北京地区重霾污染过程颗粒物酸度演变特征及其驱动机制

Evolution Characteristics of Particle Acidity and Its Driving Factors during Heavy Haze Pollution Episodes in Urban Beijing

基于北京秋冬季重霾污染时期细颗粒物化学组分和气态前体物的小时观测数据,通过ISORROPIA II模型模拟颗粒物酸度,并利用敏感性分析方法研究颗粒物酸度的驱动因子。结果表明:2017~2019年秋冬季,北京3次重霾污染事件颗粒物pH的平均值分别为4.52±0.52、5.19±0.28和5.03±0.79;从清洁阶段到中度污染阶段和从中度污染阶段到重度污染阶段,颗粒物pH平均值分别下降0.72~1.07和0.3~1.03,即随着污染的加重,颗粒物pH呈现下降趋势;颗粒物pH对化学组分变化的响应不同,主要受TNHx[总铵(气态氨+颗粒态铵盐)]和SO_(4)^(2−)影响,受环境温度和湿度变化的影响有限;颗粒物pH对TNHx的敏感性依次高于SO_(4)^(2−)高于NO_(3)^(−),且高的TNHx水平会降低颗粒物pH对SO_(4)^(2−)(TNHx>47μg/m^(3))和NO_(3)^(−)(TNHx>30~40μg/m^(3))的敏感性;北京重霾污染时期,大气处于富氨状态,抬升颗粒物pH至4以上;在重霾污染事件中,颗粒物pH及变化的幅度与总铵水平和颗粒物pH对SO_(4)^(2−)、NO_(3)^(−)的敏感性有关,这为厘清二次气溶胶生成机制提供有效参考。

Hourly continuous observations of the chemical compositions of fine particles and gaseous precursors were conducted during autumn and winter haze pollution episodes in urban Beijing.The ISORROPIA II model was used to simulate particle pH,and the driving factors of particle pH were studied using a sensitivity analysis method.The results show that the average particle pH calculated using the ISORROPIA II model is 4.52±0.52,5.19±0.28,and 5.03±0.79 for three heavy haze pollution events in the autumn and winter seasons of 2017,2018,and 2019,respectively.From the cleaning stage to the moderate pollution stage and from the moderate pollution stage to the severe pollution stage,the average value of particle pH decreases by 0.72–1.07 and 0.3–1.03,respectively,showing a gradually decreasing trend with the aggravation of haze pollution.The sensitivity tests demonstrate that particle pH exhibits different responses to the changes in chemical composition.Specifically,the particle pH is mainly affected by TNHx[total ammonium(gas+aerosol)]and SO_(4)^(2−),with limited impact from changes in ambient temperature and humidity.Particle pH is more sensitive to TNHx than SO_(4)^(2−)and NO_(3)^(−).In addition,high TNHx levels reduce the sensitivity of particle pH to SO_(4)^(2−)(TNHx>47μg/m^(3))and NO_(3)^(−)(TNHx>30–40μg/m^(3)).During heavy haze pollution in Beijing,the atmosphere is rich in ammonia;therefore,particle pH increases to>4.Furthermore,the results indicate that particle pH and the magnitude of the change in particle pH are related to the TNHx level and the sensitivity of particle pH to SO_(4)^(2−)and NO_(3)^(−)in heavy haze pollution events.These relations can serve as an effective reference for clarifying the formation mechanism of secondary aerosols.