邊界層層積雲對臺灣中部地表氣膠濃度的影響

Influence of Sulfate Production in PBL-top Stratocumulus on Surface Aerosol Concentration over Central Taiwan

胡永新(Yung-Hsin Hu)

指導教授 ： 陳正平

國立臺灣大學/理學院/大氣科學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103729

摘要

本研究旨在探討邊界層層積雲對近地面硫酸根氣膠的影響。首先針對氣象模式WRF 3.8.1版本所模擬的邊界層大渦紊流傳輸與層積雲形成對模式解析度的敏感度，以2018年3月28日臺灣中部地區的個案進行模擬。當水平向解析度由3公里提高到1公里後，層積雲平均液態水含量減少，同時在3公里高度以下垂直速度的變異度明顯增加。垂直速度的變異度變大代表模式能解析出較小的渦流系統因而增加垂直向傳送。當模式垂直向解析度增加時，平均的液態水含量稍微降低，而垂直速度的變異度則相差不大。以WRF的模擬結果來驅動空氣污染模式CMAQ 4.7.1版進行化學場的模擬分析顯示，在平原區由於火力發電廠穩定排放二氧化硫，因此在火力發電廠附近及下風處的硫酸根濃度較上風處高。雲的位置及高度對近地面硫酸根也有顯著影響。清晨受低雲及霧水內液態氧化反應影響使近地面硫酸根濃度上升，但9點後因熱力作用霧水消散以及海風帶入新鮮空氣使近地面硫酸根濃度下降。午後因邊界層頂層積雲內液態氧化反應而成的硫酸根因大渦紊流而下傳回近地面使近地面硫酸根濃度增加。入夜後，層積雲消散近地面硫酸根濃度下降，但午夜後低雲及霧水生成使近地面硫酸根濃度再次上升。而洋面上顯著的硫酸根濃度變化主要與陸地上的二氧化硫濃度與風場的方向有關。白天海面上的硫酸根濃度變化較不明顯，但是入夜後陸風將二氧化硫擴散到海面，再加上火力發電廠附近的低雲液態氧化產升的硫酸根一起傳輸到洋面上，使午夜後洋面的硫酸根濃度明顯上升。
將模擬與2018中央研究院環境變遷研究中心於臺灣中部進行的採樣分析資料進行比較，發現在接近火力發電廠及其風向下游處的硫酸根濃度相差不大，但內陸地區的硫酸根氣膠明顯的低估，可能原因是模式明顯低估內陸的雲水。但內陸地區的近地面二氧化硫也低於觀測，顯示總硫濃度也偏低，不只是雲化學的影響。由於火力發電廠排放的二氧化硫氣體順北北東風下游擴散而不經過內陸，內陸也沒有明顯的二氧化硫排放源，因此也可能是模式低估了上風處二氧化硫排放。
在解析度的影響方面，水平解析度1公里垂直30層的模擬平均濃度值最高，而水平解析度3公里垂直45層的平均值最低，但差異只有0.3 g m-3。將硫酸根液態氧化形機制關閉，發現三組模擬的硫酸根氣膠濃度都有明顯下降，12小時的平均濃度大約降了24至29個百分比左右，而在平原南區液態氧化所形成的硫酸根可占總濃度五至七成，代表層積雲液態氧化反應對於近地面硫酸根氣膠濃度有明顯之影響。但模式解析度也有間接影響：當水平解析度增加時，可以解析出較小的渦流系統，將低層排放的二氧化硫較快速地送入雲中進行反應；但因所解析的大渦紊流更細緻，使得硫酸根氣膠高濃度的區域範圍較窄。而當提升垂直方向的解析度為45層時，對於底層大氣掌握度較高，因此較接近實際觀測使氣溫溫度較30層高，但水氣向上的通量卻沒有顯著增加使得雲水凝結減少，因此液態氧化反應量下降，造成硫酸根濃度減少。

關鍵字

氣膠 ； 硫酸根 ； 液態氧化反應 ； 邊界層 ； 大渦紊流 ； 層積雲

並列摘要

This study investigated how planetary boundary layer top stratocumulus affects near-surface sulfate concentration. The WRF model version 3.8.1 was used to perform meteorological simulation and combine with the CMAQ model version 4.7.1 for air quality simulation. The case that occurred over Taichung on March 28th, 2018, was simulated to understand how the model performed on simulating the interaction of aerosol chemistry with planetary boundary stratocumulus.
The simulations showed that high sulfate concentrations exist downwind of the thermal power plant. Aqueous-phase oxidant reaction in low cloud and fog can cause near-surface sulfate concentration to increase over the plain area. However, after 9 a.m., fog dissipate over the plain while sea breeze brought fresh maritime air to land, causing sulfate concentration to drop. The aqueous-phase oxidant reaction in PBL-top stratocumulus cause sulfate concentration to increase again in the afternoon. In the nighttime, after PBL-top stratocumulus dissipates, the aqueous-phase oxidant reaction rate ceased to cause near-surface sulfate concentration to drop. After mid-night, low cloud or fog appeared over the plain area causing near-surface sulfate concentration to increase. Sulfate concentration over ocean is more related to SO2 concentration and wind direction on land. Sulfate concentration has no significant change in the daytime. In the nighttime, land breeze transport high concentration SO2 and sulfate to the sea to increase near sea surface sulfate concentration.
By comparing model simulation surface average concentration and 2018 observed data, sulfate concentration simulation is more accurate near the power plant and downwind areas. The simulation underestimated sulfate concentration at in-land areas, likely because of an underestimation in surface SO2 emission outside the model domain. The simulation results also showed that liquid phase oxidation in the stratocumulus clouds might contribute to the surface sulfate concentration by about 24 to 29 percent.
The model resolution tests showed that increasing model horizon resolution caused an increasing pattern on vertical velocity variance but a decrease in total liquid water content; while increasing vertical resolution caused an opposite effect on LWP in the Taichung area.
Also, a horizontal resolution of 1km produced the highest average surface sulfate aerosol concentration compared to the 3 km resolution; whereas the higher vertical resolution of 45 layers produced the lowest. The 12-hour average near-surface sulfate concentration between the simulations of different resolutions differed by about 0.3 g m-3. Increasing horizontal resolution can resolve finer large-eddy and cloud structures, which can make aqueous-phase SO2 oxidation concentrating in a smaller area and, cause a higher peak concentration near the surface. Increasing vertical resolution may cause a decrease in cloud water due to higher air temperature and thus lower humidity, leading to a lower aqueous-phase sulfate production and thus lower near-surface sulfate concentration.

並列關鍵字

Aerosol ； sulfate ； liquid phase oxidant reaction ； planetary boundary layer ； large eddy ； stratocumulus

參考文獻

陳正平、朱定中、蔡宜君，2015:104年度輔助空氣品質及境外污染物預報系統。53-56頁。EPA-104-L102-02-101

Altshuller, A. (1987). Potential contribution of sulfate production in cumulus cloud droplets to ground level particle sulfur concentrations, U.S. Environmental Protection Agency, Washington, D.C., EPA/600/J-87/076 (NTIS PB88125554).

Berg L. K., Shrivastava M., Easter R. C., Fast J. D. ,Chapman E. G., Liu Y. Ferrare R. A. (2015). A new WRF-Chem treatment for studying regional-scale impacts of cloud processes on aerosol and trace gases in parameterized cumuli, Geosci. Model Dev., 8, 409-429.

Chang,S.C., Chou,C.C.K., Chen, C.C., Lee, C.T. (2010). Temporal characteristics from continuous measurements of PM2.5 and speciation at the Taipei Aerosol Supersite from 2002 to 2008, Atmospheric Environment 44, 1088-1096.

Conklin M. H. and Hoffmann M. R. (1988). Metal ion-S(IV) chemistry III.Thermodynamics and kinetics of transient iron(III)-sulfur(IV) complexes. Envir. Sci. Technol. 22, 891-898.