青藏高原大氣和雪冰黑碳的空間分布特征

唐祥祥，王 茉，徐柏青,3

(1.中國科學(xué)院青藏高原研究所，北京 100101； 2.中國科學(xué)院大學(xué)，北京 100049； 3.中國科學(xué)院青藏高原地球科學(xué)卓越創(chuàng)新中心，北京 100101)

青藏高原大氣和雪冰黑碳的空間分布特征

唐祥祥1,2，王 茉1，徐柏青1,3

(1.中國科學(xué)院青藏高原研究所，北京 100101； 2.中國科學(xué)院大學(xué)，北京 100049； 3.中國科學(xué)院青藏高原地球科學(xué)卓越創(chuàng)新中心，北京 100101)

青藏高原為亞洲主要河流提供水資源。青藏高原周邊地區(qū)排放的黑碳氣溶膠經(jīng)大氣環(huán)流可被傳輸至高原內(nèi)部，并沉降到雪冰表面，對降水和冰川物質(zhì)平衡產(chǎn)生重要影響?；仡櫧畮啄陙砬嗖馗咴髿夂脱┍谔伎臻g分布特征。結(jié)果表明：青藏高原大氣黑碳濃度在空間分布上呈現(xiàn)出由外圍向內(nèi)部逐漸降低、由低海拔向高海拔指數(shù)降低的趨勢；從季節(jié)變化看，西風氣候區(qū)大氣黑碳濃度夏季出現(xiàn)高值，季風氣候區(qū)則冬、春季出現(xiàn)高值；青藏高原雪冰黑碳含量具有同大氣黑碳濃度相一致的季節(jié)變化特征，并在空間分布上呈現(xiàn)出由南向北增加、由低海拔向高海拔降低的趨勢。

黑碳；空間分布；季節(jié)變化；大氣；雪冰；氣溶膠；排放；青藏高原

0 引 言

青藏高原被譽為亞洲“水塔”，擁有除南、北兩極之外最多的冰川儲備，其冰川融水是亞洲眾多河流的重要淡水補給，與全球60%人口的生活生產(chǎn)息息相關(guān)[1]。然而，青藏高原周邊地區(qū)人口密集，能源結(jié)構(gòu)和燃燒技術(shù)相對落后。包括黑碳(BC)氣溶膠在內(nèi)的污染物不但對人類健康造成危害[2]，同時也會產(chǎn)生一系列的氣候環(huán)境效應(yīng)，特別是對區(qū)域地-氣輻射收支、降水時空分布和冰川物質(zhì)平衡的影響[3-5]，引起了國際社會的廣泛關(guān)注[6-7]。

由生物質(zhì)和化石燃料燃燒排放的黑碳氣溶膠對輻射具有極強的吸收作用，進而對地球系統(tǒng)的能量收支和分布具有重要影響，是氣候環(huán)境變化不可忽視的影響因子[8-10]。首先，黑碳氣溶膠懸浮在大氣中可吸收更多來自太陽和地表的輻射能，對空氣柱產(chǎn)生加熱作用[11-12]。大氣環(huán)流模型模擬顯示，南亞黑碳氣溶膠吸收輻射對大氣的加熱作用可使喜馬拉雅山脈南麓、印度恒河平原上空2～5 km大氣層升溫0.6 ℃，這可能是過去50年來喜馬拉雅地區(qū)比全球平均增溫快1倍的原因之一。另有研究發(fā)現(xiàn)，青藏高原積雪變化的空間分布特征同大氣黑碳分布具有緊密的聯(lián)系，模擬結(jié)果顯示大氣黑碳可使青藏高原地區(qū)升溫1.3 ℃[13]。其次，黑碳氣溶膠可作為云凝結(jié)核，改變云滴微物理特性[14-15]，降低云反照率，加熱云滴及其周圍環(huán)境，使云滴蒸發(fā)，減少云中液態(tài)水含量[16]，抑制有效降水[11,17]。另外，沉降在積雪和冰川表面的黑碳能夠顯著改變冰雪表面的反照率[18]，在冰雪下墊面產(chǎn)生正輻射強迫，進而導(dǎo)致近地表氣溫升高。但近期有研究指出，青藏高原某些地區(qū)沙塵對雪冰表面反照率及輻射強迫的影響顯著高于黑碳所產(chǎn)生的影響[19]。Hansen等的模擬結(jié)果顯示，在1880～2002年間，海冰和雪中的黑碳所產(chǎn)生的地表正輻射強迫可使全球地表平均升溫0.17 ℃[20]。Jacobson認為燃燒排放的黑碳和有機碳在10年中可使近地表溫度上升0.27 ℃[21]。Flanner等使用SNICAR模型模擬的結(jié)果顯示，青藏高原雪冰黑碳所引起的瞬時強迫最大可達20 W·m-2[22]。由此可見：黑碳可通過加熱大氣和降低雪冰表面反照率使區(qū)域氣候變暖，誘發(fā)冰川、積雪融化；同時，黑碳可抑制季風活動和改變降水的時空分配[23-24]，進而造成冰川物質(zhì)平衡負增長。因此，青藏高原黑碳氣溶膠不但影響區(qū)域氣候和水循環(huán)過程，也會改變青藏高原冰川物質(zhì)平衡。

綜上所述，黑碳氣溶膠的這些氣候環(huán)境效應(yīng)使其成為青藏高原冰川變化的重要影響因子。青藏高原大氣和雪冰黑碳的觀測及其空間分布特征對黑碳氣溶膠在青藏高原區(qū)域氣候環(huán)境變化研究中具有非常重要的意義。本文重點對青藏高原大氣黑碳濃度和雪冰黑碳含量的空間分布特征進行了系統(tǒng)分析和整理。

1 大氣黑碳濃度的空間分布

青藏高原地區(qū)海拔3 000 m以上區(qū)域大氣黑碳平均濃度為(0.49±0.88)μg·m-3[25-46]，海拔4 000 m以上區(qū)域大氣黑碳平均濃度為(0.15±0.10)μg·m-3[25,30,33-34,38-40,42-44,46]。青藏高原大氣黑碳濃度高于南極(0.05～20 ng·m-3)[47-48]，但低于北極(1～10 μg·m-3)[18]，這表明青藏高原大氣受人為活動影響較小。一般而言，青藏高原高海拔地區(qū)大氣黑碳濃度低于低海拔地區(qū)，偏遠區(qū)域低于人口密集的城市。此外，青藏高原大氣黑碳季節(jié)變化特征也表現(xiàn)出明顯的氣候區(qū)差異[49]。

1.1 由外圍向內(nèi)部

圖1匯總了青藏高原及周邊地區(qū)大氣黑碳濃度觀測結(jié)果。由圖1可以看出，大氣黑碳濃度由高原外部向高原內(nèi)部呈現(xiàn)出降低趨勢。在海拔3 000 m以下的南亞，受人為排放影響較大的城鎮(zhèn)、鄉(xiāng)村地區(qū)大氣黑碳平均濃度為(15.91±9.18)μg·m-3[40,48-54]，受人為排放影響較小的偏遠地區(qū)大氣黑碳平均濃度為(1.02±0.30)μg·m-3[37-39,50,55-62]；中國中西部鄉(xiāng)村地區(qū)大氣黑碳平均濃度為(3.93±0.21)μg·m-3[27]，而在中西部偏遠地區(qū)則為(0.66±0.63)μg·m-3[27,29,63-64]。值得指出的是，南亞受人為排放較小的偏遠地區(qū)通常也是位于喜馬拉雅山脈南側(cè)海拔較高的地區(qū)(約2 000 m)。在人口相對密集、排放較為強烈的城市(如拉薩)，大氣黑碳濃度相對于青藏高原內(nèi)陸其他地區(qū)高出1到2個數(shù)量級[27-28]。雖然在旅游旺季來源于青藏高原本地的人類活動影響排放增加[30,65]，但整體而言，由于青藏高原地勢高聳、人口稀薄，大氣黑碳濃度接近本底值。這也說明盡管南亞排放的污染物可在印度季風的作用下被傳輸至青藏高原內(nèi)陸區(qū)域[30,65-66]，但喜馬拉雅高聳的山脈可以有效地阻擋大部分黑碳氣溶膠向高原內(nèi)部輸入。

大氣黑碳濃度單位為μg·m-3；數(shù)據(jù)引自文獻[25]～[65]圖1 青藏高原及周邊地區(qū)大氣黑碳濃度空間分布Fig.1 Spatial Distribution of Concentration of Atmosphere Black Carbon over Qinghai-Tibet Plateau and Surrounding Area

1.2 海拔變化

青藏高原大氣黑碳主要來源于高原外部低海拔區(qū)域的排放。研究表明，在西風和印度季風作用下，青藏高原黑碳主要來源于中亞—東歐和南亞[5,67-68]。在大氣黑碳傳輸過程中，受青藏高原的機械抬升作用，大氣黑碳濃度由低海拔向高海拔呈現(xiàn)出指數(shù)降低的變化趨勢(圖2)。此外，由圖2可以看出，盡管拉薩海拔較高，但受當?shù)厝藶榕欧庞绊?，大氣黑碳濃度顯著高于青藏高原其他地區(qū)。

數(shù)據(jù)引自文獻[25]～[65]圖2 大氣黑碳濃度隨海拔高度的變化Fig.2 Variations of Concentration of Atmosphere Black Carbon Corresponding to Elevation

1.3 季節(jié)變化

青藏高原大氣黑碳濃度的季節(jié)變化受排放源、大氣環(huán)流和當?shù)爻两禇l件等因素影響。在受人為活動影響較大的城市，大氣黑碳濃度在冬、春季出現(xiàn)最高值，這主要來源于當?shù)鼐用袢紵?、牲畜糞便等致使黑碳排放量增加，同時干燥的區(qū)域氣候條件不利于大氣顆粒物沉降；在較為偏遠的地區(qū)，大氣黑碳濃度季節(jié)變化主要取決于大氣環(huán)流和當?shù)爻两禇l件。本文僅討論受人為活動影響較小的觀測點大氣黑碳濃度的季節(jié)變化。

青藏高原南部至東南部區(qū)域，受季風影響顯著，氣候濕潤、季風盛行的夏季降水量高，在此稱之為季風氣候區(qū)；青藏高原西北部和內(nèi)陸北部地區(qū)，常年受西風急流影響，降水少，氣候干燥，在此稱之為西風氣候區(qū)。已有研究顯示：西風氣候區(qū)大氣黑碳濃度呈現(xiàn)出夏季較高的趨勢(如慕士塔格、塔克拉瑪干、祁連山和北麓河)；季風氣候區(qū)則呈現(xiàn)夏季較低的季節(jié)變化特征(如薩拉斯瓦蒂、庫魯、馬諾拉峰、穆格代斯沃爾、NCO-P、林芝、然烏和瓦里關(guān))[25-64](圖3、4)。西風氣候區(qū)大氣黑碳濃度的高值主要同大氣邊界層活動活躍、地表污染物傳輸增強相關(guān)聯(lián)；季風氣候區(qū)則主要與區(qū)域季節(jié)性降水增強、大氣污染物濕沉降增加相關(guān)。

數(shù)據(jù)引自文獻[25]～[64]圖3 大氣黑碳月平均濃度季節(jié)變化Fig.3 Seasonal Variations of Monthly Mean Concentrations of Atmosphere Black Carbon

圖4 觀測點地理位置Fig.4 Locations of Observation Sites

2 雪冰黑碳含量的空間分布

同兩極相比，青藏高原雪冰黑碳含量(質(zhì)量分數(shù)，下同)略高((45～50)×10-9)。北極地區(qū)雪冰黑碳含量為30×10-9[69]，格陵蘭地區(qū)為3.0×10-9[20]，而南極雪冰黑碳僅為0.2×10-9[18]。在水平方向上，青藏高原雪冰黑碳含量由南向北呈現(xiàn)微弱增加趨勢；在垂直方向上則呈現(xiàn)出隨海拔升高而降低的趨勢。

2.1 區(qū)域分布

雪冰黑碳含量單位為10-9；數(shù)據(jù)引自文獻[3]、[5]、 [63]、[68]、[72]～[79]圖5 雪冰黑碳含量空間分布Fig.5 Spatial Distribution of Contents of Snow Black Carbon

青藏高原雪冰黑碳含量具有較大的空間分布差異(圖5)。納木那尼冰川表雪的雪冰黑碳含量僅為4.3×10-9，而在烏魯木齊1號冰川表雪的雪冰黑碳含量達155.5×10-9，二者相差2個數(shù)量級。從表1可以看出：各區(qū)域平均雪冰黑碳含量從小到大依次為藏東南地區(qū)、喜馬拉雅山地區(qū)、祁連山地區(qū)、帕米爾高原地區(qū)、青藏高原中部地區(qū)、天山山脈地區(qū)；青藏高原雪冰黑碳含量在緯度梯度上呈現(xiàn)出由南向北升高的趨勢(圖6)。南亞密集的人類活動排放是青藏高原大氣和雪冰黑碳的重要來源，而雪冰黑碳含量在緯度上的這種變化趨勢表明，雪冰黑碳含量不但同大氣環(huán)流和排放源相關(guān)，同時也與當?shù)氐慕邓烤o密相關(guān)。在青藏高原季風氣候區(qū)年降雪量比西風氣候區(qū)高，特別典型的是在藏東南地區(qū)年降雪量可達3.5 m[70]，而在帕米爾高原地區(qū)年降雪量約為0.6 m[71]。在青藏高原緯度相對較低、受季風降水影響較大的區(qū)域，降水量顯著高于青藏高原內(nèi)陸和北部區(qū)域，這對雪冰黑碳具有“稀釋作用”，使得雪冰黑碳含量呈現(xiàn)較低值。隨著緯度升高，季風帶來的降水所產(chǎn)生的“稀釋作用”逐漸減弱，雪冰黑碳含量隨之升高。從本文調(diào)查數(shù)據(jù)來看，在北緯30.42°以南區(qū)域，雪冰黑碳含量隨緯度增加而升高；而在北緯30.45°以北區(qū)域，隨南亞季風輸送能力減弱，雪冰黑碳含量呈現(xiàn)出下降趨勢；天山山脈雪冰黑碳同青藏高原其他地區(qū)相比具有更高的含量，這很可能緣于其四周為地表干旱的荒漠，地表對底層大氣的加熱作用使得大氣對流活動加強，可更有效地將地表黑碳傳輸至高空大氣，進而形成天山山脈雪冰黑碳含量高值區(qū)(圖6)。

表1 青藏高原各區(qū)域雪冰黑碳平均含量Tab.1 Mean Contents of Snow Black Carbon inDifferent Regions of Qinghai-Tibet Plateau

天山山脈緯度范圍較窄，觀測數(shù)據(jù)較少，未標出擬合線； 數(shù)據(jù)引自文獻[3]、[5]、[70]、[73]～[77]圖6 雪冰黑碳含量緯度變化Fig.6 Variations of Contents of Snow Black Carbon Corresponding to Latitude

此外，青藏高原典型氣候區(qū)的雪冰黑碳含量具有同大氣黑碳濃度相一致的季節(jié)性變化特征。在典型的季風氣候區(qū)(藏東南)冰芯記錄中，黑碳呈現(xiàn)出顯著的冬、春季高，夏季低的季節(jié)變化特征[5,70]；而在典型的西風氣候區(qū)(東帕米爾高原)冰芯記錄中，黑碳呈現(xiàn)出夏季高、冬季低的季節(jié)變化特征[76]。在藏東南地區(qū)，冰芯黑碳冬、春季高值反映了南亞大氣棕色云的爆發(fā)以及西風帶南支和南亞季風的輸送，而夏季黑碳低值則反映了充沛的季風降水對區(qū)域大氣環(huán)境的清潔作用以及對雪冰黑碳的“稀釋作用”；在東帕米爾高原地區(qū)，冰芯黑碳夏季高值反映了夏季中亞地區(qū)頻繁的大氣對流活動對地表黑碳更有效的輸送[43,72]，而冬季低值則反映了較弱的大氣對流活動。

2.2 沿海拔梯度的變化

青藏高原雪冰黑碳含量在海拔高度3 500～6 500 m范圍內(nèi)表現(xiàn)出顯著地隨海拔升高而降低的趨勢[圖7(a)]。對于同一條冰川，表雪的雪冰黑碳含量同樣具有隨海拔升高而降低的變化特征[圖7(b)]。雪冰黑碳含量的這種沿海拔梯度分布特征主要由排放源的空間距離和污染物大氣動力學(xué)傳輸高度決定。另外，在冰川低海拔區(qū)，隨著積雪消融增強，融水流失，具有憎水特性的黑碳被保留在殘留的雪中，由此形成富集。黑碳沉積后在表雪中的富集作用可使雪冰黑碳含量增長約20倍，極值可達90倍[5,75]。雪冰黑碳的這種沉積后作用可顯著加強雪冰黑碳含量沿海拔增高而降低的空間分布特征。

數(shù)據(jù)引自文獻[3]、[5]、[70]、[73]～[77]圖7 不同海拔高度雪冰黑碳含量變化Fig.7 Variations of Contents of Snow Black Carbon Corresponding to Elevation

3 結(jié) 語

(1)青藏高原大氣黑碳濃度呈現(xiàn)出由外向內(nèi)降低的趨勢，體現(xiàn)出青藏高原周邊低海拔地區(qū)人為活動排放向高原內(nèi)陸傳輸?shù)慕Y(jié)果；另外，在青藏高原地勢機械阻擋和抬升作用下，大氣黑碳濃度呈現(xiàn)出由低海拔向高海拔指數(shù)降低的趨勢。

(2)在青藏高原受人為活動較小的偏遠地區(qū)，大氣黑碳濃度的季節(jié)性變化主要取決于大氣環(huán)流和區(qū)域降水條件。季風氣候區(qū)由于夏季降水顯著增強，大氣黑碳濃度呈現(xiàn)出冬、春季高，夏季較低的季節(jié)變化特征；西風氣候區(qū)則由于春、夏季近地表垂直對流活動加強，大氣傳輸效率增加，大氣黑碳濃度呈現(xiàn)出春、夏季較高，冬季較低的季節(jié)變化特征。

(3)青藏高原雪冰黑碳含量在緯度梯度上主要受降水條件影響，表現(xiàn)出由南至北含量升高的空間變化特征；而在海拔梯度上則主要受控于傳輸距離和傳輸高度，雪冰黑碳含量表現(xiàn)出隨海拔升高而降低的特征，同時低海拔地區(qū)黑碳沉積后的富集作用可顯著加強這種雪冰黑碳含量沿海拔增高而降低的分布特征。

(4)冰芯和表雪中雪冰黑碳含量具有與大氣黑碳濃度相一致的季節(jié)變化特征，即季風氣候區(qū)雪冰黑碳含量在冬、春季出現(xiàn)高值，西風氣候區(qū)則在春、夏季出現(xiàn)高值。

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SpatialDistributionCharacteristicsofAtmosphereandSnowBlackCarbonsinQinghai-TibetPlateau

TANG Xiang-xiang1,2, WANG Mo1, XU Bai-qing1,3

(1. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China; 3. Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China)

Qinghai-Tibet Plateau is the source of major rivers in Asia. Black carbon (BC) aerosol emits from surrounding regions can be transported to the inner Qinghai-Tibet Plateau by atmospheric circulation and consequently deposited in snow, which can significantly influence precipitation and mass balance of glaciers. Spatial distribution of atmosphere and snow black carbons in Qinghai-Tibet Plateau was reviewed. The results show that the concentrations of atmosphere black carbon gradually decrease from the outside to the inner of Qinghai-Tibet Plateau, and exponentially decrease with the increase of elevation; the concentrations of atmosphere black carbon in the westerly region of Qinghai-Tibet Plateau show high values in summer and low values in winter, whereas those in the monsoon region present high values in winter and spring seasons, and low values in summer; the contents of snow black carbon have the same seasonal characteristics of the concentrations of atmosphere black carbon, and decrease from the southern to the northern part of Qinghai-Tibet Plateau, and linearly decrease with the increase of elevation.

black carbon; spatial distribution; seasonal variation; atmosphere; snow; aerosol; emission; Qinghai-Tibet Plateau

X131.1；P427.2

A

1672-6561(2017)05-0695-09

2017-05-05

唐祥祥(1989-)，男，安徽阜陽人，中國科學(xué)院大學(xué)理學(xué)碩士研究生，E-mail:15321611017@163.com。

徐柏青(1969-)，男，安徽冬至人，研究員，博士研究生導(dǎo)師，理學(xué)博士，E-mail：baiqing@itpcas.ac.cn。