冬季NH3和液态水含量对PM2.5中SNA形成的影响与敏感性分析

为了解冬季不同污染等级下NH₃和AWC（Aerosol Water Content，气溶胶液态水含量）对PM_2.5中水溶性二次离子形成的影响，对保定市冬季颗粒物浓度、二次离子及前体物（SO₂、NO₂、NH₃）浓度进行了分析，并利用ISORROPIA-Ⅱ计算了PM_2.5中的AWC和pH.结果表明:①2017-2018年冬季保定市重污染期（AQI>200）ρ（PM_2.5）、ρ（SO₂）、ρ（NO₂）和ρ（NH₃）较优良期（AQI < 100）分别升高了3.0、1.1、1.3和0.8倍，气态前体物的二次转化是污染形成的重要原因之一.重污染期ρ（NH₄+）、ρ（SO₄2-）、ρ（NO₃-）较冬季平均值分别升高了1.2、0.9、1.3倍，其中ρ（NO₃-）升幅最大，其次为ρ（NH₄+）.②保定市大气中过剩NH₃指数为0.1 μmol/m3，采样期间为富氨环境，NO₃-的生成主要受HNO₃限制.③重污染期PM_2.5中AWC高达93.6 μg/m3，是优良期的20.6倍，观测期间保定市SO₄2-的二次生成以颗粒物表面液相氧化为主，即SO₂被NO₂和NH₃氧化，NO₃-的二次生成包括NH₃参与的非均相转化和N₂O₅的非均相水解过程.④整体而言，pH变化的敏感性表现为TA（总氨）浓度> TS（总硫）浓度> TN（总氮）浓度≈TA+TS+TN浓度（同时改变相同比例的总氨、总硫、总氮浓度），随污染等级的升高，pH对TS、TA浓度变化的敏感性减弱，对TN浓度变化的敏感性增强；单独改变TS、TN、TA浓度时AWC敏感性弱，同时改变TS、TN、TA浓度时AWC敏感性较强，AWC变化与二次离子浓度密切相关.研究显示，保定市冬季污染期SNA的形成以液态水参与的液相氧化为主，NH₃可以维持颗粒物的高pH，保持氧化过程. 

Influences of NH3 and AWC on the Formation of SNA in PM2.5 in Winter and Sensitivity Analysis

In order to understand the influence of NH₃ and aerosol liquid water content (AWC) under different pollution levels in winter on the formation of water-soluble secondary ions in PM_2.5, the concentrations of particulate matter, secondary ions and precursors (SO₂, NO₂ and NH₃) in Baoding City were analyzed. The ISORROPIA-Ⅱ was used to calculate the AWC and pH in PM_2.5. The results showed that: (1) The concentrations of PM_2.5, SO₂, NO₂, and NH₃ during the heavy pollution (AQI>200) in Baoding City were 3.0, 1.1, 1.3 and 0.8 times than those during non-pollution period (AQI < 100) in winter from 2017 to 2018. The secondary formation of gaseous precursors was one of the important reasons for this pollution formation process. During the heavy pollution period, NO₃- concentration showed the largest increase (1.3 times than corresponding average concentration), followed by NH₄+(1.2 times), and SO₄2- (0.9 times). (2) The excess NH₃ index (0.1) indicated that the atmosphere of Baoding City was under ammonia-rich condition, and the formation of NO₃- was mainly limited by HNO₃. (3) The concentration of AWC in PM_2.5 during the heavy pollution period reached 93.6 μg/m3, which is 20.6 times that of non-pollution period. During the observation period, the secondary formation of SO₄2- in Baoding was mainly caused by the oxidation of SO₂ by NO₂ and NH₃ in the liquid phase on the particle surface. The secondary formation of NO₃- included the heterogeneous transformation of NH₃ and the heterogeneous hydrolysis of N₂O₅. (4) The strongest sensitivity of pH was observed after controlling TA (total ammonia), followed by TS (total sulfur), TN (total nitrogen), and TA+TS+TN (synchronously change the concentration of total ammonia, total sulfur, total nitrogen). As the pollution intensified, the pH sensitivity of TS and TA decreased, while pH sensitivity of TN increased. The sensitivity of AWC is weak when the concentration of TS, TN and TA is changed separately, but stronger when the concentration of TS, TN and TA is changed simultaneously. The change of AWC is closely related to the concentration of secondary ions. The study shows that the formation of SNA in winter pollution period in Baoding City is dominated by the liquid phase oxidation of water, and NH₃ can maintain the oxidation process of high-pH particulate matter.