臺中都會區揮發性有機物之光化反應對臭氧與二次有機氣膠生成潛勢及其來源與健康風險之探討

Abstract

揮發性有機物（VOCs）因其對環境及人類健康產生不利影響已被世界各地廣泛關注，且VOCs作為臭氧（O3）和二次有機氣膠（SOA）的主要前驅物之一，有必要了解其生成O3和SOA之潛勢。然而，VOCs在大氣中為高反應性的化合物，因此，在評估其生成O3和SOA時，若忽略VOCs的光化學反應，可能會低估其貢獻；此外，目前對於都會區VOCs之健康風險評估及相關文獻之探討仍然有限，尤其是在臺灣。本研究於臺中都會區分別利用鋼瓶及高量採樣器對VOCs和PM2.5進行四季及日夜的採樣。本研究的主要目的為探討VOCs之光化學損失對O3和SOA生成潛勢之來源解析及比較在O3事件日及非事件日下VOCs的健康風險。在本研究中，利用觀測VOCs/NOX之比值能更為準確地推估O3生成敏感性之閾值；結果顯示，臺中地區的O3生成敏感性為VOCs控制（VOCs-limited）。為了進一步探討VOCs與O3間之關係，分別由VOCs觀測值濃度（VOCsobs）及VOCs初始排放濃度（VOCsini）推估臭氧生成潛勢(O3 formation potential, OFP)。OFPini（由VOCsini所推估）與O3之間呈正相關，顯示考慮VOCs之光化學反應對於研究O3十分重要。SOA分別透過EC tracer方法（SOATracer）和yield方法（SOAYield）進行估算。由SOATracer與大氣氧化力之間的正相關以及與相對溼度及氣膠含水率之間的負相關性可推估，SOA的生成機制可能由VOCs的光化學反應所主導。來源解析由正矩陣因子受體模式進行分析，工業相關源（即溶劑使用、工業排放和合成橡膠行業）對OFPini和SOAYield的貢獻超過50%。最後，在VOCs健康風險評估方面，雖然在O3事件日和非事件日時VOCsobs之平均濃度相當，但在O3非事件日，由VOCs造成之致癌和非致癌風險水平皆高於事件日。此外，在採樣期間，溶劑使用和車輛排放對致癌風險的貢獻超過70%，顯示其管制的重要性。本研究將有助於相關機構制定有效的VOCs排放控制策略，從而降低空氣污染並促進公眾健康。

Volatile organic compounds (VOCs) have become a major concern worldwide due to their adverse effects on the environment and human health. As one of the primary precursors of ozone (O3) and secondary organic aerosol (SOA), it is crucial to thoroughly characterize VOCs and assess their contribution to O3 and SOA formation. Given the high reactivity of VOC species in ambient air, assessing source contribution to O3 and SOA without considering the photochemistry of VOCs could be inappropriate. Additionally, the health risk assessment of exposure to VOCs in an urban area, especially in Taiwan, remains limited. In this study, VOCs and PM2.5 samples were collected using canisters and high-volume samplers, respectively, during daytime and nighttime across four seasons in the Taichung urban area. The aim of this study was to elucidate the effects of photochemical loss of VOCs on the source apportionment of O3 and SOA formation potential, as well as health risks of VOCs under different levels of O3. In this study, O3 formation sensitivity is accurately diagnosed by deriving regional threshold values based on observed VOCs/NOX ratios. Results showed that the O3 formation sensitivity was VOC-limited regime during the sampling periods. To further investigate the relationship between VOCs and O3, O3 formation potential (OFP) was estimated by both initial (VOCsini) and observed mixing ratio of VOCs (VOCsobs), which were OFPini and OFPobs, respectively. A positive and significant correlation was found between OFPini and O3, indicating the importance of considering the photochemistry of VOCs when investigating O3 formation. In addition, SOA was estimated by both EC tracer method (SOATracer) and yield method (SOAYield). The primary formation mechanism of SOA may be driven by the photochemical reaction of VOCs, supported by a strong positive correlation between SOATracer and atmospheric oxidation capacity, along with a negative correlation between relative humidity and aerosol liquid water content. Source apportionment analysis based on positive matrix factorization model reveled that industrial-related sources, namely solvent usage, industrial emissions and synthetic rubber industry contributed to more than 50% of OFPini and SOAYield, respectively. In terms of health risks associated with VOCs, although the averaged mixing ratio of VOCs on O3-polluted days was similar to that on non-polluted days, the levels of both carcinogenic and non-carcinogenic risks were higher on O3 non-polluted days. Additionally, solvent usage and vehicle exhaust contributed more than 70% to the carcinogenic risks during sampling periods, underscoring the priority for control. This research can aid regulatory agencies in formulating effective control strategies for VOCs emissions, thereby mitigating air pollution and improving public health.