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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.

High contents of reactive nitrogen components aggravate air pollution and could also impact ecosystem structures and functioning across the terrestrial–aquatic–marine continuum. However, the long-term historical trends and future predictions of reactive nitrogen components at the global scale still remain highly uncertain. In our study, field observations, satellite products, model outputs, and many other covariates were integrated into the multi-stage machine-learning model to capture the global patterns of reactive nitrogen components during 2000–2019. In order to decrease the estimate uncertainties in the future scenarios, the constructed reactive nitrogen component dataset for the historical period was utilised as the constraint to calibrate the CMIP6 dataset in four scenarios. The results suggested that the cross-validation (CV) R2 values of four species showed satisfying performance (R2>0.55). The concentrations of estimated reactive nitrogen components in China experienced persistent increases during 2000–2013, while they suffered drastic decreases from 2013, except for NH3. This might be associated with the impact of clean-air policies. However, in Europe and the United States, these compounds have remained relatively stable since 2000. In the future scenarios, SSP3-7.0 (traditional-energy scenario) and SSP1-2.6 (carbon neutrality scenario) showed the highest and lowest reactive nitrogen component concentrations, respectively. Although the reactive nitrogen concentrations in some heavy-pollution scenarios (SSP3-7.0) also experienced decreases during 2020–2100, SSP1-2.6 and SSP2-4.5 (middle-emission scenario) still showed more rapidly decreasing trends. Our results emphasise the need for carbon neutrality pathways to reduce global atmospheric N pollution. [ABSTRACT FROM AUTHOR]

Nitrated aromatic compounds (NACs) and oxygenated derivatives of polycyclic aromatic hydrocarbons (OPAHs) play vital roles within brown carbon (BrC), influencing both climate dynamics and human health to a certain degree. The concentrations of these drug classes were analyzed in PM 2.5 from an urban area in Beijing during the autumn and winter of 2017 and 2018. There were four heavy haze episodes during the campaign, two of which happened prior to heating and the other two during heating. During the entire course of sampling, the mean total concentrations of the nine NACs and the eight OPAHs were 1.2–263 and 2.1–234 ng m -3 , respectively. The concentrations of both NACs and OPAHs were approximately 2–3 times higher in the heating period than before heating. For NACs, the relative molecular composition did not change significantly before and during heating, with 4-nitrocatechol and 4-nitrophenol demonstrating the highest abundance. For OPAHs, 1-naphthaldehyde was the most abundant species before and during heating, while the relative proportion of anthraquinone increased by more than twice, from 13 % before heating to 31 % during the heating. In Beijing's urban area during autumn and winter, significant sources of NACs and OPAHs have been traced back to automobile emissions and biomass burning activities. Interestingly, it was observed that the contribution from coal combustion increased notably during heating. It is worth noting that the secondary generation of BrC was important throughout the whole sampling period, which was manifested by the photochemical reaction before heating and the aqueous reaction during heating. It was further found that the haze in autumn and winter was nitrate-driven before heating and secondary organic carbon (SOC)-driven during heating, and the secondary formation of BrC increased significantly in pollution events, particularly during heating. [ABSTRACT FROM AUTHOR]