Impacts of synoptic circulation on surface ozone pollution in a coastal eco-city in Southeastern China during 2014-2019

The coastal eco-city of Fuzhou in Southeastern China has experienced severe ozone(O_(3))episodes at times in recent years.In this study,three typical synoptic circulations types(CTs)that influenced more than 80%of O_(3) polluted days in Fuzhou during 2014-2019 were identified using a subjective approach.The characteristics of meteorological conditions linked to photochemical formation and transport of O_(3) under the three CTs were summarized.Comprehensive Air Quality Model with extensions was applied to simulate O_(3) episodes and to quantify O_(3) sources from different regions in Fuzhou.When Fuzhou was located to the west of a high-pressure system(classified as“East-ridge”),more warm southwesterly currents flowed to Fuzhou,and the effects of cross-regional transport from Guangdong province and high local production promoted the occurrence of O_(3) episodes.Under a uniform pressure field with a low-pressure system occurring to the east of Fuzhou(defined as“East-low”),stagnant weather conditions caused the strongest local production of O_(3) in the atmospheric boundary layer.Controlled by high-pressure systems over the mainland(categorized as“Inland-high”),northerly airflows enhanced the contribution of cross-regional transport to O_(3) in Fuzhou.The abnormal increases of the“East-ridge”and“Inland-high”were closely related to O_(3) pollution in Fuzhou in April and May 2018,resulting in the annual maximum number of O_(3) polluted days during recent years.Furthermore,the rising number of autumn O_(3) episodes in 2017-2019 was mainly related to the“Inland-high”,indicating the aggravation of cross-regional transport and highlighting the necessity of enhanced regional collaboration and efforts in combating O_(3) pollution.

我国典型城市群O_3污染成因和关键VOCs活性解析

京津冀、长三角、珠三角和成渝地区四大城市群在夏秋季大气臭氧(O_3)污染问题严重.为了深入认识O_3的区域污染特征及其成因,在上述4个城市群于2014~2016年夏秋季节,开展了大气O_3及其前体物(氮氧化物(NOx)、挥发性有机物(VOCs))等的综合观测研究.观测发现,臭氧污染区域性明显,臭氧最大8h值在59~146(10^(-9)V/V),整体上城市站点臭氧浓度水平较郊区站点低,臭氧前体物NO2平均浓度水平在4~22(10^(-9)V/V),VOCs浓度为11~53(10^(-9)V/V),VOCs活性水平在1.6~10.5s^(-1).使用基于观测的盒子模型(OBM)分析臭氧生成控制区,发现四大城市群的O_3生成多数处于人为源VOCs控制区或者过渡区,且对烯烃和芳香烃的敏感性最强.运用PMF受体模型对城市站点VOCs来源进行解析,结果表明机动车尾气排放和汽油挥发是城市VOCs主要来源,占比30%~50%;溶剂涂料使用其次,占比为10%~20%.综合而言,我国现阶段臭氧污染防控应以VOCs控制为主,其中应着重控制机动车尾气排放和溶剂涂料使用.

Characteristics of volatile organic compounds,NO_2,and effects on ozone formation at a site with high ozone level in Chengdu

Chengdu is a megacity in the southwest of China with high ozone(O_3) mixing ratio.Observation of volatile organic compounds(VOCs), NO_2 and O_3 with high temporal resolution was conducted in Chengdu to investigate the chemical processes and causes of high O_3 levels. The hourly mixing ratios of VOCs, NO2, and O_3 were monitored by an online system from 28 August to 7 October, 2016. According to meteorological conditions,Chengdu, with relative warm weather and low wind speed, is favorable to O_3 formation.Part of the O_3 in Chengdu may be transported from the downtown area. In O_3 episodes,the average mixing ratios of NO_2 and O_3 were 20.20 ppbv and 47.95 ppbv, respectively. In non-O_3 episodes, the average mixing ratios of NO_2 and O_3 were 16.38 ppbv and 35.15 ppbv,respectively. The average mixing ratio of total VOCs(TVOCs) was 40.29 ppbv in non-O_3 episodes, which was lower than that in O_3 episodes(53.19 ppbv). Alkenes comprised51.7% of the total O_3 formation potential(OFP) in Chengdu, followed by aromatics which accounted for 24.2%. Ethylene, trans-pentene, propene, and BTEX(benzene, ethylbenzene,toluene, m/p-xylene, o-xylene) were also major contributors to the OFP in Chengdu. In O_3 episodes, intensive secondary formations were observed during the campaign. Oxygenated VOCs(OVOCs), such as acetone, Methylethylketone(MEK), and Methylvinylketone(MVK)were abundant. Isoprene rapidly converted to MVK and Methacrolein(MACR) during O_3 episodes. Acetone was mainly the oxidant of C3-C5 hydrocarbons.

Spatiotemporal pattern and regional characteristics of visibility in China during 1976–2010

Degradation in visibility occurs extensively across China,creating the urgent need to better understand visibility patterns and characteristics in China.Using daily observation data from the U.S.National Climatic Data Center(NCDC)during 1976–2010,the spatial distribution and temporal trends of visibility in China,as well as seasonal and regional characteristics,were determined.The distribution of visibility was characterized by a longitudinal gradient between west and east China.The northwest of China generally experienced consistent high visibility of[20 km,while central and eastern China experienced visibility degradation and increase in haziness.The 133stations with significant downward trends in visibility were clustered around central-east China and the average reduction rate was-1.65 km/decade(from 18.5 km).The96 stations with increasing trends of visibility were mostly on the Qinghai–Tibet Plateau and Xinjiang and were closely related to a reduction in local sand storms.The average visibility in 12 key regions in China varied between9.9–17.8 km(except for 23.9 km in Urumqi),which was lower than the national average of 18.7 km.The Taiyuan–Hohhot,Guanzhong,Yangtze River Delta,Pearl River Delta and Fuzhou–Xiamen areas all encountered significant decreases in visibility,while minor increases were observed in Beijing–Tianjin–Hebei and the Cheng-Yu region.At most stations,the worst visibility was measured in winter,but the season with best visibility varied between regions.A comparison of visibility trends with the emission trends of various pollutants revealed that SO2emissions dominated the visibility degradation in eastern Chinawith NOxand volatile organic compounds playing ascending roles during the 2000s.

Estimating emissions from crop residue open burning in China based on statistics and MODIS fire products

With the objective of reducing the large uncertainties in the estimations of emissions from crop residue open burning, an improved method for establishing emission inventories of crop residue open burning at a high spatial resolution of 0.25°× 0.25° and a temporal resolution of1 month was established based on the moderate resolution imaging spectroradiometer（MODIS） Thermal Anomalies/Fire Daily Level3 Global Product（MOD/MYD14A1）. Agriculture mechanization ratios and regional crop-specific grain-to-straw ratios were introduced to improve the accuracy of related activity data. Locally observed emission factors were used to calculate the primary pollutant emissions. MODIS satellite data were modified by combining them with county-level agricultural statistical data, which reduced the influence of missing fire counts caused by their small size and cloud cover. The annual emissions of CO2, CO, CH4,nonmethane volatile organic compounds（NMVOCs）, N2O, NOx, NH3, SO2, fine particles（PM2.5）,organic carbon（OC）, and black carbon（BC） were 150.40, 6.70, 0.51, 0.88, 0.01, 0.13, 0.07, 0.43,1.09, 0.34, and 0.06 Tg, respectively, in 2012. Crop residue open burning emissions displayed typical seasonal and spatial variation. The highest emission regions were the Yellow-Huai River and Yangtse-Huai River areas, and the monthly emissions were highest in June（37%）.Uncertainties in the emission estimates, measured as 95% confidence intervals, range from a low of within ±126% for N2O to a high of within ± 169% for NH3.

Seasonal variations of transport pathways and potential sources of PM2.5 in Chengdu, China （2012-2013）

Seasonal pattem of transport pathways and potential sources of PM2.5 in Chengdu during 2012 2013 were investigated based on hourly PM2.5 data, backward trajectories, clustering analysis, potential source contribution function （PSCF）, and concentration-weighted trajectory （CWT） method. The annual hourly mean PM2.5 concentration in Chengdu was 97.4 μg. rn . 5, 5, 5 and 3 mean clusters were generated in four seasons, respectively. Short-distance air masses, which travelled within the Sichuan Basin with no specific source direction and relatively high PM2.5 loadings （〉80 μg.m 3） appeared as important pathways in all seasons. These short pathways indicated that emissions from both local and surrounding regions of Chengdu contributed significantly to PM2.5 pollution. The cities in southern Chengdu were major potential sources with PSCF〉0.6 and CWT〉90 /g.m-3.