With Tibetan Plateau higher than 4 km to the west, the location of Sichuan Basin is unique all around the world and provides a good platform to study air pollution in the urban agglomerations over the complex terrain. To fill in the blanks on vertical distributions of PM1 (the particles smaller than 1 mu m) and carbonaceous aerosols within the basin, by means of high topographic relief, PM1 were off-line sampled during 20 January to 2 February 2018 at eight sites with increasing altitudes from the basin to southeastern margins of the Tibetan Plateau. The regional potential sources for each site were revealed by Hybrid-Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model and concentration-weighted trajectory (CWT) method. The lowest carbonaceous aerosol levels occurred at Lixian, while the highest OC (organic carbon) (EC, elemental carbon) was at Hongyuan (the altitude of 3500 m) (Ande, a rural site) due to more primary emissions. The pollutants inside the basin can be transported the altitudes from 2 km to 3 km by vertical dispersion, but they cannot be dispersed to higher altitudes. The vertical stratification of the pollutants was obvious and easily formed high-low-high pattern from Sichuan Basin to southeastern Tibetan Plateau, especially during highly polluted episodes. The regional potential sources significantly varied as the increased altitudes. Regional pollution was significant inside the basin. The sources at the altitudes from 2 km to 3 km originated from southeastern margins of the Plateau and surrounding cities, while those at higher altitudes were transported from southeastern margins of the Plateau. The impact of basic meteorological variables (temperature, wind speed and vapor pressure) on carbonaceous aerosols was opposite between the basin and Plateau sites. This study was essential to understanding formation mechanisms of severe pollution episodes and thus to making control measures for the urban agglomerations inside the mountainous terrain.

Owing to a lack of vertical observations, the impacts of black carbon (BC) on radiative forcing (RF) have typically been analyzed using ground observations and assumed profiles. In this study, a UAV platform was used to measure high-resolution in-situ vertical profiles of BC, fine partides (PM2.5), and relevant meteorological parameters in the boundary layer (BL). Further, a series of calculations using actual vertical profiles of BC were conducted to determine its impact on RF and heating rate (HR). The results show that the vertical distributions of BC were strongly affected by atmospheric thermodynamics and transport. Moreover. Three main types of profiles were revealed: Type I, Type II, Type III, which correspond to homogenous profiles (HO), negative gradient profiles (NG), and positive gradient profiles (PG), respectively. Types I and II were related to the diurnal evolution of the BL, and Type III was caused by surrounding emissions from high stacks and regional transport. There were no obvious differences in RF calculated for HO profiles and corresponding surface BC concentrations, unlike for NG and PG profiles. RF values calculated using surface BC concentrations led to an overestimate of 13.2 W m(-2) (27.5%, surface) and 18.2 W m(-2) (33.4%, atmosphere) compared to those calculated using actual NG profiles, and an underestimate of approximately 15.4 W m(-2) (35.0%, surface) and 16.1 W m(-2) (29.9%, atmosphere) compared to those calculated using actual PG profiles. In addition, the vertical distributions of BC HR exhibited dear sensitivity to BC profile types. Daytime PG profiles resulted in a positive vertical gradient of HR, which may strengthen temperature inversion at high altitudes. These findings indicate that calculations that use BC surface concentrations and ignore the vertical distribution of BC will lead to substantial uncertainties in the effects of BC on RE and HR. (C) 2020 Elsevier B.V. All rights reserved.

Spectral aerosol optical depth (AOD) measurements obtained from multi-wavelength radiometer under cloudless conditions over Doon Valley, in the foothills of the western Himalayas, are analysed during the period January 2007 to December 2012. High AOD values of 0.46 +/- A 0.08 and 0.52 +/- A 0.1 at 500 nm, along with low values of ngstrom exponent (0.49 +/- A 0.01 and 0.44 +/- A 0.03) during spring (March-May) and summer (June-August), respectively, suggest a flat AOD spectrum indicative of coarse-mode aerosol abundance compared with winter (December-February) and autumn (September-November), which are mostly dominated by fine aerosols from urban/industrial emissions and biomass burning. The columnar size distributions (CSD) retrieved from the King's inversion of spectral AOD exhibit bimodal size patterns during spring and autumn, while combinations of the power-law and unimodal distributions better simulate the retrieved CSDs during winter and summer. High values of extinction coefficient near the surface (similar to 0.8-1.0 km(-1) at 532 nm) and a steep decreasing gradient above are observed via CALIPSO profiles in autumn and winter, while spring and summer exhibit elevated aerosol layers between similar to 1.5 and 3.5 km due to the presence of dust. The particle depolarisation ratio shows a slight increasing trend with altitude, with higher values in spring and summer indicative of non-spherical particles of dust origin. The aerosol-climate implications are evaluated via the aerosol radiative forcing (ARF), which is estimated via the synergy of OPAC and SBDART models. On the monthly basis, the ARF values range from similar to -30 to -90 W m(-2) at the surface, while aerosols cause an overall cooling effect at the top of atmosphere (approx. -5 to -15 W m(-2)). The atmospheric heating via aerosol absorption results in heating rates of 1.2-1.6 K day(-1) during March-June, which may contribute to changes in monsoon circulation over northern India and the Himalayas.

[1] Chemical, physical, and optical measurements of aerosol particle properties within an aged biomass-burning plume were performed on board a research aircraft during a profile descent over a ground-based site in northeastern Greece (40degrees24'N, 23degrees57'E; 170 m asl) where continuous measurements of the spectral downwelling solar irradiance (global, direct, and diffuse) are being made. The aerosol optical depth measured at the ground during the time of overflight was significantly enhanced (0.39 at a wavelength of 500 nm) due to a haze layer between 1 and 3.5 km altitude. The dry particle scattering coefficient within the layer was around 80 Mm(-1), and the particle absorption coefficient was around 15 Mm(-1), giving a single scattering albedo of 0.89 at 500 nm (dry state). The black carbon fraction is estimated to account for 6-9% of the total accumulation mode particle mass (<1 mu m diameter). The increase of the particle scattering coefficient with increasing relative humidity at 500 nm is of the order of 40% for a change in relative humidity from 30 to 80%. The dry, altitude-dependent, particle number size distribution is used as input parameter for radiative transfer calculations of the spectral short-wave, downwelling irradiance at the surface. The agreement between the calculated irradiances and the experimental results from the ground-based radiometer is within 10%, both for the direct and the diffuse components (at 415, 501, and 615 nm). Calculations of the net radiative forcing at the surface and at the top of the atmosphere (TOA) show that due to particle absorption the effect of aerosols is much stronger at the surface than at the TOA. Over sea the net short-wave radiative forcing (daytime average) between 280 nm and 4 mu m is up to -64 W m(-2) at the surface and up to -22 W m(-2) at the TOA.