这项研究调查了台湾中部市区每小时的无机气溶胶化学及其对大气能见度的影响,依靠气溶胶消光的测量,无机气体,和PM2.5水溶性离子(WSI),和热力学平衡模型的模拟。平均而言,硫酸盐(SO42-),硝酸盐(NO3-),铵(NH4+)组分(SNA)贡献了90%的WSI浓度,这又占PM2.5质量的50%左右。在整个观察期间,PM2.5和SNA浓度,气溶胶pH值,气溶胶液态水含量(ALWC),硫和氮的转化率都随着能见度的降低而增加。特别是,NO3-对PM2.5的贡献增加,而SO42-贡献下降,能见度下降。上述参数的昼夜变化表明,NO3-和ALWC之间的相互作用和可能的相互促进增强了吸湿性和有利于NO3-形成的水相反应,从而导致能见度严重受损。研究区域的高相对湿度(RH)(平均为70.7%)是导致NO3-形成增强的必要因素,但不是唯一因素。这与升高的ALWC和气溶胶pH值更直接相关。来自热力学模型的模拟描绘了研究区域中的无机气溶胶系统的特征在于在富含NH3的环境中完全中和的SO42-(即能见度降低的饱和因子)和过量的NH4+。因此,PM2.5成分对气相HNO3最敏感,因此对NOx最敏感,对NH3相对不敏感。因此,NOx的减少将导致NO3-的瞬时削减,PM2.5和ALWC,从而提高了能见度。另一方面,需要大量的NH3减少(>70%)来降低气溶胶的pH值,驱动50%以上的颗粒相NO3-到气相,从而使NH3成为改变PM2.5组成的限制因素。
This study investigated the hourly inorganic aerosol chemistry and its impact on atmospheric visibility over an urban area in Central Taiwan, by relying on measurements of aerosol light extinction, inorganic gases, and PM2.5 water-soluble ions (WSIs), and simulations from a thermodynamic equilibrium model. On average, the sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+) components (SNA) contributed ∼90% of WSI concentrations, which in turn made up about 50% of the PM2.5 mass. During the entire observation period, PM2.5 and SNA concentrations, aerosol pH, aerosol liquid water content (ALWC), and sulfur and nitrogen conversion ratios all increased with decreasing visibility. In particular, the NO3- contribution to PM2.5 increased, whereas the SO42- contribution decreased, with decreasing visibility. The diurnal variations of the above parameters indicate that the interaction and likely mutual promotion between NO3- and ALWC enhanced the hygroscopicity and aqueous-phase reactions conducive for NO3- formation, thus led to severely impaired visibility. The high relative humidity (RH) at the study area (average 70.7%) was a necessary but not sole factor leading to enhanced NO3- formation, which was more directly associated with elevated ALWC and aerosol pH. Simulations from the thermodynamic model depict that the inorganic aerosol system in the study area was characterized by fully neutralized SO42- (i.e. a saturated factor in visibility reduction) and excess NH4+ amidst a NH3-rich environment. As a result, PM2.5 composition was most sensitive to gas-phase HNO3, and hence NOx, and relatively insensitive to NH3. Consequently, a reduction of NOx would result in instantaneous cuts of NO3-, PM2.5, and ALWC, and hence improved visibility. On the other hand, a substantial amount of NH3 reduction (>70%) would be required to lower the aerosol pH, driving more than 50% of the particulate phase NO3- to the gas phase, thereby making NH3 a limiting factor in shifting PM2.5 composition.