北京夏季大气主要含氮无机化合物的变化规律与相互作用

利用SJAC-MOBIC/FIA在2006年8月16日—9月9日在线测量了北京城市大气细颗粒物中主要水溶性含氮离子组分(NO₃^-和 NH₄⁺),与重要含氮气态污染物(HNO₃、HNO₂和 NH₃),以追踪细颗粒物中含氮二次无机组分和含氮气态污染物的变化规律及其相互作用。观测期间,NO₃^- 和 NH₄⁺的平均浓度分别为13.08和11.93 μg/m³,它们与SO₄^2-浓度之和在细颗粒物(PM_2.5)中的平均比例为55%,明显高于其他季节;污染过程中,积聚模态颗粒物体积浓度及其与爱根核模态颗粒物体积浓度比值逐渐增加,说明二次转化是北京夏季细颗粒物的重要来源。白天HONO迅速光解产生OH自由基,而OH自由基是生成HNO₃的重要物种,因此HONO和HNO₃具有相反的日变化规律。在温度较高的白天,大气环境不利于NH₄NO₃的生成与存在;夜间低温高湿的条件下硝酸铵理论平衡系数Ke与气态氨和硝酸的乘积Km相当或低于后者,较有利于NH₄NO₃的生成。北京夏季大气具有足量气态NH₃以中和硫酸盐;但在NO₃^-与阳离子的电荷平衡中,金属阳离子也非常重要。

Variation and Interaction of Major Azotic Inorganic Compounds in the Summer in Beijing

Simultaneously on-line measurements of major water-soluble azotic inorganic ions of NO₃^- and NH₄⁺ in PM_2.5 and azotic gaseous pollutants of HNO₃,HONO,and NH₃ were performed using SJAC-MOBIC/FIA from August 16 to September 9,2006 at an urban site in the mega-city of Beijing,in order to track their variation and interaction. The average concentrations of NO₃^- and NH₄⁺ were 13.08mg/m³ and 11.93mg/m³,respectively. The combination of them and sulfate accounted for 55% in PM_2.5 on average,which was significantly higher than in other seasons. The volume concentration of accumulation mode particles and its ratio to Aitken mode particles increased during pollution episodes. These results suggested that secondary transformation was an important source of fine particles in the summer. During the daytime,fast photolysis of HONO produces OH radical,a key reagent in the production of HNO₃.Therefore,HONO and HNO₃ showed inverse diurnal variations. NH₄NO₃ was not predicted to be formed under high temperature during the daytime. The theory equilibrium constant Ke had a good agreement with Km=NH₃]HNO₃] during the night with low temperature and high relative humidity. There was more than enough NH₄⁺ to combine with SO₄^2- in fine particles in this campaign. However,in the charge balance of NO₃^-,metal cations also played an important role.