| 摘要: | 本研究主要目的為估算2001 年3 月TRACE-P (TRAnsport and Chemical Evolution over the Pacific)實驗期間，亞洲生質燃燒氣膠的輻 射衝擊。吾人結合中尺度氣象模式MM5、大氣傳送模式HYSPLIT 和太陽輻射傳送模式CLIRAD-SW，模擬生質燃燒所排放的黑碳 (Black Carbon, BC)和有機碳(Organic Carbon, OC)氣膠之空間分布特 性，進一步模擬其所造成的直接輻射驅動力(direct radiative forcing)和 測試其對氣象場的反饋作用。BC 和OC 氣膠的日排放源資料具有高 度空間和時間解析度，有助於提高模式模擬的準確性。 結果顯示，南亞地區(70o–110oE, 5o–30oN)的生質燃燒氣膠月平均 濃度為1.2 μg m-3，最大值為14.1 μg m-3，發生在緬甸西部，同時發 現一個3 km 厚的生質燃燒氣膠層存在南亞地區上空，且延伸至印度 洋和西太平洋。生質燃燒氣膠光學厚度(Aerosol Optical Depth, AOD) 最大值出現在緬甸西岸達0.14，其中BC 氣膠的貢獻較OC 氣膠顯著， 尤其發生在排放源區附近時。在晴天空的情況下，生質燃燒氣膠所造 成的月平均大氣層頂輻射驅動力介於-1.81(海洋上)－1.08(陸地上) W m-2 之間，地表輻射驅動力介於-0.04－-9.48 W m-2 之間。全天空的情 況下，生質燃燒氣膠所造成的月平均的大氣層頂輻射驅動介於 -1.65(海洋上)－1.42(陸地上) W m-2 之間，地表輻射驅動力介於-0.03 －-9.06 W m-2 之間。比較晴天空和全天空的結果顯示，雲的存在有助 於增加大氣層頂和地表之輻射驅動力。大氣層頂輻射驅動力的海陸分 布特性主要與BC 和OC 氣膠光學特性、OC 與BC 之AOD 比值、地 表反照率有關。 生質燃燒氣膠減少抵達地表的太陽輻射，並加熱大氣，形成地表 局部性的冷卻及低層大氣增溫，其中以BC 氣膠的影響更為顯著，所 造成地表輻射驅動力為OC 氣膠的5-7 倍，且有較高的大氣加熱率。 生質燃燒氣膠造成的大氣加熱率改變量可達6 ºC month-1，發生在排 放源區附近地表至4 公里的高空，海洋和陸地上的大氣間存在著很強 的大氣加熱率改變量水平梯度，可能影響局部環流之發展。經由模擬 生質燃燒氣膠地表直接輻射驅動力對氣象場的反饋作用測試，顯示排 放源區附近地表溫度降溫可達2 ºC month-1，然而，但目前仍然缺乏 一個完整且具雙向反饋機制之模式，以探討大氣中動力與熱動力過程 將如何改變，及其對區域氣象場之衝擊，此可做為未來工作重點。地 表輻射量的改變將造成氣象場在空間上重新分布，海平面氣壓場有 -2.5－0.5 hPa month-1 的變異，月平均雲量分布有±20 %的變異，月累 積雨量達±500 mm 的變異，凸顯生質燃燒氣膠對於區域氣候變遷有不 可忽視之影響。 The purpose of this study is to estimate the regional radiative impact of Asian biomass burning aerosols during the experimental period of Transport and Chemical Evolution over the Pacific (TRACE-P) in March, 2001. Integration of the Fifth-Generation NCAR / Penn State Mesoscale Model (MM5), USA NOAA Hybrid Single-Particle Lagrangian Integrated Transport model (HYSPLIT) and a solar radiative transfer model (CLIRAD-SW) allow us to simulate the spatial and temporal distributions of black carbon (BC) and organic carbon (OC) aerosols from biomass burning. It also allows us to estimate further their aerosol optical properties and radiative forcing. Emissions of BC and OC aerosols from biomass burning sources were based on a higher spatial and temportal resolution of emissions during TRACE-P. The results show that the monthly mean surface concentration of OC and BC is 1.2 μg m-3 in the South Asian region (70o–110oE, 5o–30oN). Western Myanmar has the maximum value, with the concentration reaching 14.1 μg m-3. There is a persistent aerosol layer with a thickness of 3 km over most of the South Asian region, and the plumes of biomass burning aerosols extend far to the Indian Ocean and the western Pacific Ocean. The aerosol optical depth (AOD) of the biomass burning carbon aerosols reaches a maximum value of 0.14 over western Myanmar. Compared to the OC aerosol, the BC aerosol makes a remarkable contribution to the AOD, especially in the source region. The monthly mean clear-sky direct shortwave radiative forcing ranges from -1.81 (sea) to 1.08 (land) W m-2 at the top of the atmosphere and from -0.04 to -9.48 W m-2 at surface. The monthly mean all-sky direct shortwave radiative forcing ranges from -1.65 (sea) to 1.42 (land) W m-2 at the top of the atmosphere and from -0.03 to -9.06 W m-2 at surface. The existence of cloud contributes a positive increase of radiative forcing. Owing to the spatial distributions of the AOD ratio (OC/BC) and the surface albedo, there is a sea-land distribution of radiative forcing at the top of the atmosphere. Biomass burning aerosols result in less solar irradiance reaching the Earth’s surface, but greater heating in the lower atmosphere, particularly for the BC aerosols, which have stronger atmosphere radiative forcing and the atmospheric heating rate. The BC aerosols cause surface radiative forcing 5-7 times more than that due to the OC aerosols. The biomass burning aerosols result in an increase of the atmospheric heating rate up to 6ºC month-1 in lower atmosphere of the source region. There is a strong horizontal gradient of heating rate near the source regions, which may modify local circulations. We test the regional meteorological feedback due to biomass burning aerosol surface radiative forcing, and find that the surface temperature decreases 2ºC month-1 in the same region. Meanwhile, monthly sea level pressure and cloud mounts in the domain vary in -2.5－0.5 hPa and 20 %, respectively. The accumulate precipitation varies more than ± 500 mm in the southern Asia in March, 2001. The results imply the biomass burning aerosols have significant influences on the regional climate change. However, a more complete feedback mechanism included in the model is needed for a rationable investigation on how the radiative forcing affects the regional meteorological variations. |
