基于CMAQ-MCM模型的长三角地区夏季臭氧和大气氧化性影响的研究

Summertime ozone and atmospheric oxidation capacity over the Yangtze River Delta using the CMAQ-MCM model

传统的空气质量模型多使用简化的光化学反应机制来模拟大气污染物的形成.这些机制主要基于烟雾箱实验拟合的反应速率和产物来模拟二次产物(如臭氧(O3))前体物的氧化反应,具有一定的不确定性,导致模拟结果产生偏差.针对该问题,本研究将详细的大气化学机理(MCMv3.3.1)与美国国家环境保护局研制的第三代空气质量预报和评估系统CMAQ相结合(CMAQ-MCM),模拟研究长三角地区2015年8月27—9月5日臭氧高发时段的空气质量.CMAQ-MCM模型可以较好地模拟长三角地区6个代表城市O3和其前体物随时间的变化趋势.对模拟的O3日最大8 h平均浓度的统计分析表明,徐州表现最好(标准平均误差=-0.15,标准平均偏差=0.23).在长三角地区,居民源对挥发性有机物(VOCs)的贡献最大,占39.08%,其次是交通运输(33.25%)和工业(25.56%).能源对总VOCs的贡献最小,约为2.11%.对活性氧化氮(NOy)的分析表明,其主要组分是NOx(80%),其次是硝酸(HNO3)(<10%).O3的空间分布与NOy和NOx非常相似.HCHO等其他氧化产物的分布与NOx相似,这很可能是由于在高NOx条件下VOCs氧化产生的产物.甲基乙烯基酮(MVK)和甲基丙烯醛(MACR)的空间分布与自然源VOCs(BVOCs)非常相似,表明长三角地区MVK和MACR主要由BVOCs氧化生成.长三角地区受到人为源和自然源排放相互作用的影响.

Air quality has always been a public concern,especially when regional air pollution episodes occurred in several typical regions of China,such as the Beijing-Tianjin-Hebei region,the Yangtze River Delta(YRD),and the Pearl River Delta.However,the decrease of fine particulate matter(PM2.5)would somehow stimulate surface ozone production.Traditional air quality models usually use a simplified photochemical reaction mechanism to simulate the ozone concentration and its precursors with fitted kinetics and products based on chamber experiments,which leads to large discrepancies between model predictions and actual observations.To solve this problem,a near-explicit photochemical mechanism,the Master Chemical Mechanism(MCM)was implemented into the Community Multi-scale Air Quality model(CMAQ)to investigate a high ozone episode during August 27th to September 5th of 2015 over the YRD.The model can generally reproduce the temporal variation of ozone and its precursors in six representative cities.Statistical analysis of the maximum daily averaged 8-hour O3 showed that both the normalized mean bias(NMB)and normalized mean error(NME)met the criteria,with the best performances in Xuzhou(NMB=-0.15 and NME=0.23).In the YRD,residential source contributed most to the total VOCs,accounting for 39.08%,followed by transportation(33.25%)and industry(25.56%).Power plants contributed least to the total VOCs for about 2.11%.Further analysis of reactive oxidized nitrogen(NOy),which is the reservoirs of NOx,indicated that its majority was NOx(80%),followed by the nitric acid(HNO3<10%)in the YRD.The spatial distribution of O3 was very similar to those of NOy and NOx.The distributions of other oxidized products,such as HCHO was similar to that of NOx,perhaps due to the production by VOCs oxidation under high NOx conditions.Methyl vinyl ketone(MVK)and methacrolein(MACR)were possibly formed by oxidation of biogenic VOCs,estimated from their similar spatial distributions with that of the BVOCs.The air pollutants in the YRD is affected by strong interactions between anthropogenic and biogenic emissions.