降雨洗滌對空氣污染物濃度的影響

Abstract

降雨洗滌乃空氣污染物自淨的重要機制之一，通常在空氣品質模式中以洗滌係數表示污染物受降雨洗滌之影響。而文獻資料顯示,與降雨洗滌相關之研究雖不缺乏，但探討各洗滌係數式應用於空氣品質模式中之適用性之研究卻非常有限。本研究即以四種不同之降雨洗滌模式，包括固定洗滌係數、洗滌比值公式及兩種質傳理論式，將之代入軌跡網格模式，以模擬降雨期間各洗滌模式對空氣污染物濃度之影響，並對模擬結果進行分析比較。 本研究之結果顯示，以固定洗滌係數值(1.5x10-5 s-1)表示受降雨洗滌之影響時，其SO2模擬濃度有高估之情形。以洗滌比值公式計算之洗滌係數值為各洗滌模式之最大者，當降雨強度低於3 mm/hr時，其SO2模擬濃度較其他模式接近實際觀測值；而在中大雨時則模擬濃度值趨近於零而呈現低估之狀況。以平均雨滴粒徑所得之質傳理論式(I)在不同降雨強度下之模擬結果均與固定洗滌係數值差異不大；而考慮雨滴粒徑分佈函數之質傳理論式(II)在中大雨量時之SO2模擬結果最好。而針對NOx之降雨洗滌模擬結果顯示，無論在小、中、大雨時利用質傳理論式(II)之模擬效果均較洗滌比值為佳。在污染物受降雨洗滌之影響探討可知，SO2之濃度在降雨強度約大於10 mm/hr以上時即可達幾乎完全洗滌之效果，而NOx受降雨洗滌之結果較不明顯，其濃度在大雨時約維持在20∼50 ppb。此外，若要以固定之洗滌係數值表示降雨洗滌之影響，則本研究建議其範圍針對SO2應為10-2~10-4 s-1，針對NOx則應為10-3~10-5 s-1。

Scavenging process is one of the important mechanisms that remove air pollutants from atmosphere. The scavenging coefficient is usually used for describing the effect of scavenging on air pollutants in the air quality model. However, although there are numerous studies on the scavenging process, limited information is available on the applicability of various scavenging models as they are incorporated into the air quality model. In this study, four scavenging models including the constant scavenging coefficient, the scavenging ratio equation and two mass transfer models were incorporated into a trajectory grid air quality model to analysis the scavenging effect. The results indicated that using a constant value of scavenging coefficient tends to overestimate the SO2 concentrations. The scavenging coefficient computed from scavenging ratio equation is the largest among the four scavenging models. Its modeled concentrations are more close to the observed values than others when the rain intensity is less then 3 mm/hr. When the rain intensity is larger than 3 mm/hr, the concentrations tend to be underestimated. The mass transfer theory (I) considered the volume-mean raindrop diameter has the results similar to what calculated from constant scavenging coefficient. And the mass transfer theory (II) using a complete raindrop size distribution results in good agreement between the modeled and observed SO2 concentrations when rain intensity is larger than 3 mm/hr. for the scavenging effect on the NOx concentration, the mass transfer theory model (II) appeared to have a better prediction than the scavenging ratio equation. The results also showed that SO2 could be almost completed scavenged as the rain intensity is larger than around 10 mm/hr. But the NOx concentrations after heavy rain events are still around 20~50 ppb. The reasonable scavenging coefficients range in this study is10-2~10-4 s-1 for SO2 and10-3~10-5 s-1 for NOx, they increase as the rain intensity increases.