以微粒粒徑分佈之受體模式源解析解構大氣能見度劣化

Abstract

大氣能見度在臺灣一直受到公眾關注，因為能見度是人眼可直接感知空氣品質優劣的方式。氣膠消光係數被用於量化能見度且強烈地受到微粒粒徑分佈 (PSD) 的影響。有鑒於此，本研究在臺灣臺中地區架設IMPACT監測站，以探討在不同的能見度條件下，PSD的特徵及其來源。首先運用簡易去除氣象影響的方法於源解析模式，也就是擴散正歸化正矩陣因子法 (DN-PMF)。將11.8至2,500奈米的微粒數目粒徑分佈 (PNSD) 結合氣狀污染物、化學成分和環境氣膠消光係數 (bext, amb) 應用到源解析模型中，以估算微粒數目、表面積、體積和環境氣膠消光係數的來源貢獻。 DN-PMF一共分析出六個因子，包括新鮮的交通排放 (F1)、交通相關的來源 (F2)、老化的交通排放/工業排放 (F3)、富含硝酸鹽的二次氣膠/燃燒源 (F4)、臭氧相關的二次氣膠 (F5) 和受污染的海洋性氣膠 (F6)。DN-PMF在F4、F5和F6的晝夜變化行為中，呈現出比固有的PMF更明顯的來源特徵，為因子解讀上提供更可靠的證據。平均而言，F1（43.3%）、F2（32.3%）和F3（15.8%）共同主導微粒數目濃度，而F3、F4、F5和F6則在微粒表面積或體積濃度扮演重要角色，暗示F3、F4、F5和F6可能對微粒質量濃度及能見度具有一定程度的影響。此外，環境觀測和源解析的結果都表明積聚模範圍的微粒是能見度劣化的主因。表面積加權幾何平均粒徑的上升、表面積加權幾何標準差的下降及在300-1,000奈米最高的濃度增幅微粒，共同證實微粒傾向於集中在積聚模的粒徑範圍內。F3和F4對積聚模範圍的微粒貢獻最大，且兩者對bext, amb的合計貢獻接近80%。此結果顯示F3和F4是能見度劣化的主要貢獻來源，應優先實施針對這些來源的減量措施以改善能見度。最後，案例分析揭示了高壓東移及高壓迴流的綜觀天氣型態對能見度下降的重要作用，而通風係數 (VC) 的下降可以用來預警可能會發生能見度劣化事件。

Atmospheric visibility has been receiving public attention in Taiwan since it is a perceptible parameter of air quality by human eyes. Aerosol extinction coefficient (bext) is a strong function of the particle size distribution (PSD) and was used to quantify visibility. Given that, the IMPACT monitoring station was erected to investigate the characteristics and sources of PSDs under different visibility conditions from September 4th, 2020, to May 31st, 2021, in Taichung, Taiwan. A simple approach of removing weather influences, namely the Dispersion normalized PMF (DN-PMF), was applied to the particle number size distributions (PNSDs) from 11.8 to 2,500 nm coupling with the gaseous pollutants, chemical compositions, and ambient aerosol extinction coefficient (bext, amb) for estimating the source contributions of particle number, surface area, volume, and bext, amb. Apportioned factors are comprised of fresh traffic (F1), traffic-related source (F2), aged traffic/industrial emissions (F3), nitrate-rich secondary aerosol/combustion sources (F4), O3-associated secondary aerosols (F5), and contaminated marine aerosols (F6). The DN-PMF showed more distinct diel patterns in F4, F5, and F6 than the PMF, which provides more robust evidence for the factors' interpretation. On average, F1 (43.3%), F2 (32.3%), and F3 (15.8%) together dominated the particle number concentrations, while F3, F4, F5, and F6 play a significant part in either particle volume or surface area concentration. This implied that F3, F4, F5, and F6 might largely impact the particle mass concentrations or visibility. Furthermore, both observation and source apportionment results showed that the accumulation-mode particles were responsible for the visibility degradation. The increased surface area-weighted GMD, decreased surface area-weighted GSD, and highest concentration enhancement in 300-1,000 nm proved that particles tended to concentrate and reside in the accumulation mode size range. F3 and F4 contributed the largest proportion on the accumulation-mode particles and are attributed to nearly 80% of bext, amb. The results indicated that F3 and F4 are the main reason for visibility degradation, and the abatement targeting these sources should be prioritized for implementation. Finally, the case study revealed that the eastward movement of the high-pressure system and the High Pressure Peripheral Circulation (HPPC) type of synoptic weather also play a vital role in visibility degradation, and the decline in the ventilation coefficient (VC) can serve as an indicator of potential deterioration in visibility.