Department of Chemistry, University of Toronto, Canada.
Faraday Discuss. 2017 Aug 24;200:379-395. doi: 10.1039/c7fd00086c.
In many parts of the world, the implementation of air quality regulations has led to significant decreases in SO emissions with minimal impact on NH emissions. In Canada and the United States, the molar ratio of NH : SO emissions has increased dramatically between 1990 and 2014. In many regions of North America, this will lead the molar ratio of NH : SO, where NH is the sum of particle phase NH and gas phase NH, and SO is the sum of particle phase HSO and SO, to exceed 2. A thermodynamic model (E-AIM model II) is used to investigate the sensitivity of particle pH, and the gas-particle partitioning of NH and inorganic nitrate, to the atmospheric NH : SO ratio. Steep increases in pH and the gas fraction of NH are found as NH : SO varies from below 1 to above 2. The sensitivity of the gas fraction of nitrate also depends strongly on temperature. The results show that if NH : SO exceeds 2, and the gas and particle phase NH are in equilibrium, the particle pH will be above 2. Observations of the composition of particulate matter and gas phase NH from two field campaigns in southern Canada in 2007 and 2012 have median NH : SO ratios of 3.8 and 25, respectively. These campaigns exhibited similar amounts of NH, but very different particle phase loadings. Under these conditions, the pH values calculated using the observations as input to the E-AIM model were in the range of 1-4. The pH values were typically higher at night because the higher relative humidity increased the particle water content, diluting the acidity. The assumption of equilibration between the gas and particle phase NH was evaluated by comparing the observed and modelled gas fraction of NH. In general, E-AIM was able to reproduce the partitioning well, suggesting that the dominant constituents contributing to particle acidity were measured, and that the estimated pH values were realistic. These results suggest that regions of the world where the ratio of NH : SO emissions is beginning to exceed 2 on a molar basis may be experiencing rapid increases in aerosol pH of 1-3 pH units. This could have important consequences for the rates of condensed phase reactions that are acid-catalyzed.
在世界许多地区，空气质量法规的实施已经导致 SO 排放显著减少，而 NH 排放的影响最小。在加拿大和美国，1990 年至 2014 年间，NH：SO 的摩尔比急剧增加。在北美许多地区，这将导致 NH：SO 的摩尔比（其中 NH 是颗粒相 NH 和气相 NH 的总和，SO 是颗粒相 HSO 和 SO 的总和）超过 2。一个热力学模型（E-AIM 模型 II）用于研究颗粒 pH 以及 NH 和无机硝酸盐的气粒分配对大气 NH：SO 比的敏感性。随着 NH：SO 从低于 1 到高于 2 的变化，发现 pH 值和 NH 的气相分数急剧增加。硝酸盐气相分数的敏感性也强烈依赖于温度。结果表明，如果 NH：SO 超过 2，并且气相和颗粒相 NH 处于平衡状态，那么颗粒 pH 值将高于 2。2007 年和 2012 年在加拿大南部进行的两次野外考察中对颗粒物和气相 NH 的组成的观测结果表明，NH：SO 的中值比分别为 3.8 和 25。这些考察表现出相似的 NH 量，但颗粒相负荷非常不同。在这些条件下，使用观测结果作为输入输入到 E-AIM 模型中计算的 pH 值在 1-4 范围内。由于相对湿度较高增加了颗粒含水量，从而稀释了酸度，因此夜间 pH 值通常较高。通过比较观测到的和模拟的 NH 气相分数来评估气相和颗粒相 NH 之间平衡的假设。一般来说，E-AIM 能够很好地再现分配，这表明测量了对颗粒酸度有贡献的主要成分，并且估计的 pH 值是现实的。这些结果表明，在世界上 NH：SO 排放摩尔比开始以摩尔为基础超过 2 的地区，气溶胶 pH 值可能会迅速增加 1-3 pH 单位。这可能会对酸催化的凝聚相反应速率产生重要影响。
