The Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) is widely used as an advanced model dataset for the understanding of global climate change. However, independent validation and comparison of MERRA-2 are both insufficient and always desired. Therefore, in this study, the quantitative evaluation of MERRA-2 aerosol products was conducted over China for Aerosol Optical Depth (AOD), Angstrom exponent (AE), absorbing AOD (AAOD) and chemical components (black carbon (BC) and dust (DU)) using Sun sky radiometer Observation NETwork (SONET) ground-based measurements and POLDER-3/PARASOL satellite products generated by the GRASP algorithm. The available MERRA-2 monthly dataset and PARASOL/GRASP monthly and seasonal products were intercompared over China. MERRA-2 AOD (550 nm) show general good agreement with SONET and PARASOL/GRASP. For example, the correlation coefficients are usually 0.6–0.85 with SONET and 0.75–0.85 with PARASOL/GRASP, the bias is usually −0.293 to +0.008 with SONET. For AE and AAOD, the agreement is still reasonable. MERRA-2 is found to overestimate fine mode AE and to display a general underestimation of aerosol absorption over China. In addition, MERRA-2 BC and DU mass concentrations show spatial and quantitative consistency with PARASOL/GRASP components climatological products. The relatively high columnar BC mass concentration is observed around 1.5–2 mg/m³ over the East China industrial region and high DU mass concentration is around 150 mg/m³ near Taklimakan desert. MERRA-2 shows slightly higher BC and lower DU concentration than PARASOL/GRASP over East China. The evaluations with in situ BC measurements near surface verify the overestimation (MAE = +0.44 µg/m²) of MERRA-2 and underestimation (MAE = −0.38 µg/m²) of PARASOL/GRASP. The analysis demonstrates multi-source datasets, such as ground-based, space-borne remote sensing, in situ measurements, model simulation as well as reanalysis data, complement each other and can be used to refine aerosol characterization.

Time series of Collection 6.1 of the MODerate resolution Imaging Spectroradiometer Multi-Angle Implementation of Atmospheric Correction (MODIS/MAIAC C6.1) aerosol optical depth (AOD) and model-simulated AOD were used to determine the contributions of meteorological and anthropogenic effects to spatiotemporal AOD variations over five representative areas in China during the period from January 2010 to September 2024. The time series confirm the effective reduction in AOD between 2010 and 2018, with an additional but smaller reduction thereafter. The overall AOD reduction is mainly attributable to emission reduction policies, although with substantial meteorological effects. The respective total reduction and meteorological contributions during the whole study period and the meteorological contributions before and after 2018 over the five regions were as follows (all in %): 68, 12, 16, and 52 for the North China Plain (NCP); 62, 17, 28, and 14 for the Yangtze River Delta (YRD); 70, 33, 31, and 43 for the Pearl River Delta (PRD); 55, 16, 25, and 21 for Hunan and Hubei (HNB); and 57, 10, 14, and 38 for the Sichuan Basin (SCB). Meteorological effects for each of these periods and each region are discussed in detail. As an example, the above data show that the meteorological effects over the YRD and HNB after 2018 are smaller than before 2018; this can be explained by the occurrence of strong effects in the earlier period and the choice of the period over which effects were calculated. Monthly mean AOD patterns were distinctly different before and after 2016, suggesting that aerosol properties changed in response to the emission reduction policies. In summary, this study highlights the complex interplay between meteorological and anthropogenic factors in shaping AOD variations across China and demonstrates the increasing significance of meteorological conditions in modulating China's AOD.

With the acceleration of global climate change and urbanization, air pollution, particularly ozone pollution, has become a critical environmental issue, especially in the Beijing–Tianjin–Hebei region of China. This study investigates the spatiotemporal distribution of ozone pollution and its precursors, focusing on formaldehyde as a key indicator of volatile organic compounds. Utilizing high-resolution remote sensing data from the China High-Resolution Air Pollutants dataset and TROPOMI HCHO observations from 2013 to 2022, we employed advanced techniques such as the Kolmogorov–Zurbenko filter and high-value area identification to analyze ozone pollution trends, meteorological influences, and the spatial distribution of HCHO concentrations. Our findings reveal a significant increase in ozone concentrations across BTH, with an annual growth rate of 2.51 μg/m³, peaking during the summer months. The KZ filter decomposition highlighted that short-term and seasonal variations dominate ozone fluctuations, driven by meteorological factors such as solar radiation and temperature. Furthermore, the identification of HCHO HVAs demonstrated that urban agglomeration and expansion zones exhibit higher HCHO concentrations, with VOCs-limited zones showing the most pronounced HCHO levels. The study also introduced the PHV (Percentage Higher than Vicinity) index to quantify anomalous HCHO emissions, providing a robust tool for pinpointing pollution hotspots. Based on these insights, we propose targeted emission control strategies for key regions, including urban expansion zones in Zhangjiakou and non-urban zones in Qinhuangdao, to mitigate ozone pollution effectively. This research offers valuable scientific support for regional air quality management and the formulation of precise pollution control measures in the Beijing–Tianjin–Hebei region.

Time series of MODIS/MAIAC C6.1 aerosol optical depth (AOD) and model-simulated AOD over China were used to determine contributions of meteorological and anthropogenic effects on aerosol variations on monthly and interannual scales. The study covered the period January 2010–September 2024 with the main focus on five representative areas: NCP, YRD, PRD, HNB and SCB. The time series confirm that emission reduction policy has resulted in the effective reduction of the AOD over China. The large increase of the AOD over the YRD during 2018–2021 shows that meteorological effects have increasingly larger influences on the aerosol load as AOD decreases. During this period, the potential decrease of the AOD over the NCP was effectively cancelled by unfavorable meteorological effects. Meteorological effects, and their variations, are different over each region. For instance, over the NCP, meteorological effects were mostly unfavorable while in contrast, over the PRD, meteorological effects were initially unfavorable but had a strong effect after 2016 when they reinforced anthropogenic effects, resulting in a substantial reduction of the AOD. In addition, the data show a strong AOD minimum in 2022, attributed to favorable anthropogenic effects, over some areas reinforced by favorable meteorological effects. Monthly mean AOD patterns were distinctly different before and after 2016, suggesting that aerosol properties changed in response to emission reduction policy. In summary, this study highlights the complex interplay between meteorological and anthropogenic factors in shaping AOD variations across China and demonstrates the increasing significance of meteorological conditions in modulating China’s AOD.

Satellite observations showed the increase of the aerosol optical depth (AOD, a measure for the aerosol burden), over China at the end of the previous century, continuing until about 2007 and decreasing after 2011/2014. The initial increase, in response to economic growth and urbanization, was mitigated by the successive implementation of a series of emission reduction policy measures that resulted in strong AOD variations between 2007 and 2014, followed by a substantial AOD decrease after the implementation of the 2013 - 2017 Clean Air Action Plan. However, AOD time series show that the reductions were cancelled or even reversed over extended periods of time when the AOD increased. Model simulations show that these variations can be attributed to influences of unfavorable meteorological effects on the AOD which become stronger as AOD decreases. Further analysis shows the different effects of the occurrences of El Niño and La Niña on the AOD, in addition to economic effects. Furthermore, the emission reductions result not only in the decrease of aerosols but also affect the concentrations of precursor gases, both direct and through the chemical balance which effects the oxidative capacity of the atmosphere. As a result, aerosol composition changes occur which in turn affect aerosol optical properties. Changes in both concentrations and optical properties provide a plausible explanation for satellite observations of changes in AOD patterns. The reduction of aerosol concentrations reduces both the direct effect of aerosols and indirect effects on the Earth radiative balance.

Time series of MODIS/MAIAC C6.1 aerosol optical depth (AOD) and model-simulated AOD over China were used to determine contributions of meteorological and anthropogenic effects on aerosol variations on monthly and interannual scales. The study covered the period January 2010–September 2024 with the main focus on five representative areas: NCP, YRD, PRD, HNB and SCB. The time series confirm that emission reduction policy has resulted in the effective reduction of the AOD over China. The large increase of the AOD over the YRD during 2018–2021 shows that meteorological effects have increasingly larger influences on the aerosol load as AOD decreases. During this period, the potential decrease of the AOD over the NCP was effectively cancelled by unfavorable meteorological effects. Meteorological effects, and their variations, are different over each region. For instance, over the NCP, meteorological effects were mostly unfavorable while in contrast, over the PRD, meteorological effects were initially unfavorable but had a strong effect after 2016 when they reinforced anthropogenic effects, resulting in a substantial reduction of the AOD. In addition, the data show a strong AOD minimum in 2022, attributed to favorable anthropogenic effects, over some areas reinforced by favorable meteorological effects. Monthly mean AOD patterns were distinctly different before and after 2016, suggesting that aerosol properties changed in response to emission reduction policy. In summary, this study highlights the complex interplay between meteorological and anthropogenic factors in shaping AOD variations across China and demonstrates the increasing significance of meteorological conditions in modulating China’s AOD.