气态活性氮排放的环境影响研究进展

赵晨旭 徐 鹏 廖雅君 栾胜基,2#

(1.北京大学深圳研究生院环境与能源学院，城市人居环境科学与技术深圳市重点实验室，广东 深圳 518055； 2.深港产学研基地环境模拟与污染控制重点实验室，广东 深圳 518057)

气态活性氮排放的环境影响研究进展

赵晨旭1徐 鹏1廖雅君1栾胜基1,2#

(1.北京大学深圳研究生院环境与能源学院，城市人居环境科学与技术深圳市重点实验室，广东 深圳 518055； 2.深港产学研基地环境模拟与污染控制重点实验室，广东 深圳 518057)

大气中过量的气态活性氮积累导致大气环境污染，对气候变化、人体和生态系统健康都产生了不利影响。综述了气态活性氮排放清单估算方法的研究现状。从气候变化和大气环境质量两个方面讨论了N2O、NH3和NOx等气态活性氮的环境效应，并对未来气态活性氮的研究方向进行了展望。

气态活性氮 排放清单 气候变化 大气环境质量 减控

氮循环是自然界中重要的物质循环之一，而氮循环中的氮元素主要以不活泼的N2存在。在自然界中，不活泼的N2转化成可被生物利用的活性氮(Nr)，主要通过闪电固氮和生物固氮完成；20世纪初，人类成功实现了人工合成氨(NH3)。自此以后，人类为了满足不断增长的粮食需求和能源需求，开始极大地改变自然氮循环过程，向大气环境排放的Nr越来越多。据估算，1860年全球人类活动排放的Nr仅为15 Tg，1990年增长到了156 Tg[1]。

大气环境中的Nr主要以气态形式存在，虽然能提高农田生态系统的生产力，但同时也会带来一系列不良的环境影响。大气中的气态Nr参与各种大气化学反应，促进气溶胶、PM2.5等的形成。气态Nr主要包括N2O、NH3和NOx。N2O在大气中滞留时间长且具有温室气体，NO2和NH3又是PM2.5的重要前驱物，对雾霾的形成有重要影响[2]。NOx在雾霾及光化学烟雾的形成中起重要作用。因此，本研究综述了气态Nr的排放清单估算方法研究现状，从气候变化和大气环境质量两个方面分析了气态Nr的环境影响。

1 气态Nr排放清单估算方法研究现状

气态Nr排放与经济发展、产业结构、环境和气候特征等都密切相关，国内外学者在综合考虑相关影响因素的基础上，构建了一系列不同尺度的气态Nr排放清单估算方法，主要有直接测量法和模型计算法两种。常用的农业源气态Nr排放直接测量法有静态箱法[3]、风洞法[4]、反向扩散反演法[5]等；工业源气态Nr排放清直接测量法有台架实验法[6]、道路车载测试法[7]、道路遥感测试法[8]、隧道实验法[9]等，主要针对移动排放源。这些直接测量法在时间和空间上受到限制，影响其应用，国内外学者提出诸多计算模型。计算模型有两类，一类是经验模型又称“黑箱模型”，不涉及复杂的污染物迁移转化和函数方程，如NARSES模型[10]、RAINS模型[11]、MOBILE系列模型[12]等；一类是过程机制模型，能够对气态Nr产生的机制、迁移转化以及较为复杂的时空传输过程进行具体的模拟和本土化修正，适用范围更广，如DYNAMO动态NH3模型[13]、DNDC反硝化分解模型[14]、生物地球化学循环CENTURY/DAYCENT模型[15]等。近年来，一些学者将卫星遥感反演法和高精度空气质量模型法也应用于气态Nr排放清单的估算。

21世纪以来，学者们也越来越重视中国气态Nr排放清单的研究，应用相关估算方法对中国的各类人为源气态Nr排放清单进行了研究(见表1)。目前，气态Nr排放清单估算方法研究中，源解析不清晰、排放源低估和忽视等是中国气态Nr定量估算的主要不确定性来源。通过气态Nr排放清单的估算发现，气态Nr污染正在成为比CO2排放更为严重的大气环境问题。王跃思等[36]对京津冀地区大气污染成分进行了研究，发现北京NH3浓度居高不下。

2 气态Nr对气候变化的影响

气候变化受地面、大气、海洋中的一系列物理、化学和生物过程影响，人类对于全球气候变化的影响主要是温室气体(GHGs)的排放。气态Nr对气候变化的直接和间接影响如图1所示。NOx和NH3能促进气溶胶的生成，间接导致降温作用。NOx还能通过与CH4、O3反应对气候变化产生间接降温作用。但是N2O在大气中的停留时间长达114年，而且单分子N2O增温潜势是CO2的298倍，所以尽管其在大气中含量很低，但增温作用不可忽视。1998年以来，全球大气中的N2O正在以每年0.26%(质量分数)的速度增长；截至2005年，N2O增加导致的全球辐射强度的增加量就有(0.16±0.02) W/m2[37]。大气氮沉降和氮肥施用会增加土壤Nr的富集，土壤Nr可以使土壤固碳能力增强，从而抑制土壤CO2的释放，但CH4释放量会增加[38]。此外，NOx还会促进近地面O3生成，导致植物的碳吸收能力减弱，相当于间接增加了大气中的CO2。

目前，N2O对气候变化的影响研究较多，相对也研究的比较透彻，主要是通过不同排放源N2O排放清单的建立实现对其温室效应的定量评估。TIAN等[39]对中国陆地生态系统N2O排放通量及其全球增温效应进行评估发现，中国东南地区对N2O排放贡献最大。TIAN等[26]对中国农田氮肥施用导致的N2O排放与气候变化关系的研究表明，氮肥施用导致的N2O排放增加可以使土壤固碳能力增强，减少释放CO2，两者对气候变化影响的效应可以相互抵消。但近几年，氮肥施用量增加导致土壤N2O排放迅速增加，不能与土壤固碳能力增强导致的降温作用相抵消。工业生产过程和化石燃料燃烧对N2O的排放贡献分别约为9.8%、9.1%，其增温效应不容忽视[40]。中国工业生产过程排放的N2O未来10年的减排潜力约为1.54 Tg，可见中国工业生产过程的N2O减排潜力巨大[41]。

表1 21世纪以来中国各类气态Nr排放清单主要研究工作

注：(+)、(-)分别表示升温作用和降温作用。图1 气态Nr对气候变化的直接和间接影响Fig.1 The direct and indirect effects of gaseous Nr on climate change

3 气态Nr对大气环境质量的影响

NH3是大气中重要的碱性气体，对大气酸沉降起了至关重要的影响作用[42]。NOx是大气PM2.5的重要前驱物[43]。PM2.5是城市大气二次污染物的标志性污染物[44]，对雾霾的形成有重要影响[45]。此外，NOx参与大气化学过程可形成硝酸和硝酸盐颗粒物，形成酸雨[46]。NOx还会与平流层O3反应，引起平流层O3枯竭。NOx与挥发性有机物(VOCs)反应可引起光化学烟雾[47]。

近年来，中国学者对不同区域的NH3和NOx排放造成的大气污染进行了不少研究。WANG等[48]建议，将城市移动源的NH3排放纳入大气PM2.5污染的减控对象。WEI等[49]发现，NH3对PM2.5的形成有很大的促进作用。WANG等[50]研究表明，区域大气污染控制要对气态Nr和相关前驱物同时进行减控。

4 展 望

(1) 加强气态Nr污染源的识别和污染特征分析。一方面，需要加强对氮污染严重地区的气态Nr监测，识别主要排放源；另一方面，要分析这些排放源的排放特征及其造成的环境污染特征。

(2) 结合区域大气污染问题对气态Nr的大气环境效应进行深入研究。开展对复杂氮循环的生物地球化学过程机制研究，重点要结合中国的实际情况有针对性地从气态Nr排放的角度来缓解中国区域大气环境污染。

(3) 推进气态Nr减控措施的研究和实施。在农村，可从技术层面对农业气态Nr进行减控，如合理施肥、动物饲料合理配比等；也可从政策层面对农业气态Nr进行减控，如实施种养结合制度、畜牧业结构优化等。在城市，可从工业源、交通源等源头排放进行减控。

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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)

赵晨旭，女，1993年生，硕士研究生，研究方向为环境规划与管理、大气污染与防治。#

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10.15985/j.cnki.1001-3865.2017.05.021