劉改過,曾 勇*,閆鐵柱

城市河流溫室氣體濃度及排放通量的時空特征

劉改過1,曾 勇1*,閆鐵柱2

(1.中國石油大學(xué)(北京)化學(xué)工程與環(huán)境學(xué)院,北京 102249；2.中國農(nóng)業(yè)科學(xué)院農(nóng)業(yè)資源與農(nóng)業(yè)區(qū)劃研究所,農(nóng)業(yè)農(nóng)村部面源污染控制重點實驗室,北京 100081)

采用頂空氣相色譜法和模型法,研究北運河冬夏季的二氧化碳(CO2)、甲烷(CH4)、氧化亞氮(N2O)濃度和水氣界面排放通量,同時監(jiān)測河流的理化指標.結(jié)果表明,河流中CO2、CH4和N2O的濃度范圍為2.31～189.69, 0.05～6.11, 0.02～0.28μmol/L, 通量范圍為27.60～548.02, 1.10～12.43, 0.181～0.500mg/(m2·d), CH4和N2O的濃度及通量高于非城市河流.北運河CO2、CH4濃度及通量表現(xiàn)出時空特異性,濃度和通量最高的區(qū)域是混合區(qū)Ⅱ,且夏季高于冬季;N2O的濃度及通量在冬季農(nóng)業(yè)區(qū)較高,其季節(jié)特征不明顯.河流CO2濃度及通量與水溫、銨態(tài)氮(NH4-N)呈顯著正相關(guān),與溶解氧(DO)、pH值呈負相關(guān);CH4濃度及通量與DO、硝態(tài)氮(NO3-N)呈顯著負相關(guān),與水溫、NH4-N呈正相關(guān);鹽度、總?cè)芙庑怨腆w(TDS)、電導(dǎo)率、總磷(TP)是影響河流N2O濃度及通量的主要因素.

溫室氣體；北運河；時空特征；主導(dǎo)因子

溫室效應(yīng)是全人類面臨的環(huán)境問題,大氣中溫室氣體濃度的不斷增加是引起溫室效應(yīng)的主要原因[1].全球河流每年CO2、CH4和N2O的排放量分別為18億t碳、0.23億t碳和7.28萬t氮[2-4],其中CO2和CH4的排放量分別占內(nèi)陸水域(河流、湖泊、水庫)的86%和15%[2,5],N2O約占全球人為排放的10%[6],因此河流是大氣溫室氣體的重要排放源,但較少受到重視[7].

受人類活動影響的城市河流碳、氮負荷增加,成為溫室氣體的人為排放源[8-10],城市河段的水體CO2和CH4濃度約為非城市河段的2倍,上游為農(nóng)業(yè)區(qū)而下游為城市區(qū)的河段溫室氣體濃度明顯升高[11].河流理化性質(zhì)、人口密集程度、農(nóng)業(yè)生產(chǎn)等會影響河流溫室氣體的濃度,導(dǎo)致其溫室氣體的排放通量具有顯著時空特征[12-18].高水溫促使有機物分解,降低水中氣體的溶解度;高流速及風速通常會增強空氣與水體的交換[17,19];較低的pH值使得水體CO2易形成飽和狀態(tài);均會促進溫室氣體的排放.DO和營養(yǎng)鹽濃度分別通過影響有機物的降解和水生生物的生長代謝間接影響溫室氣體的排放[12,20].河流CO2和CH4排放量通常夏季最高[21],而N2O的濃度及通量呈現(xiàn)秋冬季較高的模式[17].目前內(nèi)陸水域溫室氣體的研究大多集中在水庫[22-25]、自然河流和農(nóng)業(yè)河流[26-29],城市河流溫室氣體的數(shù)據(jù)記錄較少.河流溫室氣體的濃度和排放通量在空間和時間上都是可變的[30],與流域面積不是簡單的正比例關(guān)系[31],IPCC推薦計算N2O排放量的間接排放因子(EF5r)是基于有限的實地研究[32],沒有對地形、氣候、流域景觀進行區(qū)分,因此使用統(tǒng)一的EF5r可能會使城市河流N2O的排放量被嚴重低估[33-35].因此開展河流溫室氣體排放的測量工作,對全球溫室氣體排放量的核算與評估有重要的作用.

北運河是典型的城市河流,是北京市內(nèi)面積最大且支流最多的水系[36],隨著城市化不斷推進,人為輸入的污染負荷不斷累積,成為當?shù)販厥覛怏w排放的潛在來源.本文以北運河為研究對象,核算城市河流的CO2、CH4和N2O濃度及水氣界面排放通量,分析其時空特征;同步監(jiān)測水溫、pH值、鹽度、DO、TDS、電導(dǎo)率和氧化還原電位等河流理化性質(zhì),探討影響城市河流溫室氣體排放的主導(dǎo)環(huán)境因子.

1 材料與方法

1.1 研究區(qū)概況

北運河水系(115°53'36.25"～116°55'38.96"E, 39°35'55.72"～40°26'28.32"N)發(fā)源于昌平燕山南麓,北京段流域面積約4250km2[37],主河道長約89.4km[36].河流以北關(guān)閘為界,上游為溫榆河,下游為北運河,沙河、清河、壩河、通惠河和涼水河是其主要支流,如圖1(源自國家基礎(chǔ)地理信息中心和Globeland30全球地理信息網(wǎng)站,經(jīng)ArcGIS軟件處理).流域內(nèi)不同河段的土地利用格局存在顯著差異[38],河流上游主要分布有農(nóng)田和村鎮(zhèn),中游主要分布城鎮(zhèn)市區(qū),下游主要分布工業(yè)區(qū)、農(nóng)田及村鎮(zhèn).流域地處溫帶大陸性季風氣候,冬季寒冷干燥,夏季炎熱多雨,平均氣溫24.8℃,多年平均降水量約600mm[39].

圖1 北運河位置和采樣點分布

1.2 采樣點設(shè)置

通過相關(guān)文獻資料及實地考察,以流域內(nèi)土地利用類型為主要依據(jù),將北運河流域分為4個研究區(qū)域:(1)人為影響較少、農(nóng)田和城鎮(zhèn)分布的河流上游區(qū)(混合區(qū)Ⅰ);(2)幾乎全部城鎮(zhèn)化的流域區(qū)域(城市區(qū));(3)農(nóng)田和城鎮(zhèn)及工廠密集分布的河流中下游區(qū)(混合區(qū)Ⅱ);(4)大量農(nóng)田為主的河流區(qū)域(農(nóng)業(yè)區(qū)).本次共設(shè)置20個采樣點(圖1),具體信息如表1所示.

表1 采樣點信息

1.3 樣品采集及分析

分別于2021年7月與2022年1月進行野外采樣,采集河流水面以下20cm處的水樣100mL于頂空瓶,設(shè)置3個平行樣于4℃以下環(huán)境保存.

使用頂空平衡-氣相色譜法測量河流溫室氣體濃度.首先將頂空瓶中的水樣排出50mL,注入50mL的高純氮氣(濃度大于99.999%),使氣液體積比為1:1,然后將頂空瓶放入振蕩器振蕩10min,再靜置30min至氣液平衡,取瓶內(nèi)的氣體26mL于氣相色譜進樣瓶,使用氣相色譜儀進行CO2、CH4、N2O濃度檢測.氣相色譜儀(島津GC2020plus)工作時的色譜柱溫度為50℃,載氣為高純氮氣,CO2和CH4測定采用氫火焰(FID)檢測器,工作溫度250°C,N2O測定用電子捕獲檢測器(ECD),工作溫度250℃.

采集河流水面上方2m的氣體26mL于頂空瓶保存,使用氣相色譜儀分析大氣中CO2、CH4、N2O的濃度.同時采集500mL水樣于聚乙烯瓶中,用于測定水質(zhì)參數(shù).河流的銨態(tài)氮(NH4-N)、硝態(tài)氮(NO3-N)、總氮(TN)、總磷(TP)濃度采用連續(xù)流動化學(xué)分析儀分析,溶解性有機碳(DOC)濃度使用總有機碳分析儀測定.河流的水溫、pH值、鹽度、DO、TDS、電導(dǎo)率和氧化還原電位使用便攜式水質(zhì)分析儀測量.氣溫和風速來源中央氣象網(wǎng)站(https: //weather.cma.cn/).

1.4 數(shù)據(jù)分析與計算

河流CO2、CH4、N2O濃度計算公式如下[40-42]:

bunsen系數(shù)是在一個標準大氣壓的條件下,單位體積的純?nèi)軇┤芙鈿怏w溶質(zhì)的最大體積,的計算公式為[41]:

式中:H為采樣時水溫和標準大氣壓條件下的CO2、CH4、N2O的Henry常數(shù),μmol/(L·Pa),其計算公式如下:

河流水氣界面溫室氣體的排放通量基于擴散氣體交換的薄邊界模型法進行計算,計算公式為:

式中:s是由采樣點水溫修正的溫室氣體的施密特數(shù),具體計算方法見文獻[46];10是大氣高空10m處的長期風速,m/s.

2 結(jié)果與分析

2.1 河流溫室氣體濃度與通量

北運河CO2濃度及水氣界面排放通量見圖2(a,b),河流CO2濃度范圍為2.31～189.69μmol/L,平均值為(59.83±56.38)μmol/L;CO2排放通量范圍為27.60～548.02mg/(m2·d),平均值為(237.58±220.82) mg/(m2·d).夏季CO2平均濃度及通量均為冬季的10倍左右;對CO2濃度及通量,混合區(qū)Ⅱ的平均值稍高于其他區(qū),但其差異不顯著.

圖2 北運河流域冬夏季CO2、CH4、N2O濃度及水氣界面通量

圖(b)(d)(f)中誤差棒為數(shù)據(jù)的標準偏差,文中以(平均值±標準偏差)給出,a、b表示流域之間95%置信水平的統(tǒng)計差異

北運河CH4濃度及水氣界面排放通量見圖2(c,d).河流CH4濃度范圍為0.05～6.11μmol/L,平均值為(1.16±1.42)μmol/L;CH4排放通量范圍為1.10～ 12.43mg/(m2·d),平均值為(4.45±5.42)mg/(m2·d).夏季CH4濃度及通量均高于冬季,夏季約為冬季的4.5倍;夏季混合區(qū)Ⅱ的CH4濃度及通量高于其他區(qū)域,并且和混合區(qū)Ⅰ有顯著差異,而冬季各區(qū)差異不明顯.

北運河N2O濃度及水氣界面排放通量見圖2(e,f).河流N2O濃度范圍為0.022～0.276μmol/L,平均值為(0.088±0.049)μmol/L;N2O排放通量的范圍為0.181～ 0.500mg/(m2·d),平均值為(0.349±0.190)mg/(m2·d). N2O濃度及通量的季節(jié)差異不明顯;夏季混合區(qū)的排放通量略高,冬季農(nóng)業(yè)區(qū)較高,但無顯著差異.

2.2 河流溫室氣體濃度及排放通量影響因素

河流水氣界面溫室氣體濃度及排放通量受多種環(huán)境因子影響,根據(jù)Pearson相關(guān)性分析(表2),河流CO2濃度與水溫、NH4-N呈顯著正相關(guān)(<0.01),與DO、pH值、NO3-N、總氮呈顯著負相關(guān)(<0.01);河流CH4濃度與溶解氧、pH值、NO3-N顯著負相關(guān)(<0.01),和水溫、氧化還原電位、NH4-N顯著正相關(guān)(<0.01);河流N2O濃度的影響因素較少,與鹽度、TDS、TP呈正相關(guān)關(guān)系.分析結(jié)果表明影響溫室氣體濃度的因子,會進一步影響溫室氣體的排放通量,CO2、CH4、N2O通量的環(huán)境影響因子與濃度的影響因子基本相同,相比N2O濃度,N2O通量與TP無顯著相關(guān)關(guān)系.

表2 河流溫室氣體濃度和通量與環(huán)境變量的Pearson相關(guān)性系數(shù)

注:表示濃度,表示河流水氣界面排放通量(觀測樣本數(shù)=40);*表示雙尾檢驗在0.05級別顯著相關(guān),**表示雙尾檢驗在0.01級別顯著相關(guān).

3 討論

3.1 城市河流溫室氣體濃度及排放通量與其他河流比較

如表3所示,就溫室氣體濃度來說,除南苕溪外,北運河的CO2濃度略小于其他城市及非城市河流,CH4濃度與其他河流數(shù)值相近,N2O濃度明顯高于非城市河流,說明河流N2O濃度受人為活動影響更明顯;就排放通量來說,北運河CO2、CH4、N2O的排放通量遠小于巢湖流域[7]、錫林河[47]、Zambezi[15]等非城市河流,也小于海河[48]、梁灘河[11]和Korogoro Creek[49]等城市河流.

表3 不同研究中河流溫室氣體的濃度及排放通量

同一研究中,城市河流溫室氣體高于非城市的河流[7,50].城市河流人為排入的污水中直接含有CH4、N2O等氣體[9],并且有大量促使溫室氣體產(chǎn)生的有機質(zhì),使得城市成為溫室氣體排放的熱點.一些河流的溫室氣體排放量出現(xiàn)負值[48,51],相較其他研究,北運河流域溫室氣體排放量較低,但均為正值,是大氣溫室氣體的“源”,因此城市河流溫室氣體的排放仍是不可忽視的.

3.2 城市河流溫室氣體濃度及通量的時空特征

夏季CO2、CH4濃度及通量表現(xiàn)出空間特異性,混合區(qū)Ⅱ的CO2、CH4濃度及通量高于其他區(qū)域.其主要原因有以下幾點:(1)城市區(qū)人口分布密集且發(fā)展迅速,污水排放多,但該區(qū)溫室氣體的濃度及通量的水平不高,其原因是入河C、N污染物分解的滯后性[27].混合區(qū)Ⅱ處城市區(qū)下游,接受來自市區(qū)的污染物,大量有機物及夏季高溫為河流CO2、CH4的產(chǎn)生提供條件,導(dǎo)致CO2、CH4濃度及排放通量較高.(2)混合區(qū)Ⅱ有大量的污水處理廠[38],處理廠排入河流的污水本身含有高濃度溫室氣體[52-53].(3)混合區(qū)Ⅱ周圍分布密集的農(nóng)田和工業(yè)建設(shè)開發(fā)區(qū),降雨導(dǎo)致的地表徑流使得河流營養(yǎng)負荷增加.以上所述原因均使得混合區(qū)Ⅱ的CO2、CH4濃度及通量高于其他區(qū)域.冬季農(nóng)業(yè)區(qū)N2O的濃度及通量明顯高于其他區(qū)域,其主要原因與耕地面積和農(nóng)業(yè)活動有關(guān)[35,54],積雪融化會將農(nóng)業(yè)土壤中的過量的氮肥帶入河流[55],增加河流溶解無機氮的負荷.

北運河流域CO2、CH4濃度及排放通量具有明顯的季節(jié)特征.北運河夏季CO2的平均濃度及排放通量約為冬季的10倍,CH4的平均濃度和通量約為冬季的4.5倍,具有夏高冬低的特點,其他河流也得出類似結(jié)論[13,15,21].夏季流域CO2、CH4的濃度及通量遠高于冬季,其根本原因是溫度季節(jié)性變化.夏季高溫會降低氣體溶解度、激發(fā)微生物活性、提高有機物的降解速率、藻類和水生生物的生物量增加產(chǎn)生較強的呼吸作用,因此CO2和CH4濃度及排放量升高[56-57].北運河冬夏季N2O的平均濃度和通量相當,季節(jié)模式不明顯.

3.3 河流溫室氣體排放的主導(dǎo)因子分析

河流CO2、CH4濃度及通量的主導(dǎo)因子為水溫、DO、NH4-N[11,58-60],此外pH值還是水體CO2的主要影響因子.水溫通過直接(氣體的溶解度)和間接作用(水體生物活動、有機物降解)影響河流CO2、CH4的排放.河流碳、氮等營養(yǎng)物質(zhì)輸入及高溫會刺激異養(yǎng)代謝,增加DO的消耗,促進CO2的生成,反之,水體DO含量決定水體中有機物降解途徑及其產(chǎn)物[50],厭氧狀態(tài)時,河流的有機碳在微生物作用下分解為CO2、H2和乙酸,H2及乙酸被產(chǎn)甲烷菌利用生成CH4[20],DO濃度升高時,厭氧環(huán)境被破壞,產(chǎn)甲烷菌活性減弱,水體中CH4被氧化,CH4濃度降低.NH4-N會影響水生植物初級生產(chǎn)力和浮游生物新陳代謝過程,間接影響水體CO2含量[18];NH4+與CH4具有相似結(jié)構(gòu),會在甲烷單加氧酶催化作用中取代CH4,而且氨氧化菌與甲烷氧化菌爭奪氧氣,從而抑制CH4氧化[61],減少CH4消耗,因此CO2、CH4濃度及通量均與NH4-N含量顯著正相關(guān)[62],和上海城市河流[63]、太湖[44]、天津河流[50]的研究結(jié)論一致.pH值會影響河流碳酸鹽平衡,進而影響CO2濃度[11].

河流N2O濃度及通量的主要影響因子為鹽度、TDS、電導(dǎo)率和TP.水體N2O主要作為副產(chǎn)物在微生物硝化、反硝化、硝化-反硝化耦合作用等過程產(chǎn)生的[64-65].硝化作用是微生物將氨(NH3)或NH4-N氧化為NO3-,在NH3或NH4-N氧化成NO2-時,N2O作為中間產(chǎn)物形成[66];反硝化是氮氧化物(NO3-或NO2-)還原為氣態(tài)(NO、N2O和N2)的過程[67].其復(fù)雜的形成機制導(dǎo)致N2O濃度與環(huán)境變量之間影響關(guān)系難以識別.充足的N是形成N2O的先決條件,鹽度、TDS、電導(dǎo)率通過影響水體N元素的遷移轉(zhuǎn)化,從而間接影響河流N2O的產(chǎn)生[68].研究表明N2O濃度受浮游植物生物量的影響[69],而磷是初級生產(chǎn)的限制因子,因此N2O與TP濃度有相關(guān)關(guān)系.N2O主要是微生物的活動產(chǎn)生的,而水溫影響微生物的活性,因此普遍推測河流N2O的濃度及通量與水溫可能有相關(guān)關(guān)系[20],但本研究發(fā)現(xiàn)水溫并不是N2O的主導(dǎo)因子,和多數(shù)學(xué)者得出的研究結(jié)論一致[11,17,70-71],其主要原因是河流N2O的產(chǎn)生更受水體中有效性N的影響.當水體中的NH4-N和NO3-N水平保持相對恒定時,水溫是N2O重要的主導(dǎo)因子,但是如果水體中的有效性N元素含量是可變的,并且不受溫度的影響,則水溫和水體N2O的產(chǎn)生無相關(guān)關(guān)系[72].本研究的采樣點分布在不同區(qū)域,水體的NH4-N和NO3-N濃度差異較大,因此水溫和N2O的濃度及通量無相關(guān)關(guān)系,進一步證實Quick等[68]提出的,相比水溫,N元素的有效性對微生物代謝的控制更強這一結(jié)論.有研究發(fā)現(xiàn)河流中N2O的濃度與TN濃度呈正相關(guān)[54],但本文并未發(fā)現(xiàn)N2O與含氮物質(zhì)有顯著相關(guān)關(guān)系,說明硝化和反硝化作用對水體N2O生成的主導(dǎo)作用仍不明確,可能與水體中NH4-N、NO3-N含量的比例有關(guān),是接下來的研究方向.

4 結(jié)論

4.1 北運河CO2、CH4和N2O的濃度范圍為2.31～ 189.69, 0.05～6.11, 0.02～0.28μmol/L,通量范圍為27.60～548.02, 1.10～12.43, 0.181～0.500mg/(m2·d),說明該河流向大氣排放氣體,是大氣溫室氣體的“源”.

4.2 北運河夏季CO2和CH4濃度及通量表現(xiàn)出明顯的空間特征,混合區(qū)Ⅱ的河流CO2、CH4濃度及通量最高;冬季農(nóng)業(yè)區(qū)N2O的濃度及通量明顯高于其他區(qū)域.CO2、CH4的濃度及排放通量具有夏高冬低的季節(jié)特征,而N2O的季節(jié)模式不明顯.

4.3 影響北運河CO2濃度及通量的環(huán)境因子為水溫、DO、pH值、NH4-N;影響CH4濃度及通量的因子為水溫、DO、NH4-N、NO3-N;影響N2O濃度及通量的因子有鹽度、TDS、電導(dǎo)率和TP.

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Seasonal and spatial variability of greenhouse gas concentration and emission flux in the urban river.

LIU Gai-guo1, ZENG Yong1*, YAN Tie-zhu2

(1.College of Chemical Engineering and Environment, China University of Petroleum, Beijing 102249, China；2.Key Laboratory of Nonpoint Source Pollution Control, Ministry of Agriculture and Rural Affairs, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China)., 2023,43(8)：4409～4417

The concentration and water-air interface emission flux of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) were investigated in both winter and summer seasons in the Beiyun River using headspace gas chromatography and model method. And physicochemical indicators of the river were also monitored. The results show that the concentration ranges of CO2, CH4, and N2O in the river were 2.31 to 189.69, 0.05 to 6.11, and 0.02 to 0.28μmol/L, respectively, while the flux ranges were 27.60 to 548.02, 1.10 to 12.43, and 0.181 to 0.500mg/(m2·d). The concentration and flux of CH4and N2O were higher in the urban river compared to non-urban rivers. The CO2and CH4concentration and flux in the Beiyun River showed spatio-temporal heterogeneity. The highest concentration and flux were found in the mixed area II, with higher concentration and flux observed in summer compared to winter. In winter, higher N2O concentration and flux were found in agricultural area, while N2O's seasonal variation remained insignificant. Additionally, the CO2concentration and flux of the river showed a significantly positive correlation with water temperature and ammonium nitrogen (NH4-N), and a negative relationship with dissolved oxygen (DO) and pH. CH4concentration and flux had a negative correlation with DO and nitrate (NO3-N), but a positive correlation with water temperature and NH4-N. Salinity, total dissolved solids (TDS), electrical conductivity, and total phosphorus (TP) were the primary factors affecting N2O concentration and flux in the river.

greenhouse gas；Beiyun River；temporal and spatial characteristics；main factors

X511

A

1000-6923(2023)08-4409-09

劉改過(1997-),女,寧夏固原人,中國石油大學(xué)(北京)碩士研究生,研究方向為生態(tài)水文與水體溫室氣體.liugaiguo2002@163.com.

劉改過,曾 勇,閆鐵柱.城市河流溫室氣體濃度及排放通量的時空特征 [J]. 中國環(huán)境科學(xué), 2023,43(8):4409-4417.

Liu G G, Zeng Y, Yan T Z.Seasonal and spatial variability of greenhouse gas concentration and emission flux in the urban river [J]. China Environmental Science, 2023,43(8):4409-4417.

2023-01-05

國家自然科學(xué)基金資助項目(52270203)

* 責任作者, 副教授, yongzeng1974@163.com