大氣能見度劣化期間衍生性污染物的特徵及季節變化

Abstract

近年來由於大氣能見度的劣化成為大眾所關注之議題，許多研究利用消光係數以客觀方式定義能見度，且衍生性污染物已被證實為消光主要的貢獻者，因此其生成機制與來源對於改善能見度至關重要。本研究透過即時量測微粒化學組成成分與觀測各氣象參數，以分析台中地區導致能見度劣化的影響因子與衍生性污染物於不同季節之生成差異，並利用正矩陣因子法(PMF)分析污染來源及其分別對大氣氧化能力(AOC)之貢獻。 研究結果顯示，台中地區PM2.5之質量濃度與能見度呈現非線性之反向關係，且亦發現相較於乾淨期，二次無機鹽類於能見度劣化期間會有顯著增加的趨勢。其中，較強之AOC與較高之相對濕度(RH)環境下，會導致能見度劣化並伴隨高微粒質量濃度之污染事件。此外，夏季能見度劣化期間，較強之AOC有利於硫酸鹽經光化反應形成，而較高之RH有助於硝酸鹽與二次有機物透過液相反應來生成；冬季時，能見度劣化則多受較差的大氣擴散情況主導，導致污染物容易累積並有利於氧化反應發生，促使衍生性污染物生成。而利用硝酸鹽之分配比例發現，較高之微粒含水量有利於硝酸鹽以微粒的形式存在於環境中，並透過ISORROPIA之模擬結果顯示，若能降低環境中的氨，即能有效抑制硝酸鹽以微粒的形式存在。在污染來源解析方面，能見度劣化時二次氣膠/煤碳燃燒與交通排放為PM2.5主要之貢獻者。在AOC組成中，NO2在夏季與冬季皆以交通排放為主要來源；而在O3生成方面，夏季以工業/鋼鐵工業與二次氣膠/煤碳燃燒為主要之相關貢獻源，冬季則以工業/鋼鐵工業的排放為主要可能產生來源。由於冬季NO2對AOC之貢獻較為顯著(48.5 ± 22.9%)，表示若要透過降低AOC來避免衍生性污染物之生成，必須針對不同來源進行控制。因此，能見度改善策略必須因應季節而有所不同。

In recent years, visibility impairment has received public attention and has become a political issue in Taiwan. This study employs the extinction coefficient as the objective surrogate of atmospheric visibility. It is affected by the chemical composition of particulate matter (PM) as well as particle size. Studies have found that both are related to secondary aerosols, suggesting that identifying the sources and the formation mechanisms of secondary aerosols is important for improving visibility. Hence, online high-temporal-resolution measurements of the chemical composition of PM2.5, gaseous pollutants, and meteorological conditions are conducted in Taichung city. The influencing factors of visibility degradation are analyzed, and the seasonal impacts of secondary aerosol formation on visibility impairment are investigated. Moreover, the positive matrix factorization (PMF) approach is applied to apportion the contributions from emissions sources in different seasons and to quantify the atmospheric oxidation capacity (AOC) contributions. It was found that the significant enhancement of secondary inorganic aerosols (SIA) is observed as visibility degraded, and the high AOC and relative humidity (RH) are associated with low visibility and severe PM pollution events. In summer, the stronger AOC favors sulfate formation through photochemical reactions, and nocturnal high RH enhances nitrate and SOC generation by heterogeneous oxidations. In winter, the weak dispersion, resulting in pollutant and precursor accumulation, is the main reason for visibility deterioration. On the other hand, the PMF results show that the secondary aerosols/coal combustion sources and the traffic emissions are the main contributors to PM2.5 mass concentration during visibility impairment. Traffic emissions are the major sources of NO2, regardless of the season, while the generation of O3 varies between seasons. In addition, NO2 contributes 48.6 ± 22.9% to AOC in winter, and O3 contributes 67.8 ± 16.4% in summer. These findings suggest that the seasons-depending control strategy should be implemented to inhibit AOC and secondary aerosol formation. Furthermore, the simulation by ISORROPIA confirms that the decrease of NH3 will inhibit the nitrate in the particulate phase, which sheds light on improving visibility by intervening with secondary aerosol formation.