| 摘要: | 本研究探討南台灣地區2005年5月至2005年12月底止，包括梅雨季節、颱風降雨與颱風外圍環流、夏季及冬季降雨之濕沉降特性，主要探討降雨的酸鹼度、無機鹽類及低分子量之二元有機酸之當量濃度變化及其相關性。
在南台灣不同季節降雨型態之平均降雨，酸強度由高至低依序為梅雨季節、颱風雨水、颱風外圍環流、夏季陣雨及冬雨，其pH值分別為4.92、5.35、5.34、5.43及6.69，而降雨的pH是由酸性離子SO42-、NO3-、HCO3-及鹼性離子NH4+、Ca2+所共同決定。不同季節降雨型態的前、中、後期中，梅雨季節的單位降雨強度之最大陰陽離子物種當量濃度，在陰離子為SO42-分別為12.5 meq L-1 mm-1 hr、4.9 meq L-1 mm-1 hr、7.1 meq L-1 mm-1 hr，在陽離子為Na+，分別為16.0 meq L-1 mm-1 hr、9.4 meq L-1 mm-1 hr、7.0 meq L-1 mm-1 hr，颱風降雨的單位降雨強度之最大陰陽離子物種當量濃度，在陰離子為Cl-，在降雨的前、中、後期分別為71.8 meq L-1 mm-1 hr、38.8 meq L-1 mm-1 hr、158.1 meq L-1 mm-1 hr，在陽離子為Na+，分別為56.9 meq L-1 mm-1 hr、35.9 meq L-1 mm-1 hr、138.0 meq L-1 mm-1 hr，颱風外圍環流降雨的單位降雨強度之最大陰陽離子物種當量濃度，在陰離子為Cl-，在降雨的前、中、後期分別為22.5 meq L-1 mm-1 hr、5.0 meq L-1 mm-1 hr、1.8 meq L-1 mm-1 hr，在陽離子為Na+，分別為29.4 meq L-1 mm-1 hr、5.7 meq L-1 mm-1 hr、1.9 meq L-1 mm-1 hr。各季節降雨中物種組成成份，所佔百分比不同，其中在颱風濕沉降中Cl- (40.43±26.69%)、Na+ (35.14±21.20%)及Mg2+ (7.45±5.04%)佔降雨物種當量濃度比例最高，在夏季濕沉降中，SO42- (16.69±21.55%)佔降雨物種當量濃度比例很高，而在冬季濕沉降中，NH4+ (19.93±7.98%)佔降雨物種當量濃度比例最高。此外五種濕沉降型態中，平均而言，二元有機酸佔物種組成成份的0.11－0.87%，二元有機酸均以oxalic acid為主要物種。
在三次颱風與二次颱風外圍環流濕沉降中，二元有機酸單位降雨強度之物種當量濃度oxalic acid (C2)、malonic acid (C3)和succinic acid (C4)均有一段時間達到穩定的最低值，而C3/C2和C4/C2之比值成定值，但其值皆不一樣，與颱風所走的路徑和暴風半徑影響範圍有關。三種二元有機酸在各降雨型態中，以冬季濕沉降所佔已分析物種組成比例為最高，主要乃因冬季時期長期沒有降雨，而在降雨時，降雨雨量少及降雨強度小，導致濕沉降中之三種二元有機酸當量濃度較其他降雨型態為高。比較不同季節型態之二元有機酸之malonic acid:succinic acid (M:S)比值，在梅雨季節、颱風降雨及夏季降雨型態的M:S比值，分別為2.1、2.0及1.2，其值介於低比值(0.3－0.5)與高比值3之間，由梅雨季節、颱風降雨及夏季降雨型態逆軌跡圖所示，顯示降雨物種經長程傳輸，從海上吹經高雄市區，降雨之部份來源為市區交通的污染排放和在海洋上產生的光化產物溶入降雨中，在外圍環流及冬季降雨型態則有相同的M:S比值為3.4，其M:S的比值高於3，由外圍環流及冬季降雨型態逆軌跡圖顯示降雨中之物種來源主要為交通車輛排放。
In this study, characteristics of the wet precipitation of the Mei-yu rain, typhoon precipitation and typhoon peripheral circumfluence in southern Taiwan from May 2005 till December 2005 were investigated. The investigation covered the acidity, inorganic salts and low-molecular-weight dicarboxylic acids (low-Mw DCAs) to study their equivalent concentration variations and correlations.
For the various seasonal precipitation patterns in southern Taiwan, according to the acidity decreasing magnitudes, the rainwater pH values are 4.92 for the Mei-yu rainy season, 5.35 for typhoon precipitation, 5.34 for typhoon peripheral circumfluence, 5.43 for summer shower and 6.69 for winter precipitation. The pH is dependent on presence of anions, e.g. SO42-, NO3- and HCO3- as well as cations, e.g. NH4+ and Ca2+ in the rainwater. For the Mei-yu rainy season, SO42- has the maximum equivalent concentration intensities of 12.5 meq L-1 mm-1 hr, 4.9 meq L-1 mm-1 hr and 7.1 meq L-1 mm-1 hr during the initial precipitation period, mid-rainfall period, post precipitation period, respectively. Among all anions, Na+ has the maximum equivalent concentration intensities of 16.0 meq L-1 mm-1 hr, 9.4 meq L-1 mm-1 hr and 7.0 meq L-1 mm-1 hr. For typhoon precipitation, Cl- has the highest concentration among all anions; its equivalent concentration intensities are 71.8 meq L-1 mm-1 hr, 38.8 meq L-1 mm-1 hr and 158.1 meq L-1 mm-1 hr during the initial precipitation period, mid-rainfall period, post precipitation period, respectively. Among all cations, Na+ has the highest concentration of 56.9 meq L-1 mm-1 hr, 35.9 meq L-1 mm-1 hr and 138.0 meq L-1 mm-1 hr during the initial precipitation period, mid-rainfall period, post precipitation period, respectively. For typhoon peripheral circumfluence, Cl- has the highest concentration among all anions; its equivalent concentration intensities are 22.5 meq L-1 mm-1 hr, 1.0 meq L-1 mm-1 hr and 5.8 meq L-1 mm-1 hr during the initial precipitation period, mid-rainfall period, post precipitation period, respectively. Among all cations, Na+ has the highest concentration of 29.4 meq L-1 mm-1 hr, 5.7 meq L-1 mm-1 hr and 1.9 meq L-1 mm-1 hr during the initial precipitation period, mid-rainfall period, post precipitation period, respectively.
Further, as the percentage compositions of all chemical species in the various precipitation patterns are concerned, the typhoon wet precipitation has the highest percentages of Cl- (40.43±26.69%), Na+ (35.14±21.20%) and Mg2+ (7.45±5.04%). In summer wet precipitation, SO42- has the highest equivalent concentration percentage of 16.69±21.55% while in winter wet precipitation, NH4+ has the highest equivalent concentration percentage of 19.93±7.98%.
Additionally, in all 5 types of wet precipitations, the average percentage of low-Mw DCAs constitutes 0.87±0.43% of all chemical species. Oxalic acid is the major constituent among all low-Mw DCAs.
Among the 3 typhoons and 2 typhoon peripheral circumfluence wet precipitations, the equivalent concentrations per unit precipitation of low-Mw DCAs, e.g. oxalic acid (C2); malonic acid (C3) and succinic acid (C4), reach stable but low values after a certain period as seen by stable but low C3/C2 and C4/C2 ratios. This observation is related to the path and cyclonic radius of a typhoon. These three low-Mw DCAs have the highest percentages of all analyzed chemical species in winter wet precipitation. This is mainly because that there is little precipitation in winter and the occasional precipitations have small quantities and intensities thus leading to higher observed equivalent concentrations of low-Mw DCAs than observed for other patterns of precipitation. The ratios of malonic acid to succinic acid (M:S ratio) are 2.1 for the Mei-yu rainy season, 2.0 for typhoon precipitation and 1.2 for summer precipitation. All these ratios are between the low ratio (0.3－0.5) and the high ratio (3) ranges. Based on the backward trajectories for these three precipitation patterns, the chemical species contained in the rainwater have been subject to long-range transportation to be blown from sea to Kaohsiung downtown. Some chemical species originated from local municipal traffic-related pollution react photo-chemically with the species from the ocean and the products are dissolved in the rainwater. Both peripheral circumfluence and the winter precipitation have similar M:S ratios of 3.4, which is higher than 3. The backward trajectories for both precipitation patterns indicate that the main chemical species in the rainwater mainly come from traffic emission. |
