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Beijing suffers from poor air quality, particularly during wintertime haze episodes when concentrations of PM 2.5 (particulate matter with a diameter < 2.5 µ m) can peak at > 400 µ g m -3. Black carbon (BC), an aerosol which strongly absorbs solar radiation, can make up to 10 % of PM 2.5 in Beijing. BC is of interest due to its climatic and health impacts. BC has also been found to impact planetary boundary layer (PBL) meteorology. Through interacting with radiation and altering the thermal profile of the lower atmosphere, BC can either suppress or enhance PBL development depending on the properties and altitude of the BC layer. Previous research assessing the impact of BC on PBL meteorology has been investigated through the use of regional models, which are limited both by resolution and the chosen boundary layer schemes. In this work, we apply a high-resolution model (UCLALES-SALSA) that couples an aerosol and radiative transfer model with large-eddy simulation (LES) to quantify the impact of BC at different altitudes on PBL dynamics using conditions from a specific haze episode which occurred from 1–4 December 2016 in Beijing. Results presented in this paper quantify the heating rate of BC at various altitudes to be between 0.01 and 0.016 K/h per µ g/m 3 of BC, increasing with altitude but decreasing around PBL top. Through utilising a high-resolution model which explicitly calculates turbulent dynamics, this paper showcases the impact of BC on PBL dynamics both within and above the PBL. These results show that BC within the PBL increases maximum PBL height by 0.4 % but that the same loading of BC above the PBL can suppress PBL height by 6.5 %. Furthermore, when BC is present throughout the column, the impact of BC suppressing PBL development is further maximised, with BC causing a 17 % decrease in maximum PBL height compared to only scattering aerosols. Assessing the impact of these opposite effects, in this paper, we present a mechanism through which BC may play a prominent role in the intensity and longevity of Beijing's pollution episodes. [ABSTRACT FROM AUTHOR]

As has been the case in North America and western Europe, the SO2 emissions have substantially reduced in the North China Plain (NCP) in recent years. Differential rates of reduction in SO2 and NOx concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate (pNO3-) over the NCP. In this study, we observed a polluted episode with the particulate nitrate mass fraction in nonrefractory PM 1 (NR-PM 1) being up to 44 % during wintertime in Beijing. Based on this typical pNO3- -dominated haze event, the linkage between aerosol water uptake and pNO3- enhancement, further impacting on visibility degradation, has been investigated based on field observations and theoretical calculations. During haze development, as ambient relative humidity (RH) increased from ∼10 % to 70 %, the aerosol particle liquid water increased from ∼1 µgm-3 at the beginning to ∼75 µgm-3 in the fully developed haze period. The aerosol liquid water further increased the aerosol surface area and volume, enhancing the condensational loss of N2O5 over particles. From the beginning to the fully developed haze, the condensational loss of N2O5 increased by a factor of 20 when only considering aerosol surface area and volume of dry particles, while increasing by a factor of 25 when considering extra surface area and volume due to water uptake. Furthermore, aerosol liquid water favored the thermodynamic equilibrium of HNO3 in the particle phase under the supersaturated HNO3 and NH3 in the atmosphere. All the above results demonstrated that pNO3- is enhanced by aerosol water uptake with elevated ambient RH during haze development, in turn facilitating the aerosol take-up of water due to the hygroscopicity of particulate nitrate salt. Such mutual promotion between aerosol particle liquid water and particulate nitrate enhancement can rapidly degrade air quality and halve visibility within 1 d. Reduction of nitrogen-containing gaseous precursors, e.g., by control of traffic emissions, is essential in mitigating severe haze events in the NCP. [ABSTRACT FROM AUTHOR]

Atmospheric aerosol concentrations have increased globally mainly due to anthropogenic activities such as industry, heating, transport and cooking. Increased aerosol concentrations can result in pollution events, particularly in urban mega-cities. The severity of pollution events is often defined by concentrations of PM2.5 (particulate matter with a diameter of 2.5 μm or less), which has an impact on both the climate and human health. Beijing, is a megacity which is well-known for poor air quality, often subjected to heavy pollution events termed 'haze', of which PM2.5 is a major contributor. The extent to which aerosols absorb or scatter radiation is dependent on their size and composition; the impact of this interaction can lead to atmospheric warming or cooling. Meteorological measurements in Beijing show that polluted periods are accompanied by a decrease in temperature in the lower atmosphere and an increase in the upper atmosphere, reducing turbulent mixing due to buoyancy and lowering the planetary boundary layer height. Water vapour and aerosols become more concentrated in a smaller volume and so the aerosols swell and interact with radiation to a greater extent. These interactions between aerosols, radiation and meteorology thus form a positive feedback loop, exacerbating pollution episodes. Observations in several cities have established the existence of this feedback loop and several regional modelling studies have examined the importance of aerosol-radiation interactions in enhancing polluted episodes. Through the use of a Large Eddy Scale model coupled to a Sectional Aerosol Module (UCLALES-SALSA), this work allows for much higher sensitivity studies on the impact of aerosol size and composition upon meteorology. Through the use of both meteorological and aerosol measurements taken in November-December 2016, I will present case studies quantifying the impact of changing aerosol characteristics on meteorology for a polluted multi-day period. This work will also identify whether aerosol size or composition is of more importance in prolonging and enhancing polluted episodes. [ABSTRACT FROM AUTHOR]