The impact of the aerosol reduction on the worsening ozone pollution over the Beijing-Tianjin-Hebei region via influencing photolysis rates

Distinct responses of urban and rural O3 pollution with secondary particle changes to anthropogenic emission reductions: Insights from a case study over North China

Ozone (O3) pollution with excessive near-surface O3 levels has been an important environmental issue in China, although the anthropogenic emission reductions (AER) have improved air quality since 2013. In this study, we investigated the sensitivities of atmospheric chemical environment with the urban and rural changes to the AER targeting a typical O3 pollution episode over North China in summer 2019, by conducting two WRF-Chem simulation experiments under two scenarios of anthropogenic emission inventories of years 2012 and 2019 with the meteorological conditions in the 2019 summertime O3 pollution episode for excluding the meteorological impacts on O3 pollution. The results show that the unbalanced AER aroused more serious O3 pollution in urban and rural areas. The intense NO reduction was responsible for the significant increments of urban O3, while the falling NO2 and NO synergistically devoted to the slight O3 variations in rural areas. Induced by the recent-year AER, the urban O3 production was governed by VOCs-limited and transition regime, whereas the NOx-limited regime dominated over rural areas in North China. Also, the AER reinforced the atmospheric oxidation capacity with the elevations of atmospheric oxidants O3 and ROx radicals, strengthening the chemical conversions to secondary inorganic particles. In both urban and rural areas, the sharp drop in SO2 caused a decrease in sulfate fraction, while the enhanced AOC accelerated the transformation to nitrate even when NOx was reduced. The AER induced nitrate to occupy the principal position in secondary PM2.5 in urban and rural areas. The AER promoted daytime and suppressed nighttime the nitrate production in urban areas, and more vigorous conversion of secondary aerosols were found in rural areas with much lower AOC increments. This study provides insights from a case study over North China in distinct responses of urban and rural O3 pollution with secondary particle changes to AER in urban and rural atmospheric environment changes, with implications for an effective abatement strategy on O3 pollution.

Air quality and health benefits of achieving carbon-neutrality in building sector over Beijing, China

To meet the goal of the Paris Agreement, China pledges to realize the "Dual Carbon" targets by 2060. As the capital of China, Beijing plays a leading role in becoming zero-emission or carbon neutral in the future. We project the pollutants emissions of building sector based on current strict clean air policies (PO scenario) and China's carbon neutrality target by 2060 (CN scenario) from 2019 to 2050. Results show that PM2.5 concentration will increase by 2.62 μg/m3 under PO scenario; under the CN scenario, ozone concentration will increase by 2.53 μg/m3 but PM2.5 concentration will reduce by 9.04 μg/m3. It is projected that China carbon neutrality goals could avoid 11.12% of PM2.5-related health burden; With strict clean air policies, health burdens of ozone (3.9%) and PM2.5 (4.1%) could be avoided, respectively. This study highlights the importance of achieving co-benefits of air quality and public health.

Control of fine particulate nitrate during severe winter haze in "2+26" cities

The "2+26" cities, suffering the most severe winter haze pollution, have been the key region for air quality improvement in China. Increasing prominent nitrate pollution is one of the most challenging environmental issues in this region, necessitating development of an effective control strategy. Herein, we use observations, and state-of-the-art model simulations with scenario analysis and process analysis to quantify the effectiveness of the future SO2-NOX-VOC-NH3 emission control on nitrate pollution mitigation in "2+26" cities. Focusing on a serious winter haze episode, we find that limited NOX emission reduction alone in the short-term period is a less effective choice than VOC or NH3 emission reduction alone to decrease nitrate concentrations, due to the accelerated NOX-HNO3 conversion by atmospheric oxidants and the enhanced HNO3 to NO3- partition by ammonia, although deep NOX emission reduction is essential in the long-term period. The synergistic NH3 and VOC emission control is strongly recommended, which can counteract the adverse effects of nonlinear photochemistry and aerosol chemical feedback to decrease nitrate more. Such extra benefits will be reduced if the synergistic NH3 and VOC reduction is delayed, and thus reducing emission of multiple precursors is urgently required for the effective control of increasingly severe winter nitrate pollution in "2+26" cities.

Insights into quantitative evaluation technology of PM2.5 transport at multi-perspective and multi-spatial and temporal scales in the north China plain

Cross-border transport is a crucial factor affecting air quality, while how to quantify the transport contribution through different technologies at multi-perspective and multi-scale have not been fully understood. This study established three quantification techniques, and conducted a systematic assessment of PM2.5 transport over the North China Plain (NCP) based on numerical simulations and vertical observations. Results suggested that the annual local emissions, inter-urban and outer-regional transport contributed 44.5%-64.6%, 15.2%-27.9% and 18.0%-28.2% of total surface PM2.5 concentrations, respectively, with transport intensity stronger in July and April, yet weaker in January and October. The southwest-northeast, northeast-southwest, and southeast-northwest were three prevailing transport directions near the surface. By comparison, the annual PM2.5 transport contribution below the atmospheric boundary layer height increased by 16.8%-24.5% in Beijing, Tianjin and Shijiazhuang, with inter-urban and outer-regional contribution of 29.8%-32.1% and 18.5%-23.1%. Furthermore, observed fluxes from fixed-point and vehicle-based mobile lidar were in good agreement with the simulated flux. PM2.5 net flux intensity varied with height, with generally larger at the middle- and high-altitude layer than that of low-altitude layer. In the early, during and late period of haze peak formation (Stage Ⅰ, Ⅱ, Ⅲ, respectively), the largest absolute flux intensity on average was Stage Ⅱ (566.7 t/d), followed by Stage Ⅲ (307.0 t/d) and Ⅰ (191.4 t/d). Besides, external transport may dominate the second concentration peak, while local emissions may play a more vital role in the first and third peaks. It has been noted that joint prevention and control measures should be proposed 1-2 days before reaching PM2.5 extremes. These findings could improve our understanding of transport influence mechanism of PM2.5 and propose effective emission reduction measures in the NCP region.

Changes in tropospheric air quality related to the protection of stratospheric ozone in a changing climate

Ultraviolet (UV) radiation drives the net production of tropospheric ozone (O3) and a large fraction of particulate matter (PM) including sulfate, nitrate, and secondary organic aerosols. Ground-level O3 and PM are detrimental to human health, leading to several million premature deaths per year globally, and have adverse effects on plants and the yields of crops. The Montreal Protocol has prevented large increases in UV radiation that would have had major impacts on air quality. Future scenarios in which stratospheric O3 returns to 1980 values or even exceeds them (the so-called super-recovery) will tend to ameliorate urban ground-level O3 slightly but worsen it in rural areas. Furthermore, recovery of stratospheric O3 is expected to increase the amount of O3 transported into the troposphere by meteorological processes that are sensitive to climate change. UV radiation also generates hydroxyl radicals (OH) that control the amounts of many environmentally important chemicals in the atmosphere including some greenhouse gases, e.g., methane (CH4), and some short-lived ozone-depleting substances (ODSs). Recent modeling studies have shown that the increases in UV radiation associated with the depletion of stratospheric ozone over 1980-2020 have contributed a small increase (~ 3%) to the globally averaged concentrations of OH. Replacements for ODSs include chemicals that react with OH radicals, hence preventing the transport of these chemicals to the stratosphere. Some of these chemicals, e.g., hydrofluorocarbons that are currently being phased out, and hydrofluoroolefins now used increasingly, decompose into products whose fate in the environment warrants further investigation. One such product, trifluoroacetic acid (TFA), has no obvious pathway of degradation and might accumulate in some water bodies, but is unlikely to cause adverse effects out to 2100.

Potential Factors Contributing to Ozone Production in AQUAS-Kyoto Campaign in Summer 2020: Natural Source-Related Missing OH Reactivity and Heterogeneous HO2/RO2 Loss

This study presents total OH reactivity, ancillary trace species, HO₂ reactivity, and complex isoprene-derived RO₂ reactivity due to ambient aerosols measured during the air quality study (AQUAS)-Kyoto campaign in September, 2020. Observations were conducted during the coronavirus disease (COVID-19) pandemic (associated with reduced anthropogenic emissions). The spatial distribution of missing OH reactivity highlights that the origin of volatile organic compounds (VOCs) may be from natural-emission areas. For the first time, the real-time loss rates of HO₂ and RO₂ onto ambient aerosols were measured continuously and alternately. Ozone production sensitivity was investigated considering unknown trace species and heterogeneous loss effects of XO₂ (≡HO₂ + RO₂) radicals. Missing OH reactivity enhanced the ozone production potential by a factor of 2.5 on average. Heterogeneous loss of radicals could markedly suppress ozone production under low NO/NO_x conditions with slow gas-phase reactions of radicals and change the ozone regime from VOC- to NO_x-sensitive conditions. This study quantifies the relationship of missing OH reactivity and aerosol uptake of radicals with ozone production in Kyoto, a low-emission suburban area. The result has implications for future NO_x-reduction policies. Further studies may benefit from the combination of chemical transport models and inverse modeling over a wide spatiotemporal range.