Vertical Structure of Air Pollutant Transport Flux as Determined by Ground-Based Remote Sensing Observations in Fen-Wei Plain, China

The effect of cross-regional transport on ozone and particulate matter pollution in China: A review of methodology and current knowledge

China is currently one of the countries impacted by severe atmospheric ozone (O3) and particulate matter (PM) pollution. Due to their moderately long lifetimes, O3 and PM can be transported over long distances, cross the boundaries of source regions and contribute to air pollution in other regions. The reported contributions of cross-regional transport (CRT) to O3 and fine PM (PM2.5) concentrations often exceed those of local emissions in the major regions of China, highlighting the important role of CRT in regional air pollution. Therefore, further improvement of air quality in China requires more joint efforts among regions to ensure a proper reduction in emissions while accounting for the influence of CRT. This review summarizes the methodologies employed to assess the influence of CRT on O3 and PM pollution as well as current knowledge of CRT influence in China. Quantifying CRT contributions in proportion to O3 and PM levels and studying detailed CRT processes of O3, PM and precursors can be both based on targeted observations and/or model simulations. Reported publications indicate that CRT contributes by 40-80 % to O3 and by 10-70 % to PM2.5 in various regions of China. These contributions exhibit notable spatiotemporal variations, with differences in meteorological conditions and/or emissions often serving as main drivers of such variations. Based on trajectory-based methods, transport pathways contributing to O3 and PM pollution in major regions of China have been revealed. Recent studies also highlighted the important role of horizontal transport in the middle/high atmospheric boundary layer or low free troposphere, of vertical exchange and mixing as well as of interactions between CRT, local meteorology and chemistry in the detailed CRT processes. Drawing on the current knowledge on the influence of CRT, this paper provides recommendations for future studies that aim at supporting ongoing air pollution mitigation strategies in China.

Observations of HONO and its precursors between urban and its surrounding agricultural fields: The vertical transports, sources and contribution to OH

The insufficient study on vertical observations of main atmospheric reactive nitrogen oxides (NO2 and HONO) posed a great challenge to evaluate their intertransport between urban and agricultural areas, and to further learn the atmospheric nitrogen chemistry and the atmospheric oxidation capacity at high altitudes. A stereoscopic measurement campaign (satellite remote sensing, hyperspectral unmanned aerial vehicle (UAV) remote sensing and MAX-DOAS observation) was performed in a typical inland city Hefei and its surrounding agricultural fields from June to October 2022. Average aerosol vertical profiles exhibited a Gaussian shape above 100 m with maximum values of 0.67 km-1 and 0.55 km-1 at 300-400 m layer at Anhui University (AHU) and Changfeng (CF), respectively. The distinct layered structure was mainly attributed to regional transport. Average H2O and NO2 vertical profiles all showed a Gaussian shape and an exponential shape at AHU and CF, respectively. Moreover, the diurnal evolution of H2O profiles performed one peak and bi-peak patterns at AHU and CF, respectively, whereas the diurnal evolution of NO2 at two stations all exhibited bi-peak patterns attributed to vehicle emissions. Average HONO vertical profiles showed an exponential shape and a Gaussian shape at AHU and CF, respectively. Higher HONO (> 0.05 ppb) above 1.0 km at 14:00-16:00 was observed at CF. The transport flux analysis showed that the northern transport flux always larger than southern transport flux for aerosol and H2O. The maximum northern transport fluxes appeared at 300 m and surface for aerosol and H2O, respectively. It indicated that surrounding agricultural fields was an important source of atmospheric H2O of city. The southern transport flux was larger than northern transport flux for NO2, with a maximum net transport flux of 9.20 ppb m s-1 at 100 m. It demonstrated that NO2 transported from urban areas was an important source of NO2 in agricultural fields. For HONO, the southern transport flux was larger than northern transport flux under 100 m, whereas it was opposite above 100 m. It indicated that the HONO distributed at high altitudes at agricultural fields had potential to enhance the atmospheric oxidation capacity of urban area. The net horizontal transport fluxes of HONO of our defined cropland were 5.25 μg m-2 s-1 and -3.65 μg m-2 s-1 during non-fertilization and fertilization periods, respectively. It indicated that the cropland could obviously export HONO to surrounding atmosphere during the fertilization period. Deducing the contribution of direct emission, heterogeneous process was a major source of HONO at urban and agricultural areas. The average surface conversion rate of NO2-to-HONO (CHONO) was 0.01467 h-1, and this value decreased with the increase of height at urban station. While average surface CHONO was 0.0322 h-1 at agricultural fields, which was ~1.2-2.8 times higher than that at urban area. The CHONO at agricultural fields significantly increased with the increase of height. The average CHONO at 1.0 km was ~2.0-3.6 times higher than that at surface. That suggested that the heterogeneous process was the main HONO source at high altitudes at CF, and this process obviously correlated with aerosol and H2O. The higher OH production from HONO (P(OH)HONO) occurred at 0-200 m and 100-400 m with averaged values of 0.31 ppb h-1 and 0.39 ppb h-1 at AHU and CF, respectively. The high P(OH)HONO above 1.0 km at CF from September to October was strongly correlated with high O3 (> 80 ppb). This study emphasized the importance of the stereoscopic of HONO on the analysis of its distribution, evolution, source and atmospheric oxidizing contribution.

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.

Vertical profiles of the transport fluxes of aerosol and its precursors between Beijing and its southwest cities

The influence of regional transport on aerosol pollution has been explored in previous studies based on numerical simulation or surface observation. Nevertheless, owing to inhomogeneous vertical distribution of air pollutants, vertical observations should be conducted for a comprehensive understanding of regional transport. Here we obtained the vertical profiles of aerosol and its precursors using ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) at the Nancheng site in suburban Beijing on the southwest transport pathway of the Beijing-Tianjin-Hebei (BTH) region, China, and then estimated the vertical profiles of transport fluxes in the southwest-northeast direction. The maximum net transport fluxes per unit cross-sectional area, calculated as pollutant concentration multiply by wind speed, of aerosol extinction coefficient (AEC), NO₂, SO₂ and HCHO were 0.98 km^-1 m s^-1, 24, 14 and 8.0 μg m^-2 s^-1 from southwest to northeast, which occurred in the 200-300 m, 100-200 m, 500-600 m and 500-600 m layers, respectively, due to much higher pollutant concentrations during southwest transport than during northeast transport in these layers. The average net column transport fluxes were 1200 km^-1 m² s^-1, 38, 26 and 15 mg m^-1 s^-1 from southwest to northeast for AEC, NO₂, SO₂ and HCHO, respectively, in which the fluxes in the surface layer (0-100 m) accounted for only 2.3%-4.2%. Evaluation only based on surface observation would underestimate the influence of the transport from southwest cities to Beijing. Northeast or weak southwest transports dominated in clean conditions with PM_2.5 <75 μg m^-3 and intense southwest transport dominated in polluted conditions with PM_2.5 >75 μg m^-3. Southwest transport through the middle boundary layer was a trigger factor for aerosol pollution events in urban Beijing, because it not only directly bringing air pollutants, but also induced an inverse structure of aerosols, which resulted in stronger atmospheric stability and aggravated air pollution in urban Beijing.