In the early 21st century, Southwest China (SWC) frequently experienced extreme droughts and severe haze pollution events. Although the meteorological causes of these extreme droughts have been widely investigated, previous studies have yet to understand the causes of haze pollution events over SWC. Moreover, the associations between winter atmospheric teleconnections during drought and haze pollution event across SWC has received negligible attention and therefore warrants investigation. This study examines the associations between the atmospheric teleconnections with respect to winter droughts and winter haze pollution over SWC. Our main conclusions are as follows. (1) Winter precipitation and winter haze days (WHD) over SWC had three major fluctuations from 1959 to 2016. (2) The atmospheric circulation pattern over the Eurasian (EU) continent associated with WHD over SWC resembled that of winter droughts over SWC, where both can be characterized by an EU teleconnection pattern. The Arctic Oscillation (AO) mainly induced the atmospheric circulation pattern over the EU continent that is associated with WHD over SWC. (3) The sea surface temperature (SST) and low circulation anomalies in the Pacific and north Atlantic associated with WHD were similar to those associated with winter droughts over SWC. La Nina events and negative phases of the North Atlantic Oscillation (NAO) may induce winter drought and increase the WHD over SWC. (4) Compared with winter drought over SWC, the variation in the WHD was more complex and the factors affecting WHD were more diverse, and winter drought and its related atmospheric circulations were important factors that induced haze pollution over SWC. Overall, this study not only fills a gap in the literature with respect to the associations between the atmospheric teleconnections of winter drought and winter haze pollution over SWC, but also provides an important scientific basis for the development of potential predictions of local monthly haze pollution, which improves the forecast accuracy of local short-term haze pollution and enriches the theoretical understanding of the meteorological causes of local haze pollution. (C) 2020 Elsevier B.V. All rights reserved.

Severe haze hovered over Harbin during the heating season of 2019-2020, making it one of the ten most polluted Chinese cities in January of 2020. Here we focused on the optical properties and sources of brown carbon (BrC) during the extreme atmospheric pollution periods. Enhanced formation of secondary BrC (BrCsec) was evident as relative humidity (RH) became higher, accompanied with a decrease of ozone but concurrent increases of aerosol water content and secondary inorganic aerosols. These features were generally similar to the characteristics of haze chemistry observed during winter haze events in the North China Plain, and indicated that heterogeneous reactions involving aerosol water might be at play in the formation of BrCsec despite the low temperatures in Harbin. Although BrCsec accounted for a substantial fraction of brown carbon mass, its contribution to BrC absorption was much smaller (6 vs. 28%), pointing to a lower mass absorption efficiency (MAE) of BrCsec compared to primary BrC. In addition, emissions of biomass burning BrC (BrCBB) were inferred to increase with increasing RH, coinciding with a large drop of temperature. Since both the less absorbing BrCsec and the more absorbing BrCBB increased as RH became higher, the MAE of total BrC were largely unchanged throughout the measurement period. This study unfolded the contrast in the source apportionment results of BrC mass and absorption, and could have implications for the simulation of radiative forcing by brown carbon.

Regional atmospheric circulation patterns affect haze pollution and they change in the warming climate. Here, the characteristics of atmospheric circulation anomalies conducive to extreme haze occurrence in China and their historical and future trends are examined based on surface observations, reanalysis data, aerosol source tagging technique, and multimodel intercomparison results. December 2016 and 2017 are identified as the worst months of haze pollution over northern and southern China, featuring weakened and strengthened prevailing winds, respectively. During 1980-2019, the atmospheric pattern similar to December 2016 decreased, while that similar to 2017 increased, suggesting that severe haze formation mechanism in eastern China has been shifting from causes of local accumulation to regional transport processes. In the future, climate change under the sustainable and intermediate development scenarios are the ideal paths to reduce haze in China, while high social vulnerability and radiative forcing would cause a severe damage to the environment.

Since aerosols are an integral part of the Arctic climate system, understanding aerosol radiative properties and the relation of these properties to each other is important for constraining aerosol radiative forcing effects in this remote region where measurements are sparse. In situ measurements of aerosol size distribution, aerosol light scattering and absorption were taken near Eureka (80.05 degrees N, 86.42 degrees W), on Ellesmere Island, in the Canadian High Arctic over three consecutive years to provide insights into radiative properties of Arctic aerosols. During periods of Arctic haze, we find that the single scattering albedo (SSA) at 405 nm is generally higher and more stable than that determined at 870 nm, with values ranging between 0.90-0.99 and 0.79-0.97, respectively. Events with elevated absorption coefficients (B-abs) exhibit generally an absorption Angstrom exponent (AAE) of around 1 suggesting that black carbon (BC) is the dominant absorbing aerosol for the measurement period. AAE values close to 2 occurring with scattering Angstrom exponent (SAE) values near 0 and SAE values below 0 occasionally observed in December indicate a potential contribution from mineral dust aerosols in late fall and early winter. The apparent real and imaginary parts of the complex refractive index at 405 nm have been found to range between 1.6-1.9 and 0.002-0.02, respectively. The low imaginary component indicates very weak intrinsic absorption compared to BC-rich aerosols. Systematic variabilities between different aerosol optical and microphysical properties depend strongly on the given wavelength. SSA at 405 nm shows a strong inverse dependence with B-abs, because B-abs correlates positively with the imaginary component of the refractive index. On the other hand, SSA at 870 nm correlates with scattering coefficient (B-sca) and not with B-abs due to a greater sensitivity to the ambient particle size distribution for 870 nm scattering. Smaller particles with higher SAE that are prevalent during less polluted periods only weakly scatter at 870 nm leading to lower SSA when B-sca is also low. Lastly, FLEXPART back-trajectories show that lower aerosol SSA and higher B-abs correspond to air masses which are more influenced by Eurasian and Alaskan regions, including regions known to have important BC emissions. This work emphasizes the important variability in Arctic aerosol optical properties during winter and spring, which is likely due to changes in source regions.

The changes in aerosol optical and microphysical properties, and sub-band shortwave direct radiative forcing (DARF) in Beijing and Wuhan were compared at different haze levels in winters. The occurrence of haze is found to be governed by the wind circulation and boundary layer in Beijing where the ground wind speed and the height of boundary layer decreased significantly with the development of haze. Compared with the boundary layer and wind, the relative humidity has a stronger impact on haze in Wuhan. Especially, hygroscopic growth of aerosol particles is observed in Wuhan. The increase of fine-mode non-absorbing particles is the main characteristic of aerosol change during haze periods. With the development of haze, the larger increase in DARF at the top of atmosphere is found in Beijing, while the change in DARF at the atmosphere is more obvious in Wuhan. Efficiency of the DARF shows that the change of DARF depends highly on the single scattering albedo in Beijing due to the obvious enhancement of particle scattering, while it depends much more on particle radius over Wuhan. The increase in particle size can also change DARF proportion of each sub-band in shortwave that the DARF proportion in ultraviolet and visible decreased. However, the DARF proportion in near-infrared increased during haze, as the light in shorter wavelength is more sensitive to the change of the fine particle radius.

The Indo-Gangetic Plain (IGP) suffers from recurrent heavy air pollution during the dry season of each year. The AERONET station established in Lumbini, a semi-urban site and historically significant location as the birthplace of Buddha in the northern edge of the central IGP in Nepal, captured two events of heavy air pollution: 29-30 October (Event-I) and 1-2 December (Event-II) of 2014. Daily aerosol optical depth (AOD) was found to be 2.66, 2.44, 2.09 and 2.15 respectively with angstrom ngstrom exponent (AE) > 1 during both events. The daily AOD during two events was 3.26-4.43 times higher than average AOD observed during the monitoring period 2013-2014. Based on various aerosol parameters we concluded that the fine-mode aerosols sourced from biomass-burning and urban/industrial activities were responsible for the haze events. Aerosol absorbed 205-240 Wm(-2) of the solar radiation in the atmosphere, resulting in aerosol-induced atmospheric heating rate of 5.76-7.28 K day(-1) during the hazy days. Back-trajectory analysis of air masses reaching Lumbini showed that the air masses either originated or travelled over the regions with open fires both prior and during the events. In addition to this, meteorological conditions (e.g. prevailing winds) also played role in initiation/persistence of the haze events.

Although regional haze adversely affects human health and possibly counteracts global warming from increasing levels of greenhouse gases, the formation and radiative forcing of regional haze on climate remain uncertain. By combining field measurements, laboratory experiments, and model simulations, we show a remarkable role of black carbon (BC) particles in driving the formation and trend of regional haze. Our analysis of long-term measurements in China indicates declined frequency of heavy haze events along with significantly reduced SO2, but negligibly alleviated haze severity. Also, no improving trend exists for moderate haze events. Our complementary laboratory experiments demonstrate that SO2 oxidation is efficiently catalyzed on BC particles in the presence of NO2 and NH3, even at low SO2 and intermediate relative humidity levels. Inclusion of the BC reaction accounts for about 90-100% and 30-50% of the sulfate production during moderate and heavy haze events, respectively. Calculations using a radiative transfer model and accounting for the sulfate formation on BC yield an invariant radiative forcing of nearly zero W m(-2) on the top of the atmosphere throughout haze development, indicating small net climatic cooling/warming but large surface cooling, atmospheric heating, and air stagnation. This BC catalytic chemistry facilitates haze development and explains the observed trends of regional haze in China. Our results imply that reduction of SO2 alone is insufficient in mitigating haze occurrence and highlight the necessity of accurate representation of the BC chemical and radiative properties in predicting the formation and assessing the impacts of regional haze.

According to satellite monitoring data (MODIS/Terra), the spatial distribution of the aerosol optical depth (AOD) at a wavelength of 550 nm for the summer smog of 2007 over the North China Plain (NCP) and adjacent areas has been obtained. Areas over which the AOD is higher due to regional anthropogenic contamination sources near Beijing and Shanghai, as well as the smoke haze forming due to agricultural burning (the southwest part of the NCP), have been revealed. The similarity of optical and microphysical characteristics of aerosol in the smoke haze over the NCP and in the Russian territory has been found: (i) the decisive contribution to the optical characteristics of smoke aerosol is made by the fine mode and (ii) the attenuation spectra in the wavelength region 340-1020 nm are approximated (in logarithmic coordinates) by parabolas or fourth degree polynomials. The monitoring data at the AERONET Beijing site show that the single scattering albedo in the summer smog over the NCP is on average less (0.91) than in the smoke haze in the Russian territory (0.95-0.96). The radiative regimes of the atmosphere are significantly different: in the smog, the aerosol radiative forcing efficiency is lower approximately by 30% at the top of the atmosphere and higher by 30% at the bottom of the atmosphere than in the smoke haze.

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 (similar to 106 m a.s.l.) and a mountainous site (similar to 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 (similar to 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. (C) 2019 Elsevier B.V. All rights reserved.

Pollutants, which are usually transported from urban cities to remote glacier basins, and aerosol impurities affect the earth's temperature and climate by altering the radiative properties of the atmosphere. This work focused on the physicochemical properties of atmospheric pollutants across the urban and remote background sites in northwest China. Information on individual particles was obtained using transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDX). Particle size and age-dependent mixing structures of individual particles in clean and polluted air were investigated. Aerosols were classified into eight components: mineral dust, black carbon (soot)/fly ash, sulfates, nitrates, NaCI salt, ammonium, organic matter, and metals. Marked spatial and seasonal changes in individual particle components were observed in the study area. Aerosol particles were generally found to be in the mixing state. For example, salt-coated particles in summer accounted for 31.2-44.8% of the total particles in urban sites and 37.5-74.5% of the total particles in background sites, while in winter, almost all urban sites comprised >50%, which implies a significant effect on the radiative forcing in the study area. We found that in PM2.5 section, the internally mixed black carbon/organic matter particles clearly increased with diameter. Moreover, urban cities were characterized by atmospheric particles sourced from anthropogenic activities, whereas background locations exhibited much lower aerosol concentrations and increased particle density, originating from natural crustal sources (e.g., mineral dust and NaCI salt), which, together with air mass trajectory analysis, indicates a potential spatial transport process and routes of atmospheric transport from urban cities to background locations. Thus, this work is of importance in evaluating atmospheric conditions in northwest China and northeast Tibetan Plateau regions, to discover the transport processes and facilitate improvements in climatic patterns concerning atmospheric impurities. (C) 2018 Elsevier Ltd. All rights reserved.