The feedback effects of aerosols from different sources on the urban boundary layer in Beijing China

Vertically resolved meteorological adjustments of aerosols and trace gases in Beijing, Taiyuan, and Hefei by using RF model

Air pollution represents a complex phenomenon defined by the presence of various gases and particulate matter, leading to intricate spatio-temporal fluctuations. This study aims to enhance our understanding of how meteorological factors influence trace gases and aerosols, exacerbating air pollution in various geographical locations, specifically in Beijing's Fengtai (BJFT), Taiyuan City (SXTY), and Hefei's Science Island (HFDP). The study employs 2D-MAX-DOAS observations and utilizes the Random Forest (RF) model to decouple the influence of meteorological conditions from pollutant data. The vertical profile of nitrogen dioxide (NO2), sulfur dioxide (SO2), formaldehyde (HCHO), and aerosols at each study site was classified into four distinct layers, followed by conducting a meteorological decoupling analysis on each layer. This decoupling analysis demonstrates that meteorology significantly influences aerosols across all sites, with reductions ranging from 75 % to 95 % after de-weathering. SO2 shows minimal susceptibility, with the changes ranging from ±20 % to ±60 % after de-weathering. Among all sites, BJFT's pollutants exhibit less susceptibility overall, while pollutants at HFDP are more susceptible. The findings further reveal significant meteorological interventions in pollutants in surface layers (0.05 km and 0.2-0.4 km) at BJFT, with some exceptions at SXTY. However, pollutants, particularly NO2 and aerosols in higher layers (0.6-0.8 km and 1.0-1.2 km) at HFDP, also experience significant meteorological interferences. The findings at HFDP and SXTY reveal that removing meteorological influence also adjusts the profile shape of pollutants. For instance, the NO2 profile at HFDP during the winter season shifted from a bimodal to an exponential shape after de-weathering. Overall, this study sheds light on the complex interplay between meteorological factors and trace gases at various altitudes across different geographic locations, offering insights crucial for holistic and effective pollution mitigation strategies.

Characteristics of sub-micron aerosols above the urban canopy in Beijing during warm seasons

To understand the characteristics of atmospheric pollution above the urban canopy in warm seasons, the characteristics of sub-micron aerosol (PM1) was studied based on high-altitude observations at the Beijing 325 m meteorological tower. The PM1 at 260 m was 34, 29 and 21 μg m-3 in May 2015, June 2015, and June 2017, respectively, indicating a reduction in PM1 pollution above the urban canopy. Meanwhile, an overall decrease was also observed in the concentrations of all PM1 chemical species (excluding Chl and BC) and organic aerosol (OA) factors. Previous instances of heavy haze in Beijing often coincided with high humidity and stagnant weather conditions. However, the heightened pollution episodes in June 2017 were accompanied by high wind speeds and low relative humidity. Compared to May 2015, the contribution of secondary components to PM1 in June 2017 was more prominent, with the total proportion of SNA (sulfate, nitrate, and ammonium) and more-oxidized oxygenated OA (MO-OOA) to PM1 increased by approximately 10 %. Secondary species of NH4NO3, (NH4)2SO4, and MO-OOA, as well as black carbon, collectively contributed the vast majority of aerosol extinction coefficient (bext), with the four species contributing a total of ≥96 % to bext at 260 m. Hydrocarbon-like OA, cooking OA, and less-oxidized oxygenated OA have undergone significant reductions, so continued emphasis on controlling local sources to reduce these three aerosol species and addressing regional sources to further mitigate overall aerosol species is imperative. In lower pollution situation, the diurnal variation of PM was smoother, and its pollution sources were more regionally uniform, which might be attributed to the reduced diversity and complexity in the physical and chemical processes in air pollution.

Impact of aerosol-boundary layer interactions on PM2.5 pollution during cold air pool events in a semi-arid urban basin

Global emission reductions still must address winter fine particulate matter (PM2.5) pollution in urban basins with enclosed terrains and frequent cold air pool (CAP) events when the temperatures within the basin are colder than above it. The effects of urban basin aerosol-boundary layer interactions on PM2.5 pollution during CAP events remain unclear. Intensive boundary layer observations in January 2021 and numerical models were used to investigate this issue in the semi-arid urban Lanzhou Basin of China. The results showed that CAPs formed because of the synoptic weather system that exacerbated the warming over the basin. The CAPs in this experiment were characterized by stronger temperature inversion (TI) layers in the vertical direction and lower relative humidity, lower wind speed, and weaker turbulence at the bottom of the basin compared to other conditions. The strong TI layers below the top of the basin inhibited the vertical dispersion of pollutants in the basin and concentrated the PM2.5 within a height of 0.3 km from the bottom of the basin. During CAP events, the proportion of elemental carbon in PM2.5 increased, whereas that of secondary inorganic species decreased. Aerosol absorption increased faster than scattering during CAP events. Therefore, the mean single scattering albedo decreased from 0.85 during non-CAP periods to 0.81 during CAP events. Radiosonde-sounding observations and numerical simulations indicated that aerosols accumulating in the lower basin heated the atmosphere during the daytime and facilitated boundary layer development via the "stove effect" (absorption aerosol heats lower atmosphere to promote boundary layer development). No significant "dome effect" (absorption aerosol heats the upper boundary layer to suppress boundary layer development) occurred during the two CAP events. These findings provide a theoretical basis for scientifically-guided PM2.5 pollution control in winter in isolated urban basins.

The interactions of aerosol and planetary boundary layer over a large city in the Mongolian Plateau

The interactions of aerosol and planetary boundary layer (PBL) play a crucial role in deteriorating the air quality in vast urban agglomeration areas in eastern China. However, there remains a lacking of report regarding their performance in the hazy events in Mongolian Plateau cities in northern China. In this study, half-month (01-16 January 2020) physical and material datasets of the PBL measured by multi instrumentations mounted in downtown Hohhot, a largest Mongolian Plateau city, are statistically analyzed. Results demonstrate that the aerosol-PBL feedback is of particular importance in promoting the hazy outbreak and the statistical relationship follows PBLH = -76.14 × ln(PM2.5) + 820.61. The non-linear fitting implies that there exists a potential threshold of 76.14 μg m-3 for PM2.5, below which the PBLH decrease rapidly along with the increasing of air pollutants, defined as strong aerosol-PBL interaction phase, while beyond which there is minimal decrease for PBLH even when PM reaches to a high value, i.e., the hazy accumulation phase. Using a large-eddy simulation model named as Dutch Atmospheric Large-Eddy Simulation (DALES) initialized with the synergetical observations in a representative hazy process in 11 January 2020, we found that the DALES is effective to capture the diurnal PBLH in this region. The existence of atmospheric aerosols is essential for lowering PBLH by 51.4 % from 1090 m of clean scenario to 530 m of polluted condition, postponing the development time, and advancing the afternoon lapse time. The enhancement of aerosol absorption ability strengthens the aerosol heating rate, thereby weakening the sensible heat flux, and inhibiting the development of PBLH. While opposite elevation on PBLH is found for the scattering aerosols. These findings highlight the importance of aerosol-PBL interaction in motivating the hazy episodes in Mongolian Plateau cities, which provide scientific references for the local policy-making towards pollution reductions in future.

Retrieval uncertainty and consistency of Suomi-NPP VIIRS deep blue and dark target aerosol products under diverse aerosol loading scenarios over South Asia

Retrieval accuracy and stability of two operational aerosol retrieval algorithms, Deep Blue (DB) and Dark Target (DT), applied on Visible Infrared Imaging Radiometer Suite (VIIRS) on-board Suomi National Polar-orbiting Partnership (S-NPP) satellite were evaluated over South Asia. The region is reported to be highly challenging to accurate estimation of satellite-based aerosol optical properties due to variations in surface reflectance, complex aerosol system and regional meteorology. Performance of both algorithms were initially evaluated by comparing their ability to retrieve aerosol signal over the complex geographical region under specific air pollution emission scenario. Thereafter, retrieval accuracy was investigated against 10 AERONET sites across South Asia, selected based on their geography and predominance aerosol types, from year 2012-2021. Geo-spatial analysis indicates DB to efficiently retrieve fine aerosol features over bright arid surfaces, and for smoke/dust dominating events whereas DT was better to identify small fire events under dark vegetated surface. Both algorithms however, indicate unsatisfactory retrieval accuracy against AERONET having 56-59% of valid retrievals with high RMSE (0.30-0.33) and bias. Overall, DB slightly underpredicted AOD with -0.02 mean bias (MB) whereas DT overpredicted AOD (MB: 0.13), with seasonality in their retrieval efficiency against AERONET. Time-series analysis indicates stability in retrieving AOD and match-up number for both algorithms. Retrieval bias of DB and DT AOD against AERONET AOD under diverse aerosol loading, aerosol size, scattering/absorbing aerosol, and surface vegetation coverage scenarios revealed DT to be more influenced by these conditions. Error analysis indicates at low AOD (≤0.2), accuracy of both DB and DT were subject to underlying vegetation coverage. At AOD>0.2, DB performed well in retrieving coarse aerosols whereas DT was superior when fine aerosols dominated. Overall, accuracy of both VIIRS algorithms require further refinement to continue MODIS AOD legacy over South Asia.