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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.

A new long-term emission inventory called the Inversed Emission Inventory for Chinese Air Quality (CAQIEI) was developed in this study by assimilating surface observations from the China National Environmental Monitoring Centre (CNEMC) using an ensemble Kalman filter (EnKF) and the Nested Air Quality Prediction Modeling System. This inventory contains the constrained monthly emissions of NO x , SO 2 , CO, primary PM 2.5 , primary PM 10 , and non-methane volatile organic compounds (NMVOCs) in China from 2013 to 2020, with a horizontal resolution of 15 km × 15 km. This paper documents detailed descriptions of the assimilation system and the evaluation results for the emission inventory. The results suggest that CAQIEI can effectively reduce the biases in the a priori emission inventory, with the normalized mean biases ranging from -9.1 % to 9.5 % in the a posteriori simulation, which are significantly reduced from the biases in the a priori simulations (-45.6 % to 93.8 %). The calculated root-mean-square errors (RMSEs) (0.3 mg m -3 for CO and 9.4–21.1 µgm3 for other species, on the monthly scale) and correlation coefficients (0.76–0.94) were also improved from the a priori simulations, demonstrating good performance of the data assimilation system. Based on CAQIEI, we estimated China's total emissions (including both natural and anthropogenic emissions) of the six species in 2015 to be as follows: 25.2 Tg of NO x , 17.8 Tg of SO 2 , 465.4 Tg of CO, 15.0 Tg of PM 2.5 , 40.1 Tg of PM 10 , and 46.0 Tg of NMVOCs. From 2015 to 2020, the total emissions decreased by 54.1 % for SO 2 , 44.4 % for PM 2.5 , 33.6 % for PM 10 , 35.7 % for CO, and 15.1 % for NO x but increased by 21.0 % for NMVOCs. It is also estimated that the emission reductions were larger during 2018–2020 (from -26.6 % to -4.5 %) than during 2015–2017 (from -23.8 % to 27.6 %) for most of the species. In particular, the total Chinese NO x and NMVOC emissions were shown to increase during 2015–2017, especially over the Fenwei Plain area (FW), where the emissions of particulate matter (PM) also increased. The situation changed during 2018–2020, when the upward trends were contained and reversed to downward trends for the total emissions of both NO x and NMVOCs and the PM emissions over FW. This suggests that the emission control policies may be improved in the 2018–2020 action plan. We also compared CAQIEI with other air pollutant emission inventories in China, which verified our inversion results in terms of the total emissions of NO x , SO 2 , and NMVOCs and more importantly identified the potential uncertainties in current emission inventories. Firstly, CAQIEI suggested higher CO emissions in China, with CO emissions estimated by CAQIEI (426.8 Tg) being more than twice the amounts in previous inventories (120.7–237.7 Tg). Significantly higher emissions were also suggested over western and northeastern China for the other air pollutants. Secondly, CAQIEI suggested higher NMVOC emissions than previous emission inventories by about 30.4 %–81.4 % over the North China Plain (NCP) but suggested lower NMVOC emissions by about 27.6 %–0.0 % over southeastern China (SE). Thirdly, CAQIEI suggested lower emission reduction rates during 2015–2018 than previous emission inventories for most species, except for CO. In particular, China's NMVOC emissions were shown to have increased by 26.6 % from 2015 to 2018, especially over NCP (by 38.0 %), northeastern China (by 38.3 %), and central China (60.0 %). These results provide us with new insights into the complex variations in air pollutant emissions in China during two recent clean-air actions, which has the potential to improve our understanding of air pollutant emissions in China and their impacts on air quality. All of the datasets are available at 10.57760/sciencedb.13151 (Kong et al., 2023a). [ABSTRACT FROM AUTHOR]

Nitrogen oxides (NOx) play an important role for atmospheric chemistry and radiative forcing. However, NOx emissions from the vast northern circumpolar permafrost regions have not been studied in situ due to limitations of measurement techniques. Our goals were to validate the offline analytical technique, and based on this, to widely quantify in situ NOx emissions from peatlands in the southern Eurasian permafrost region. To this end, we conducted a comparison of online and offline flux measurements in 2018 and 2019 using the synthetic air flushing, steady‐state opaque chamber method. With differences in annual average and cumulative fluxes less than 0.1 μg N m−2 h−1 and 0.01 kg N ha−1 year−1, the online and offline fluxes were in good agreement, demonstrating the feasibility of conducting offline measurements in remote regions without power supply. The flux measurements over 2 years showed obvious NOx emissions of 0.05–0.14 and 0.13–0.30 kg N ha−1 year−1 in the hollow and hummock microtopography of permafrost peatlands, respectively. The rapid expansion of alder (Alnus sibirica) in the peatlands induced by permafrost degradation significantly increased soil mineral N contents and NOx emissions depending on the age of alder (0.64–1.74 and 1.44–2.20 kg N ha−1 year−1 from the alder forests with tree ages of 1–10 years and 11–20 years, respectively). Alder expansion also intensively altered the thermal state of permafrost including the sharp increases of soil temperatures during the non‐growing season from October to April and active layer thickness. This study provides the first in situ evidences of NOx emissions from the northern circumpolar permafrost regions and uncovers the well‐documented expansion of alders can substantially stimulate NOx emissions and thus, significantly affect air quality, radiative forcing, and ecosystem productivity in the pristine regions. [ABSTRACT FROM AUTHOR]