the monthly mean value between 0.91 and 0.96, indicating weakly to slightly strong absorptive ability at visible spectrum. The monthly mean direct aerosol radiative forcing at the Earth's surface and the top of the atmosphere varied from −40 ± 7 to −105 ± 25 and from −18 ± 4 to −49 ± 17 W m−2, respectively, and the maximum atmospheric heating was found in summer (~66 ± 12 W m−2). From a radiative point of view, during dust episode, the presence of mineral dust heated the lower atmosphere, thus promoting vertical turbulence, causing more air pollutants being transported to the upper air by the increasing PBLH. In contrast, during haze episode, a large quantity of absorbing aerosols (such as black carbon) had a cooling effect on the surface and a heating effect on the upper atmosphere, which favored the stabilization of PBL and occurrence of inversion layer, contributing to the depression of the PBLH. Unlabelled Image • Aerosol optical properties for upper planetary boundary layer were showed in Beijing. • Aerosol radiative effects to planetary boundary layer were investigated. • Mineral dust heated middle to low atmosphere, thus promoted turbulence. • Absorptive aerosol facilitated the formation of inversion layer in haze episode. [ABSTRACT FROM AUTHOR]

Abstract As one of the largest metropolitan areas in the world, Beijing has a high aerosol loading. Although the government has taken various measures to reduce emissions, the large-scale and intermittent occurrence of pollution is still a major issue. To understand the mechanisms and characteristics of pollution, Raman-Mie lidar and CE-318 sun photometer were used to study the aerosol vertical distribution and optical properties during dust, haze and clean periods in Beijing that occurred in September to October 2017. Combined with meteorological data, the sources and transmission paths of the pollution events were analyzed. The results are as follows. For the dust event, irregular particles with a large linear particle depolarization ratio played a leading role. The aerosol boundary layer height reached 3.5 km. The volume concentration of coarse particles was obviously larger in the volume size distribution. Strong northwest winds at high altitudes cause high-altitude long-distance transmission of pollutants. And this process can exacerbate air quality of Beijing. For the haze event, high-spherical water-soluble particles were dominant. The extinction coefficient exceeded 1 km−1 while the linear particle depolarization ratio was relatively small. The aerosol boundary layer height was <1 km. The range of aerosol optical depth at 440 nm varied between 0.8 and 2.4. Compared with the dust process, meteorological conditions contribute differently to the haze pollution. Aerosols from the south-west is one of the major resources of haze in Beijing. Combined with static weather such as low wind speed, the pollution level can be continuously increased. During the clean period, favorable weather conditions, such as high wind speed, provided good conditions for the diffusion of local particles. The extinction coefficient and the linear particle depolarization ratio were significantly lower. The single-scattering albedo fluctuated between 0.75 and 0.85, the scattering of light was weak and the visibility was generally high. Highlights • Dust pollution in autumn can exacerbate air quality of Beijing. • Aerosols from the south-west is one of the major resources of haze in Beijing. • Meteorological conditions contribute differently to the autumn pollution in Beijing. [ABSTRACT FROM AUTHOR]

The reduction of traffic flow in downtown areas during the Chinese National Day holiday and the fireworks during the Spring Festival provide a unique opportunity for investigating the impact of urban anthropogenic activities on aerosol optical properties during these important Chinese festivals in Beijing. The National Day in 2014 and 2015 and Spring Festival in 2015 and 2016 were selected as study periods. The aerosol optical depth (AOD) at 440 nm increased over the all holiday periods and the average AODs during the 2015 National Day, 2015 Spring Festival and 2016 Spring Festival were about 81%, 21% and 36% higher than the background levels, respectively. The average AOD in 2014 National Day holiday was lower than background level partly influenced by precipitation event. The absorption AOD (AAOD) at 440 nm showed consistent variations with the AOD and the average AAODs during the 2015 National Day, 2015 Spring Festival and 2016 Spring Festival holidays were about 75%, 19% and 23% higher than the background level, respectively. The mean values of single scattering albedo were greater than the background level during the Spring Festival holidays, whereas the values during the National Day holiday in 2015 were lower partly due to the reduction of vehicular emissions in downtown areas. Fine- and coarse-mode particle volumes during pollution periods in holidays were ~ 0.04–0.25 μm 3 and ~ 0.03–0.15 μm 3 larger than background level, respectively. The results of potential source contribution function and concentration-weighted trajectory analyses identified the areas south of Beijing as the main source regions of PM 2.5 and were responsible for the extremely high PM 2.5 concentrations in Beijing during the holiday periods. The findings of this study may aid understanding the effects of human activities on aerosol optical properties over Beijing area and contribute to improving regional air quality. [ABSTRACT FROM AUTHOR]