| dc.description.abstract | This study used a semi-automatic instrument to observe short-interval (20 minutes) PM2.5 water-soluble inorganic ions at the Chung Shan Medical University station and resolved the results by combining with monitoring data of the neighboring monitoring station of Taiwan Environmental Protection Administration in spring (4/22 to 5/6) and autumn (9/17 to 9/25) of 2020. In addition, short-term variations of NH3 were observed to evaluate the impact of NH3 in autumn.
In the early spring (4/22 to 4/27), sea-breeze events occurred, three high Na+ values with varying degrees accompanied by long-lasting westerly winds before 4/27. The Cl-/Na+ was averaged at 1.07 during the sea-breeze, and the average chlorine loss was 19.14%. Simulations from the ISORROPIA II model showed a pronounced Na2SO4 peak during the sea-breeze event. In the middle spring (4/27 to 5/2), a high-concentration event was observed with increased precursor gas concentrations such as NO2, NO, and CO, which was attributed to traffic emissions dominated by NH4NO3 from secondary reactions of NO2. In the late spring (5/2 to 5/6), the overall low concentration was related to the weakening of secondary reactions of NO2. In addition, the change of the entire airflow from the middle spring (4/27) to the late spring (5/6) enhanced the diffusion of pollutants based on flow field simulation from the website (http://earth.nullschool.net).
The data in autumn (9/17 to 9/25) showed that the concentration ratio of NH4+ gas to particle partition, ε(NH4+), decreased with an increase of NH3 in a curvelike relationship and finally reached at a relatively stable value of 0.2 for NH4+/SO42->1.5. The decrease of ε(NH4+) partition ratio is presumed to be related to the lack of HNO3 concentration in the environment, and 0.2 is the equilibrium point of the reaction. In addition, the sources of NH3 are conjectured to contribute from the evaporation of plant dew plus heavy machinery at the nearby construction site.
In the early autumn (2020 9/17 to 9/21), the first high-concentration event (ES1) was attributed to the emissions of a large amount of precursor gases from traffic activities and photochemical reactions in producing NO3-. During the second high-concentration event, SO42- was peaked in contrast to relatively flat NO3-, which was opposite to ES1. Since the photochemical activities were not as strong as ES1, the PM2.5 concentration was higher than the ES1 period, even under sufficient NH3 concentration. At night, NO3- peaks were presumed to be caused by the heterogeneous reactions of N2O5 hydrolysis when the environment was under low temperature, high humidity, and low wind speed.
During the first high-concentration event in late autumn (9/21 to 9/25), NH3 was insufficient, and with noticeable northerly winds, the rising concentration was from external transport. During the second high-concentration event, the wind speed was low to limit outside transport. NO3- dominated the increase of PM2.5 concentration, but the high concentration and high proportion of SO42- provided a stable PM2.5 base concentration. From the measurements of NO2-, the liquid phase reactions of NO2 played an essential role in NO2- formation when the relative humidity was higher than 69%. And the more potent the ability of alkalinity supply in the atmosphere, the more the formation of NO2-. However, the primary formation mechanism of NO2- should be the photolysis of NO3- when the relative humidity is lower than 55%.
In summary, this study found that the topography of the Taichung metropolis tended to accumulate local pollution for the southerly wind and conducted an external pollution transport under the northeast monsoon. Traffic pollution sources dominated the increase of PM2.5 concentrations. In addition to pollution sources and meteorological factors, NH4NO3 formed from secondary reactions of NO2 dominated the chemical reaction mechanism of high-concentration NO3- in spring. In contrast, the high NO3- concentrations in autumn were attributed to the heterogeneous reactions of N2O5 hydrolysis. The concentration of NH3 plays a crucial role in the formation of pollutants. | en_US |