Displays

China has suffered from increasing levels of ozone pollution in urban areas despite the implementation of various stringent emission reduction measures since 2013. In this study, we conducted numerical experiments with an up-to-date regional chemical transport model to assess the roles of changes in meteorology and anthropogenic emission in summer ozone variations from 2013 to 2017 over China. The model can faithfully reproduce the observed meteorological parameters and air pollutant concentrations and capture the increasing trend in the surface maximum daily 8-hour average (MDA8) ozone (O₃) from 2013 to 2017. An increase of 0.46 ppbv a^-1 (p=0.001) and a slight decrease of 0.17 ppbv a^-1 (p=0.005) in MDA8 O₃ levels were simulated from 2013 to 2017 in urban and rural areas, respectively. The meteorological influence on the ozone trend varied by region and by year and could be comparable with or even larger than the impact of changes in anthropogenic emissions. The variation in biogenic emissions during summer varied across regions and was mainly affected by temperature. China’s midlatitude areas (25°N to 40°N) experienced a significant decrease in MDA8 O₃ due to a decline in biogenic emissions, while higher temperatures in northern (north of 40°N) and southern (south of 25°N) China after 2013 led to an increase in MDA8 O₃ concentrations via an increase in biogenic emissions. We assessed the effects of changes in individual meteorological factors on ozone levels from 2013 to 2017. The results show that the wind field change made a significant contribution to the increase in surface ozone over China by transporting the ozone downward from the upper troposphere and the lower stratosphere. The long-range transport of ozone and its precursors outside the modeling domain also contributed to the increase in MDA8 O₃ on the Tibetan Plateau. The effects of changes in individual pollutant emissions on ozone were simulated. The reduction of NO_x emission increased ozone in urban areas due to non-linear NO_x-VOCs chemistry and decreased aerosol effects; the slight increase in VOCs emission enhanced ozone levels; the reduction of particulate matter(PM) emission increased ozone concentrations by enhancing the photolysis rates and reducing the loss of reactive gases on aerosol surfaces; the reduction of SO₂ emission resulted in a drastic decrease in sulfate concentrations which increase ozone levels through the aerosol effects. In contrast, the reduction of CO emissions helped decrease ozone levels in the past years. On the effects of decreasing levels of aerosol, the drop in heterogeneous uptake of reactive gasses, mainly HO₂ and O₃, was found to be more important than the increase in photolysis rates. The adverse effect on ozone of the reductions of NO_x, SO₂ and PM emissions would have been avoided with ~20% reduction of VOCs emission from 2013 to 2017. Our analysis revealed that the NO_x reduction in the past years has helped to contain the total ozone production in China. However, in order to decrease ozone concentrations in major urban and industrial areas, VOCs emission control should be added to the current NO_x-SO₂-PM policy.

How to cite: Liu, Y. and Wang, T.: Worsening urban ozone pollution in China from 2013 to 2017: The roles of meteorology and anthropogenic emission, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11046, https://doi.org/10.5194/egusphere-egu2020-11046, 2020.

Hydroxyl (OH) play an essential role in atmospheric chemistry. OH radical is an indicator of atmospheric oxidation and self-purification, which determines the removal of most trace gases in the atmosphere, such as CO, SO₂, NO₂, CH₄ and other volatile organic compounds (VOCs). A ground-based system for measurement of tropospheric OH radical by Laser Induced Fluorescence technique (AIOFM-LIF) was developed and integrated into a mobile observation platform for field observation. Ambient air expands through a 0.4 mm nozzle to low pressure. OH radical is irradiated by the 308 nm laser pulse at a repetition rate of 8.5 kHz, accompanying the release fluorescence of the A²Σ⁺(v’=0)—X²Π_i(v’’=0) transition at 308 nm with the resultant fluorescence being detected by gated photon counting. The detection sensitivity of AIOFM-LIF system was calibrated by a portable standard OH radical source based on water photolysis-ozone actinometry. Following laboratory and field calibrations to characterise the instrument sensitivity, OH radical detection limits were (1.84±0.26) × 10⁵ cm^-3 and (3.69±0.52) × 10⁵ cm^-3 at night and noon, respectively. During “A comprehensive STudy of the Ozone foRmation Mechanism in Shenzhen” (STORM) campaign, AIOFM-LIF system was deployed in Shenzhen, China, and OH radical concentration was obtained validly except for the rainy days. Mean diurnal variation of HOx radical concentration was obtained, and the peak was 6.6×10⁶ cm^-3 which appeared around 12:00 at noon. A general good agreement of OH radical concentration with j(O¹D) was observed with a high correlation (R² =0.77), which illustrates that photolysis of ozone is an important source of OH radical during this campaign. A box model was applied to simulate the concentrations of OH at this field site, the primary production of OH radical was generally dominated by photolysis of O₃, HONO, HCHO, while the other production was contributed by calculated species (OVOCs).

How to cite: Wang, F., Hu, R., Xie, P., Wang, Y., Lou, S., Lu, K., Zhang, G., Liu, J., and Liu, W.: In situ measurement for tropospheric OH radical by Laser Induced Fluorescence technique during STORM campaign, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12047, https://doi.org/10.5194/egusphere-egu2020-12047, 2020.

A differential absorption lidar was used to study the vertical structure of ozone in Jiangmen city and Yangjiang city, Guangdong Province, Southern China in summer and autumn of 2019, and analyze the two typical pollution characteristics and spatial-temporal distribution characteristics of atmospheric ozone local pollution and regional transport. The results show that the vertical concentration of ozone in Jiangmen city in the summer and autumn seasons is characterized by a single peak of ozone in the afternoon. It is mainly distributed below 600 m in summer and mainly distributed within 1.0 km in autumn. There is ozone residual in Yangjiang city in summer at night. In summer, the differences between the average ozone values in Jiangmen city and Yangjiang city are small, being 92.22 μg/m³ and 82 μg/m³, respectively. In autumn, the average ozone concentration in Jiangmen city is 1.58 times the ozone concentration in Yangjiang city, which is 122.27 μg/m³ and 77.36 μg/m³. In the process of local pollution, high-concentration ozone is mainly concentrated near the ground, and the ozone concentration of 1.5-2km tends to be uniformly distributed. In regional transport, the transport heights of the two stations are mainly in two height intervals, ranging from 0.7 to 1.1 km and above 1.1 km. And use TrajStat to perform trajectory clustering analysis on the main airflows that affect Jiangmen city and Yangjiang city in summer and autumn, the dominant directions of ozone transport at the two cities in summer and autumn are analyzed. In addition, we studied two typical pollution processes, on August 24, the lidar of both cities detected the presence of ozone input and sedimentation at a height of about 1.5 km, and found that the ozone transport came from the northeast through the backward trajectory. During September 28-30, the two cities were locally polluted by ozone and there were obvious ozone residues at night.

Keywords: Differential absorption lidar；Ozone；Local pollution；Regional transport

How to cite: Wang, X., Zhang, T., Xiang, Y., and Lv, L.: Vertical distribution of ozone in summer and autumn in Guangdong Province, Southern China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12068, https://doi.org/10.5194/egusphere-egu2020-12068, 2020.

Although the air quality in China has been improved by collaborative efforts dedicating to mitigate the haze pollution, PM2.5 concentrations still remain high levels and the issue of increasing O₃ concentration has attracted more attention of the public. The YRD region has been suffering from both the PM2.5 and O3 pollution problems. To investigate the formation mechanisms and sources of PM2.5 and O3 in this region, a comprehensive EXPLORE-YRD campaign (EXPeriment on the eLucidation of theatmospheric Oxidation capacity and aerosol foRmation, and their Effects inYangtze River Delta) was carried out in May - June 2018. In this study, we investigate the contributions of different source categories to PM2.5 and O₃. A source-oriented 3-D air quality model (CMAQ) was applied to analyze contributions of different emission sources to PM2.5 and O₃ in the YRD region. Emissions were divided into eight source categories: industry, power, transportation, residential, agriculture, biogenic, wildfire, and other countries. Contribution from individual source category was quantified. The importance of anthropogenic and natural sources to PM2.5 and O₃ was discussed.

How to cite: Hu, J., Li, L., Li, J., Wang, X., and Gong, K.: Regional source apportionment of ozone and PM2.5 during the EXPLORE-YRD campaign in the Yangtze River Delta region of China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12174, https://doi.org/10.5194/egusphere-egu2020-12174, 2020.

Ozone (O₃) is a greenhouse gas which plays different roles in stratosphere and troposphere. It also has an important role in radiative and chemical balance of the atmosphere, and thus the changes in O₃ have greater climatic implications. Although O₃ is present in trace amounts in troposphere, it is adequate to govern the oxidation processes in the Earth’s atmosphere by forming OH radicals, as the atmospheric lifetime of many gases is controlled by OH radicals. Rapidly developing countries in tropics and subtropics have realized the importance of tropospheric O₃ studies as these regions have very limited measurements of ozone and its precursor gases. Understand the variability of the surface O₃ and their association with precursors are extremely important for the policy decisions to mitigate the impacts of ozone on human health and crops and ozone air quality management issues in the region. This study investigates the variability of surface ozone (O₃) its association with its precursors (NO, NO2, NOX, CO) at time scales of annual, seasonal and diurnal scales for the duration of three years using ground-based observations from IMD Ayanagar, IMD Lodhi road, CRRI, CV Raman, IGI Palam stations in the Indian Capital region. Further, we will present the back-trajectory analysis to elucidates the transport mechanisms/ pathways on the variability of ozone for the study region.

How to cite: Kunchala, R. K., Chandel, A., Attada, R., Vellore, R. K., and Soni, V. K.: Observed variability of Surface Ozone (O3) and its Precursors over the Indian Capital region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12767, https://doi.org/10.5194/egusphere-egu2020-12767, 2020.

OH radical is the key driver of the photochemical process and closely related to ozone formation. OH reactivity is the quantification of OH radical sink in ambient air. In this study, in-situ OH reactivity measurements are carried out in Shenzhen, Chengdu and Changzhou, three typical cities in major city clusters of China, during their ozone pollution seasons. The measured OH reactivity is ranging from 5~35 s-1 under various meteorological conditions and trace gas concentrations. Aldehydes such as HCHO and acetaldehyde, mainly from the secondary formation of VOCs, are the principal contributors at day time. Primary VOCs such as toluene and biogenic VOCs such as isoprene play different roles in three measurement locations. The missing OH reactivity, which is defined as the OH reactivity that cannot be explained by trace gas measurements, are evaluated by in-situ measurement results as well as an observation-based model. Gas-phase secondary pollutants could be the main source of the missing OH reactivity. The sensitivity tests by the OBM model show ozone production in all areas is mainly VOC-limited but the key precursors of ozone are not identical, leading to different control strategies.

How to cite: Lou, S., Shen, X., Li, X., Wang, Q., and Liu, S.: OH reactivity in three major city clusters of China in ozone seasons: insights for regional ozone formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12898, https://doi.org/10.5194/egusphere-egu2020-12898, 2020.

    An instrument to detect atmospheric HO₂ radicals using fluorescence assay by gas expansion (FAGE) technique has been developed. HO₂ is measured by reaction with NO to form OH and subsequent detection of OH by laser-induced fluorescence at low pressure. The system performance has been improved by optimizing the expansion distance and pressure, and the influence factors of HO₂ conversion efficiency are also studied. The interferences of RO₂ radicals produced from OH plus some typical organic compounds were investigated by determining the conversion efficiency of RO₂ to OH during the measurement of HO₂. The dependence of the conversion of HO₂ on NO concentration was investigated, and low HO₂ conversion efficiency was selected to realize the ambient HO₂ measurement, where the conversion efficiency of RO₂ derived by propane, ethene, isoprene and methanol to OH has been reduced to no more than 6%. Furthermore, no significant interferences from PM_2.5 and NO were found in the ambient HO₂ measurement. The detection limits for HO₂ (S/N=2) are estimated to 4.8×10⁵ cm^-3 and 1.1×10⁶ cm^-3 (the conversion efficiency of HO₂ to OH, =20%) under night and noon conditions, with 60s signal integration time. The instrument was successfully deployed during STORM-2018 field campaign at Shenzhen graduate school of Peking University. The diurnal variation of HOx concentration shows that the OH maximum concentration of those days is about 5.5×10⁶ cm^-3 appearing around 12:00, while the HO₂ maximum concentration is about 5.0×10⁸ cm^-3 appearing around 13:30.

How to cite: Wang, Y., Hu, R., Xie, P., Wang, F., Liu, J., and Liu, W.: Measurement of tropospheric HO2 radical using fluorescence assay by gas expansion with low interferences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12969, https://doi.org/10.5194/egusphere-egu2020-12969, 2020.

A comprehensive field campaign was carried out in summer 2019 in Chengdu, which obtained the first complete radical dataset of Chengyu Urban Agglomeration. Observed daily concentration maxima of radicals by the laser-induced-fluorescence (LIF) technique were in the range of (2-10)×10⁶ cm^-3 for OH and (4-15)×10⁸ cm^-3 for HO₂. During daytime, OH reactivities were generally high (5-32 s^-1). The missing reactivity was not be observed within uncertainty, and inorganics, observed VOCs and the calculated oxidation products contributed about one-third in total reactivity, respectively.

The chemical box model RACM 2 was used to interpret the observed radical concentrations. The model over-predicted OH and HO₂ at noon during the O₃ polluted episode. Constraining the model by the observed HO₂ concentration, the overestimation of OH can be explained almost by the overestimation of HO₂. Besides, as in the previous field campaigns (e.g. Pennsylvania, Mexico City, New York and so on), the underestimation of the net conversion of OH into HO₂ enlarged with the increasing NO concentration, indicating the conversion of HO₂ into OH still need to be studied based on the discussion above. Different schemes to improve the agreement between observed and modelled HO₂ were explored in this work. The sensitivity tests indicated observed and modelled HO₂ can be agreed well by reducing the HO₂ yield in the reaction of OH and HCHO a half.

The oxidation rate of primary pollutants dominated by OH radicals was significantly higher than that in winter Beijing, which contributes significantly to secondary pollution, especially O₃. Besides, the atmospheric self-cleaning ability and recycling efficiency both peaked for about 600 pptv of NO, indicating small amounts of NO can help to maintain the atmospheric oxidation. The campaign emphasizes the important role of HO₂ yield in the reaction channels of OH and VOCs especially, and the need for further laboratory experiments of the HO₂ yield measurement in order to understand radical chemistry in VOC-rich air.

How to cite: Yang, X., Lu, K., Ma, X., and Zhang, Y.: The First Observation of HOx radicals in the Chengyu Urban Agglomeration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13292, https://doi.org/10.5194/egusphere-egu2020-13292, 2020.

The concentration of surface ozone in East Asia is high due to strong solar radiation, but decreases in areas affected by summer monsoons. This study analyzes the summer surface ozone variations in East Asia using meteorological and atmospheric chemistry variables in 12 models participating in Chemistry-Climate Model Initiative (CCMI) for the period of 1979 to 2010. The concentration of 850 hPa ozone was identified two modes by Empirical Orthogonal Functions (EOF) analysis. The first mode is an increase in all regions over East Asia, mainly in eastern China. This mode was associated with downward wind, weak horizontal wind speed, increase in temperatures, decrease in precipitation. The second mode showed high ozone concentrations in eastern China and low in northern Japan. In eastern China, temperatures and precipitation are decreased, and shortwave radiation reaches the surface is increased. In addition, the concentration of nitrogen oxides and carbon monoxide and the net ozone production are increased. The second mode was highly correlated with El Nino-Southern Oscillation (ENSO) and western North Pacific subtropical high (WNPSH) indices and was found to be closely associated with East Asian summer monsoons.

Acknowledgements: This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2019R1A2C1008549). We acknowledge the modeling groups for making their simulations available for this analysis, the joint WCRP SPARC/IGAC Chemistry–Climate Model Initiative (CCMI) for organizing and coordinating the model simulations and data analysis activity, and the British Atmospheric Data Centre (BADC) for collecting and archiving the CCMI model output.

How to cite: Wie, J., Park, H.-J., Lee, H., and Moon, B.-K.: Summertime surface ozone variation over East Asia in CCMI, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16374, https://doi.org/10.5194/egusphere-egu2020-16374, 2020.

Surface ozone (O₃) pollution became the main cause of atmospheric pollution over industrial regions in China since 2013, due to the effective mitigation of fine particulate matter (PM_2.5) by stringent emission controls by Air Pollution Prevention and Control Action Plan (APPCAP). O₃, as a secondary photochemical pollutant, poses a challenge to control due to its non-linear chemical relationship to precursors – nitrogen oxides (NO_x), carbon monoxide (CO) and volatile organic compounds (VOCs).

We hence investigated the differences of atmospheric chemistry environment in the main industrial regions with high emissions – North China Plain (NCP), Yangtze River Delta (YRD), Pearl River Delta (PRD) and Chongqing - in summer 2016, China by using a global climate-chemistry model, based on United Kingdom Chemistry and Aerosol (UKCA). Anthropogenic Multi-resolution Emission Inventory for China (MEIC) 2013 and Hemispheric Transport of Air Pollution (HTAP) emissions 2010 for the rest of globe were used but scaled to 2016 regionally and nationally separately. In addition, we improved the gas-phase chemistry scheme by adding more highly reactive VOC tracers to better simulate regional pollution. Diurnal cycles of O₃ and NO_x have been evaluated and the results show very good model-observation comparisons after modifying the gas-phase chemistry scheme. Radical (OH, RO₂ and HO₂), NO_x and VOC concentrations have also been examined. O₃ production rates and budgets calculated based on these show the highest production rate in YRD and the lowest in PRD due to different NO_x and VOC concentration levels.

To investigate the O₃ sensitivity — VOC limited or NO_x limited, we quantified the O₃ response to VOCs and NO_x in total 64 scenarios by scaling NO_x and VOCs emissions. O₃ isopleths suggest that most regions are VOC limited, but the sensitivities vary. O₃ in YRD is more sensitive to NO_x emission change but PRD can be effectively controlled by decreasing VOC emissions. The ratio of H₂O₂ to HNO₃ is applied to provide a quick examination method of O₃ sensitivity. The contribution of O₃ from China to the global O₃ burden compared with other continents has also been quantified. The results show that the relatively higher O₃ concentration in Asia is mainly contributed by China, and O₃ becomes more sensitive to VOCs. The model allows us to provide a quantitative assessment of different emission controls on mitigating O₃ over China and the impacts of Chinese emissions on the global O₃ burden.

How to cite: Liu, Z., M. Doherty, R., Wild, O., and M. O’Connor, F.: Different atmospheric O3 chemical environment in industrial regions of China, 2016: implications for emission control strategies and impacts on the globe in the future, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18148, https://doi.org/10.5194/egusphere-egu2020-18148, 2020.

Since 2015, the annual average ozone (O₃) concentration in Chengdu has shown significant positive trends and reached a maximum of 55.2 ppb in 2018. By 2019, the annual average O₃ value has slightly decreased to 52.9 ppb, but it is still at the highest level in the Sichuan Basin. In order to illuminate VOCs characteristics, identify critical ozone precursors and explore potential sources during ozone pollution events in Chengdu plain, we performed a comprehensive field observation campaign from 9 August to 14 September 2019. During the campaign, the averaged O₃ concentration was 29.1 ppb, and mean values of ozone precursors NOx and TVOC were 14.9 ppb and 31.3 ppb, respectively. Two severe ozone pollution events occurred in Chengdu during the observation period. In ozone pollution event 1, the ratios of the average O₃, NOx, NMHCs, and OVOCs concentration on the polluted days relative to the clean days were 4.1, 0.3, 0.6, and 1.4, respectively. In ozone pollution event 2, the ratios of the average O₃, NOx, NMHCs, and OVOCs concentration on the polluted days relative to the clean days were 3.4, 0.4, 0.6 and 2.1, respectively. The difference of the ratios indicates that there are secondary conversions of NMHCs and NOx and secondary formation of O₃ and OVOCs during the pollution period. Isoprene, Acetaldehyde, Methyl Vinyl Ketone, m/p-Xylene and 1-Butene constitute a large fraction of the L_OH during polluted days.  In this study, air mass cluster analysis, the potential source contribution function (PSCF), and positive matrix factorization (PMF) receptor models were used in combination to analyze the sources and potential source areas of VOCs during O3 pollution events.

How to cite: Song, M., Li, X., Yang, S., Yu, X., Chen, S., Dong, H., Lou, S., Lu, S., Zeng, L., Lu, K., and Zhang, Y.: Characterization and sources of volatile organic compounds (VOCs) during ozone pollution events in Chengdu plain, Southwest China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18314, https://doi.org/10.5194/egusphere-egu2020-18314, 2020.

The atmospheric oxidizing capacity (AOC) is closely related to the self-cleaning ability of the atmosphere in which the air pollutants were removed through reacting with oxidations such as OH radical. The level of OH radical is a dominant indicator to the AOC in clean regions characterized as low levels of NO_x which is another factor that influences AOC. Due to a lack of VOCs-related mechanisms in model simulation and high cost of the direct observations of OH radical, the long-term trend of OH radical in China is still unclear, especially under the circumstance of significant reduction of Chinese emissions in recent years. In this study, three methods based on a proxy gas CH₃CCl₃ from 5 regional background stations in China have been developed to investigate the long-term variation of OH radical in China. The concentration of OH radical in the background area of China is approximately (0.8±0.1)*10⁶ molecular/cm³, lower than the results in other background regions of the world. This could be explained by the larger depletion of OH radical in China due to the higher concentrations of polluted gases (i.e., NO_x, CO and CH₄). The different methods showed almost consistent results for the long-term trends of OH radical in China. From 2006 to 2017, the annual averaged OH concentration showed a slow downward but insignificant trend with the anomalous annual changes ranging from -0.1% to 0.15%. However, significant inter-annual fluctuations were also detected concurrently with a period about two years. This is consistent with the 2-3 years Quasi-biennial Oscillation (QBO) in the long-term variation of surface O₃ concentrations. These results provide the new insights into the annual variation of OH radical in China, which could help improve our understanding of the long-term characteristics of atmospheric oxidation in background areas of China.

How to cite: Wang, J., Ge, B., Yao, B., Lin, W., Liu, Y., and Wang, Z.: Investigation of long-term variation of atmospheric oxidation in background areas of China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20812, https://doi.org/10.5194/egusphere-egu2020-20812, 2020.