台中地區二次無機氣膠形成機制與影響因子之探討

Abstract

二次無機氣膠（SIA）已被許多文獻普遍認為是細懸浮微粒（PM2.5）質量濃度的重要貢獻者，其中又以硝酸鹽（NO3-） 和硫酸鹽（SO42-）為主。本研究利用奇數氧（Ox）以及氣膠液態含水量（ALWC）分別作為光化學反應和液相反應是否盛行的指標，搭配即時且長期的氣膠化學成分、氣狀污染物和氣象條件等數據，探討在不同時期硝酸鹽和硫酸鹽的形成機制。 研究結果顯示光化學反應和液相反應都會影響硝酸鹽與硫酸鹽的生成，不過發現在不同季節間兩種形成機制對於硝酸鹽產生的相對貢獻具有不同的情況。在春、夏兩季，光化學反應可能對於硝酸鹽的形成具有更為顯著的影響；在秋季時，兩種反應機制的貢獻則較為相當；至於在冬季，硝酸鹽的生成可能會受到液相反應相對明顯的影響。而在不同季節，氣相氧化反應可能都對硫酸鹽的形成擁有較為顯著的貢獻。 氣膠酸度（aerosol acidity）也是涉及SIA形成的關鍵因子之一。本研究還解析在氣膠酸度較高的條件下（pH < 5），二氧化氮（NO2）與過渡金屬離子（TMI）對於液相硫酸鹽生成的影響。結果表明NO2與TMI對於形成液相硫酸鹽的貢獻可能存在季節性的差異，在春、夏、秋三季兩個物種的貢獻較為相當。然而，在冬季卻發現液相硫酸鹽的產生很可能是依賴TMI的催化反應途徑，NO2更多的是扮演輔助的角色。 另外，根據本研究進行的PM對於前驅物變化的敏感性分析可以得知，微粒主要是分布在對硝酸氣（HNO3）敏感的區域。為了控制和減少顆粒物的形成，可以透過降低前驅物濃度和改變氣-粒相分配比例來採取措施。以台中地區為例，首要重點應該放在抑制HNO3的生成上，而在夏季，有必要同時減少HNO3和NH3的濃度；另一個策略為降低顆粒物的pH值以減少硝酸鹽的分配比例，這可以藉由降低大氣氨濃度和降低鈉、鎂、鈣離子貢獻來源而獲得改善。

Secondary Inorganic Aerosols (SIA) have been widely recognized in numerous studies as significant contributors to the mass concentration of fine particulate matter (PM2.5), primarily consisting of nitrate (NO3-) and sulfate (SO42-). This study employs odd oxygen (Ox) and aerosol liquid water content (ALWC) as indicators of photochemical reactions and aqueous-phase reactions, respectively. Coupled with real-time and long-term data on aerosol chemical compositions, gaseous pollutants, and meteorological conditions, this study investigates nitrate and sulfate formation mechanisms across different periods. The results indicate that both photochemical and aqueous-phase reactions influence the formation of nitrate and sulfate. However, it was found that the relative contributions of these two mechanisms to nitrate production vary with the seasons. In spring and summer, photochemical reactions may significantly impact nitrate formation, while in autumn, the contributions from both mechanisms are comparable. In winter, nitrate formation is likely more influenced by aqueous-phase reactions. Moreover, gas-phase oxidation reactions may consistently play an important role in sulfate formation throughout seasons. Aerosol acidity is also a key factor in SIA formation. This study also analyzes the impact of nitrogen dioxide (NO2) and transition metal ions (TMIs) on the formation of aqueous sulfate under conditions of higher aerosol acidity (pH < 5). The results indicate that the contributions of NO2 and TMIs to the formation of aqueous sulfate might vary by season. In spring, summer, and autumn, the contributions of both species are relatively comparable. However, in winter, it was found that the production of aqueous sulfate likely relies solely on the catalytic reaction pathways of TMIs, with NO2 playing more of an auxiliary role. Furthermore, based on the sensitivity analysis of PM to precursor perturbation, it is evident that the cases are primarily distributed in the nitric acid (HNO3)-sensitive domain. Measures can be taken to control and reduce particle formation by decreasing precursors concentrations and altering the gas-particle partition ratio. Specifically, in the Taichung area, the primary focus should be suppressing the formation of HNO3. Besides, during the summer, concurrently reducing the concentrations of both HNO3 and NH3 is necessary. Another approach is to lower the aerosol pH to decrease the partition ratio of nitrate, achievable through reducing atmospheric ammonia levels and suppressing the emission of sodium, magnesium, and calcium ions.