趙晨旭 徐 鵬 廖雅君 欒勝基,2#

(1.北京大學(xué)深圳研究生院環(huán)境與能源學(xué)院，城市人居環(huán)境科學(xué)與技術(shù)深圳市重點(diǎn)實(shí)驗(yàn)室，廣東 深圳 518055； 2.深港產(chǎn)學(xué)研基地環(huán)境模擬與污染控制重點(diǎn)實(shí)驗(yàn)室，廣東 深圳 518057)

氣態(tài)活性氮排放的環(huán)境影響研究進(jìn)展

趙晨旭1徐 鵬1廖雅君1欒勝基1,2#

(1.北京大學(xué)深圳研究生院環(huán)境與能源學(xué)院，城市人居環(huán)境科學(xué)與技術(shù)深圳市重點(diǎn)實(shí)驗(yàn)室，廣東 深圳 518055； 2.深港產(chǎn)學(xué)研基地環(huán)境模擬與污染控制重點(diǎn)實(shí)驗(yàn)室，廣東 深圳 518057)

大氣中過(guò)量的氣態(tài)活性氮積累導(dǎo)致大氣環(huán)境污染，對(duì)氣候變化、人體和生態(tài)系統(tǒng)健康都產(chǎn)生了不利影響。綜述了氣態(tài)活性氮排放清單估算方法的研究現(xiàn)狀。從氣候變化和大氣環(huán)境質(zhì)量?jī)蓚€(gè)方面討論了N2O、NH3和NOx等氣態(tài)活性氮的環(huán)境效應(yīng)，并對(duì)未來(lái)氣態(tài)活性氮的研究方向進(jìn)行了展望。

氣態(tài)活性氮 排放清單 氣候變化 大氣環(huán)境質(zhì)量 減控

氮循環(huán)是自然界中重要的物質(zhì)循環(huán)之一，而氮循環(huán)中的氮元素主要以不活潑的N2存在。在自然界中，不活潑的N2轉(zhuǎn)化成可被生物利用的活性氮(Nr)，主要通過(guò)閃電固氮和生物固氮完成；20世紀(jì)初，人類成功實(shí)現(xiàn)了人工合成氨(NH3)。自此以后，人類為了滿足不斷增長(zhǎng)的糧食需求和能源需求，開(kāi)始極大地改變自然氮循環(huán)過(guò)程，向大氣環(huán)境排放的Nr越來(lái)越多。據(jù)估算，1860年全球人類活動(dòng)排放的Nr僅為15 Tg，1990年增長(zhǎng)到了156 Tg[1]。

大氣環(huán)境中的Nr主要以氣態(tài)形式存在，雖然能提高農(nóng)田生態(tài)系統(tǒng)的生產(chǎn)力，但同時(shí)也會(huì)帶來(lái)一系列不良的環(huán)境影響。大氣中的氣態(tài)Nr參與各種大氣化學(xué)反應(yīng)，促進(jìn)氣溶膠、PM2.5等的形成。氣態(tài)Nr主要包括N2O、NH3和NOx。N2O在大氣中滯留時(shí)間長(zhǎng)且具有溫室氣體，NO2和NH3又是PM2.5的重要前驅(qū)物，對(duì)霧霾的形成有重要影響[2]。NOx在霧霾及光化學(xué)煙霧的形成中起重要作用。因此，本研究綜述了氣態(tài)Nr的排放清單估算方法研究現(xiàn)狀，從氣候變化和大氣環(huán)境質(zhì)量?jī)蓚€(gè)方面分析了氣態(tài)Nr的環(huán)境影響。

1 氣態(tài)Nr排放清單估算方法研究現(xiàn)狀

氣態(tài)Nr排放與經(jīng)濟(jì)發(fā)展、產(chǎn)業(yè)結(jié)構(gòu)、環(huán)境和氣候特征等都密切相關(guān)，國(guó)內(nèi)外學(xué)者在綜合考慮相關(guān)影響因素的基礎(chǔ)上，構(gòu)建了一系列不同尺度的氣態(tài)Nr排放清單估算方法，主要有直接測(cè)量法和模型計(jì)算法兩種。常用的農(nóng)業(yè)源氣態(tài)Nr排放直接測(cè)量法有靜態(tài)箱法[3]、風(fēng)洞法[4]、反向擴(kuò)散反演法[5]等；工業(yè)源氣態(tài)Nr排放清直接測(cè)量法有臺(tái)架實(shí)驗(yàn)法[6]、道路車載測(cè)試法[7]、道路遙感測(cè)試法[8]、隧道實(shí)驗(yàn)法[9]等，主要針對(duì)移動(dòng)排放源。這些直接測(cè)量法在時(shí)間和空間上受到限制，影響其應(yīng)用，國(guó)內(nèi)外學(xué)者提出諸多計(jì)算模型。計(jì)算模型有兩類，一類是經(jīng)驗(yàn)?zāi)Ｐ陀址Q“黑箱模型”，不涉及復(fù)雜的污染物遷移轉(zhuǎn)化和函數(shù)方程，如NARSES模型[10]、RAINS模型[11]、MOBILE系列模型[12]等；一類是過(guò)程機(jī)制模型，能夠?qū)鈶B(tài)Nr產(chǎn)生的機(jī)制、遷移轉(zhuǎn)化以及較為復(fù)雜的時(shí)空傳輸過(guò)程進(jìn)行具體的模擬和本土化修正，適用范圍更廣，如DYNAMO動(dòng)態(tài)NH3模型[13]、DNDC反硝化分解模型[14]、生物地球化學(xué)循環(huán)CENTURY/DAYCENT模型[15]等。近年來(lái)，一些學(xué)者將衛(wèi)星遙感反演法和高精度空氣質(zhì)量模型法也應(yīng)用于氣態(tài)Nr排放清單的估算。

21世紀(jì)以來(lái)，學(xué)者們也越來(lái)越重視中國(guó)氣態(tài)Nr排放清單的研究，應(yīng)用相關(guān)估算方法對(duì)中國(guó)的各類人為源氣態(tài)Nr排放清單進(jìn)行了研究(見(jiàn)表1)。目前，氣態(tài)Nr排放清單估算方法研究中，源解析不清晰、排放源低估和忽視等是中國(guó)氣態(tài)Nr定量估算的主要不確定性來(lái)源。通過(guò)氣態(tài)Nr排放清單的估算發(fā)現(xiàn)，氣態(tài)Nr污染正在成為比CO2排放更為嚴(yán)重的大氣環(huán)境問(wèn)題。王躍思等[36]對(duì)京津冀地區(qū)大氣污染成分進(jìn)行了研究，發(fā)現(xiàn)北京NH3濃度居高不下。

2 氣態(tài)Nr對(duì)氣候變化的影響

氣候變化受地面、大氣、海洋中的一系列物理、化學(xué)和生物過(guò)程影響，人類對(duì)于全球氣候變化的影響主要是溫室氣體(GHGs)的排放。氣態(tài)Nr對(duì)氣候變化的直接和間接影響如圖1所示。NOx和NH3能促進(jìn)氣溶膠的生成，間接導(dǎo)致降溫作用。NOx還能通過(guò)與CH4、O3反應(yīng)對(duì)氣候變化產(chǎn)生間接降溫作用。但是N2O在大氣中的停留時(shí)間長(zhǎng)達(dá)114年，而且單分子N2O增溫潛勢(shì)是CO2的298倍，所以盡管其在大氣中含量很低，但增溫作用不可忽視。1998年以來(lái)，全球大氣中的N2O正在以每年0.26%(質(zhì)量分?jǐn)?shù))的速度增長(zhǎng)；截至2005年，N2O增加導(dǎo)致的全球輻射強(qiáng)度的增加量就有(0.16±0.02) W/m2[37]。大氣氮沉降和氮肥施用會(huì)增加土壤Nr的富集，土壤Nr可以使土壤固碳能力增強(qiáng)，從而抑制土壤CO2的釋放，但CH4釋放量會(huì)增加[38]。此外，NOx還會(huì)促進(jìn)近地面O3生成，導(dǎo)致植物的碳吸收能力減弱，相當(dāng)于間接增加了大氣中的CO2。

目前，N2O對(duì)氣候變化的影響研究較多，相對(duì)也研究的比較透徹，主要是通過(guò)不同排放源N2O排放清單的建立實(shí)現(xiàn)對(duì)其溫室效應(yīng)的定量評(píng)估。TIAN等[39]對(duì)中國(guó)陸地生態(tài)系統(tǒng)N2O排放通量及其全球增溫效應(yīng)進(jìn)行評(píng)估發(fā)現(xiàn)，中國(guó)東南地區(qū)對(duì)N2O排放貢獻(xiàn)最大。TIAN等[26]對(duì)中國(guó)農(nóng)田氮肥施用導(dǎo)致的N2O排放與氣候變化關(guān)系的研究表明，氮肥施用導(dǎo)致的N2O排放增加可以使土壤固碳能力增強(qiáng)，減少釋放CO2，兩者對(duì)氣候變化影響的效應(yīng)可以相互抵消。但近幾年，氮肥施用量增加導(dǎo)致土壤N2O排放迅速增加，不能與土壤固碳能力增強(qiáng)導(dǎo)致的降溫作用相抵消。工業(yè)生產(chǎn)過(guò)程和化石燃料燃燒對(duì)N2O的排放貢獻(xiàn)分別約為9.8%、9.1%，其增溫效應(yīng)不容忽視[40]。中國(guó)工業(yè)生產(chǎn)過(guò)程排放的N2O未來(lái)10年的減排潛力約為1.54 Tg，可見(jiàn)中國(guó)工業(yè)生產(chǎn)過(guò)程的N2O減排潛力巨大[41]。

表1 21世紀(jì)以來(lái)中國(guó)各類氣態(tài)Nr排放清單主要研究工作

注：(+)、(-)分別表示升溫作用和降溫作用。圖1 氣態(tài)Nr對(duì)氣候變化的直接和間接影響Fig.1 The direct and indirect effects of gaseous Nr on climate change

3 氣態(tài)Nr對(duì)大氣環(huán)境質(zhì)量的影響

NH3是大氣中重要的堿性氣體，對(duì)大氣酸沉降起了至關(guān)重要的影響作用[42]。NOx是大氣PM2.5的重要前驅(qū)物[43]。PM2.5是城市大氣二次污染物的標(biāo)志性污染物[44]，對(duì)霧霾的形成有重要影響[45]。此外，NOx參與大氣化學(xué)過(guò)程可形成硝酸和硝酸鹽顆粒物，形成酸雨[46]。NOx還會(huì)與平流層O3反應(yīng)，引起平流層O3枯竭。NOx與揮發(fā)性有機(jī)物(VOCs)反應(yīng)可引起光化學(xué)煙霧[47]。

近年來(lái)，中國(guó)學(xué)者對(duì)不同區(qū)域的NH3和NOx排放造成的大氣污染進(jìn)行了不少研究。WANG等[48]建議，將城市移動(dòng)源的NH3排放納入大氣PM2.5污染的減控對(duì)象。WEI等[49]發(fā)現(xiàn)，NH3對(duì)PM2.5的形成有很大的促進(jìn)作用。WANG等[50]研究表明，區(qū)域大氣污染控制要對(duì)氣態(tài)Nr和相關(guān)前驅(qū)物同時(shí)進(jìn)行減控。

4 展 望

(1) 加強(qiáng)氣態(tài)Nr污染源的識(shí)別和污染特征分析。一方面，需要加強(qiáng)對(duì)氮污染嚴(yán)重地區(qū)的氣態(tài)Nr監(jiān)測(cè)，識(shí)別主要排放源；另一方面，要分析這些排放源的排放特征及其造成的環(huán)境污染特征。

(2) 結(jié)合區(qū)域大氣污染問(wèn)題對(duì)氣態(tài)Nr的大氣環(huán)境效應(yīng)進(jìn)行深入研究。開(kāi)展對(duì)復(fù)雜氮循環(huán)的生物地球化學(xué)過(guò)程機(jī)制研究，重點(diǎn)要結(jié)合中國(guó)的實(shí)際情況有針對(duì)性地從氣態(tài)Nr排放的角度來(lái)緩解中國(guó)區(qū)域大氣環(huán)境污染。

(3) 推進(jìn)氣態(tài)Nr減控措施的研究和實(shí)施。在農(nóng)村，可從技術(shù)層面對(duì)農(nóng)業(yè)氣態(tài)Nr進(jìn)行減控，如合理施肥、動(dòng)物飼料合理配比等；也可從政策層面對(duì)農(nóng)業(yè)氣態(tài)Nr進(jìn)行減控，如實(shí)施種養(yǎng)結(jié)合制度、畜牧業(yè)結(jié)構(gòu)優(yōu)化等。在城市，可從工業(yè)源、交通源等源頭排放進(jìn)行減控。

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Researchprogressonenvironmentimpactofgaseousreactivenitrogenemission

ZHAOChenxu1，XUPeng1，LIAOYajun1，LUANShengji1,2.

(1.KeyLaboratoryforUrbanHabitatEnvironmentalScienceandTechnology,SchoolofEnvironmentandEnergy,PekingUniversityShenzhenGraduateSchool,ShenzhenGuangdong518055；2.KeyLaboratoryofEnvironmentalSimulationandPollutionControl,PKU-HKUSTShenzhen-HongKongInstitution,ShenzhenGuangdong518057)

Gaseous reactive nitrogen (Nr) accumulation is leading environmental pollution,resulting in bad effects on climate change,human health and ecosystem health. In this study,the research progress of gaseous Nr emission inventory estimation methods was summarized. The environmental effects of 3 kinds of Nr (N2O,NH3and NOx) were discussed from the aspcts of climate change and atmospheric environmental quality. Finally,future research directions were proposed.

gaseous reactive nitrogen； emission inventory； climate change； atmospheric environmental quality； reduce and control

2016-07-30)

趙晨旭，女，1993年生，碩士研究生，研究方向?yàn)榄h(huán)境規(guī)劃與管理、大氣污染與防治。#

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10.15985/j.cnki.1001-3865.2017.05.021