2023, Vol. 44
2. 中国科学院大学资源与环境学院, 北京 100049;
3. 中国科学院城市环境研究所区域大气环境研究卓越创新中心, 厦门 361021;
4. 北京化工大学软物质科学与工程高精尖创新中心, 北京 100029;
5. 攀枝花学院生物与化学工程学院, 攀枝花 617000
, LIU Jun1,2 , MA Qing-xin1,2,3
, CHU Bi-wu1,2,3 , ZHANG Peng1,2 , LIU Yong-chun4 , LIU Chang-geng5 , HE Hong1,2,3
2. College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China;
3. Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China;
4. Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China;
5. School of Biological and Chemical Engineering, Panzhihua University, Panzhihua 617000, China
细颗粒物(PM2.5)对全球空气质量、气候变化和人类健康具有重要影响[1~3].有研究表明, 二次有机气溶胶(secondary organic aerosol, SOA)是PM2.5的重要组分, 在全球范围内对PM2.5的贡献可达20% ~60%[4, 5].在我国重污染时期, SOA可占PM2.5浓度的30% ~77%[6~8].近年来, 随着我国一系列减排措施的实施, 二次无机气溶胶(尤其是SO42-)的浓度显著降低[9], 相对而言, SOA对PM2.5的贡献进一步凸显, 最高可达80%[10], SOA的形成机制及其环境气候影响已成为大气环境科学研究的前沿领域.SOA主要由挥发性有机物(volatile organic compounds, VOCs)和半/中等挥发性有机物(semivolatile/intermediate volatility organic compounds, S/IVOCs)等经· OH、NO3 ·、Cl ·和O3等氧化剂气相氧化后生成低挥发性有机物[11, 12], 并进一步通过气-粒分配[13]、均相成核[14, 15]、液相反应[16~18]和非均相反应(如酸催化过程)[19, 20]等过程形成.近年来, SOA研究的热点主要集中在不同前体物对SOA的贡献评估、氧化形成机制和模型模拟等方面.随着质谱技术的发展, 对氧化产物化学组成的鉴定和识别取得了较大突破.但SOA化学成分非常复杂, 分子组成可达成千上万种, 目前已知的成分仅占10% ~20%, 更多的成分是未知的[21, 22].这主要是由于多种因素可以影响大气中SOA的生成, 如VOCs的种类与浓度[23~25]、VOCs/NOx[26, 27]、相对湿度(RH)[28]、温度(T)[29]、颗粒物种子[30, 31]、氧化剂类型与水平[32, 33]和液相化学与光化学反应老化过程[34, 35]等.目前SOA的模式预测值普遍低于外场观测结果.已有研究表明, 基于现有实验模拟结果将导致城市污染源对SOA的贡献被严重低估[36, 37], 说明目前对SOA的生成机制和来源贡献还缺乏准确认识[38, 39].一方面, 由于仪器检测水平和反应时间的限制, SOA生成的实验室模拟研究所采用的单个VOCs体积分数通常远高于实际大气中VOCs的体积分数(×10-9级或更低), 导致获得的SOA产率可能显著低于其在实际大气环境中的产率, 相差最高达一个数量级[24, 25].另一方面, 在多污染物多介质共存的复合污染条件下, 高浓度一次颗粒物(如黑碳、矿质颗粒物等)和多种共存气体污染物(如NOx、SO2、NH3等)会显著影响SOA生成过程中的气相氧化和气-粒分配过程.例如, Liu等[40, 41]和Chu等[42]研究表明, SO2和NH3的存在可以显著促进机动车尾气和甲苯SOA的生成.Deng等[43]研究也发现, 在NOx和SO2存在的复合污染条件下, 当以实际大气为反应介质时, 甲苯光氧化的SOA产率比以纯净零空气为反应介质时提高了5.6~12.9倍.因此, 研究大气复合污染条件下SOA的生成, 是准确定量表征SOA来源贡献的重要途径, 具有重要的大气环境意义.
大气中VOCs的种类繁多, 既包括植物排放等过程产生的天然源VOCs(如异戊二烯和蒎烯等), 也包括机动车排放、生物质燃烧、溶剂使用和工业过程等排放的人为源VOCs(如芳香烃VOCs和含氧VOCs等)[44, 45]. 全球范围内, VOCs排放以天然源为主, 人为源排放的VOCs总量仅为天然源的10% ~20%[46].但是, 在中小尺度的区域范围内, 尤其是城市区域, 人为源排放的VOCs对区域空气质量的影响更为显著[47].因此, 本文主要综述了我国大气复合污染条件下人为源VOCs的SOA生成研究进展, 主要包括大气复合污染条件下SOA生成的常用研究手段, 不同类型来源VOCs(包括芳香烃VOCs、汽油蒸气VOCs、生物质燃烧VOCs和含氧VOCs)和实际大气VOCs的SOA生成, 以及不同因素对SOA生成的影响机制和贡献评估.
1 SOA的常用研究手段研究SOA生成的常用手段和方法有烟雾箱系统、流动管反应器、液相反应器和傅里叶变换红外光谱等[48].其中, 最常用的研究手段是烟雾箱系统和氧化流动管反应器, 因此, 本文主要对这两种研究手段进行介绍.
1.1 烟雾箱系统烟雾箱系统主要由密闭反应器、零空气发生系统、温度控制系统、光源系统、进样与控制系统、检测系统和辅助系统等构成, 如图 1所示.烟雾箱反应器通常是由透光性能高且惰性的聚四氟乙烯膜或石英玻璃等材料搭建而成, 并可直接使用太阳光或者人造光源(如黑光灯、氙灯和氩灯等)模拟不同波长范围内的太阳光辐射.根据光源类型, 烟雾箱可分为使用人造光源的室内烟雾箱和以太阳光为光源的室外烟雾箱.从20世纪60年代以来, 国内外烟雾箱经过了长期的发展和完善, 在大气化学的发展中起到了关键作用, 对认识和解决局地、区域和全球尺度的大气污染问题发挥着至关重要的作用.具体而言, 烟雾箱系统为揭示酸雨和光化学烟雾形成机制、VOCs降解机制、臭氧(O3)和二次气溶胶生成机制做出了重要贡献, 进而为发展和改善数值模式机制以及大气污染控制措施的制定提供了科学依据[49, 50].在SOA生成研究方面, 烟雾箱系统可用于模拟不同VOCs的SOA生成过程, 评估VOCs的SOA产率和贡献, 表征SOA的组成和物理化学性质(如吸湿性和光学性质), 为深入认识SOA形成的关键过程(如气-粒分配过程、气相和颗粒相动力学过程)提供基础数据, 并最终实现SOA生成机制的不断完善[48, 51, 52].近年来, 烟雾箱模拟的实验条件趋于更加复杂化, 从单一污染物扩展到不同污染源如机动车、生物质燃烧、植物源等排放的多种污染物的大气光化学过程模拟[41, 53, 54], 也更加关注不同形态的污染物和大气化学反应产物的健康、气候和生态效应的评估[55, 56].
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改自文献[63] 图 1 RCEES-CAS烟雾箱系统构成 Fig. 1 Schematic of the RCEES-CAS smog chamber facility |
Chu等[50]综述了国内外烟雾箱系统的发展历程, 介绍和比较了全球典型烟雾箱系统的特征, 总结了依托烟雾箱模拟的主要研究方向以及在大气化学领域取得的重要研究进展.目前, 全球范围内以欧洲CLOUD(26 m3)[57]为代表的室内烟雾箱和以西班牙EUPHORE(2×200 m3)[58]、德国SAPHIR(270~370 m3)[59]和美国UCR(2×90 m3)[60]为代表的室外烟雾箱, 引领着大气化学机制研究、大气环境监测仪器研制和空气质量模型校验的前沿工作.近年来, 我国大型烟雾箱系统也得到了较快的发展, 已建有多个10 m3以上的烟雾箱, 包括中国环境科学研究院(CRAES)56 m3室外烟雾箱[61]、中国科学院广州地球化学研究所(GIG-CAS)30 m3室内烟雾箱[62]和中国科学院生态环境研究中心(RCEES-CAS)30 m3室内烟雾箱[63], 此外, 还建有一系列小型的烟雾箱[64~72].以上烟雾箱在认识我国复合污染条件下二次气溶胶与O3的生成、VOCs降解机制和动力学等方面起到了重要的作用[53, 54, 61~75].
1.2 氧化流动管近年来, 氧化流动管反应器(OFR)也被广泛用于研究SOA的生成和演化过程[76, 77], 作为一种较新的SOA研究手段快速发展[78, 79].在过去的10 a中, OFR相关研究的年度出版物数量已经快速增加到烟雾箱相关研究的约1/3, 并且仍在以较快速度呈指数增长[79].中国科学院生态环境研究中心设计搭建了由两个体积为15 L的不锈钢圆柱形反应器组成的孪生OFR系统[80, 81], 如图 2所示.OFR的主要优势在于停留时间短(达分钟级)以及可产生高浓度的· OH(达109~1010 molecules ·cm-3), 能快速模拟较长时间尺度(相当于实际大气老化几天至几周)下VOCs的老化过程.因此, OFR可用于外场观测和航测原位测定实际大气的SOA生成潜势[82~85], 也可以用于评估生物质燃烧、机动车尾气等特征排放源的SOA生成潜势[86~88].但是, OFR内远高于实际大气水平的· OH浓度, 可能导致其中的VOCs氧化反应路径和产物分布与实际大气环境中有较大差异[89].因此, VOCs氧化机制的研究主要使用烟雾箱系统, 而OFR系统在原位实际大气及特征排放源的SOA生成潜势研究方面更具优势.
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改自文献[80] 图 2 孪生氧化流动管反应器系统示意 Fig. 2 Schematic of twin oxidation flow reactor system |
前已述及, 在城市区域, 人为源VOCs对空气质量的影响更为显著[47].因此, 本文主要综述在我国复合污染条件下典型人为源VOCs的SOA生成机制及影响因素的研究进展, 部分研究结果汇总如表 1.
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表 1 典型人为源VOCs在不同影响因素条件下的SOA产率 Table 1 SOA yields of typical anthropogenic VOCs under different influencing factors |
2.1 不同前体物体积分数条件下的SOA生成
根据VOCs与· OH的反应机制可知[95], VOCs体积分数会影响烷基过氧自由基(RO2 ·)的生成, 进而影响后续多代产物和SOA的生成.Alfarra等[24]针对β-石竹烯的研究发现, 低VOCs体积分数条件下形成的SOA的氧碳比(O/C)更高, 具有更高的氧化程度.Chen等[25]对甲苯、间-二甲苯和均三甲苯等芳香烃VOCs的研究发现, VOCs光氧化过程中SOA产率存在显著的前体物体积分数效应, 低前体物体积分数(37×10-9~59×10-9)条件下的SOA产率是高体积分数条件(425×10-9~690×10-9)下双产物模型产率曲线预测值的3~4倍(图 3)[25, 29, 96, 97].这说明高体积分数VOCs模拟的结果会导致实际大气中VOCs的SOA产率低估, 可能是目前模型SOA的预测值低于外场观测值的重要原因.在高前体物体积分数条件下, 较高体积分数的前体物会与中间气相产物(IVOCs等)竞争性消耗· OH等氧化剂, 不利于中间产物进一步向低挥发性有机物(LVOCs)的转化, 导致SOA生成主要来自IVOCs的气-粒分配.因IVOCs挥发性较高, SOA的生成将受到较大限制.相对而言, 在低前体物体积分数条件下, 中间气相产物更容易转化为LVOCs等高级氧化产物, 易于通过气-粒分配生成更多SOA.在不同前体物体积分数条件下, IVOCs和LVOCs对SOA相对贡献的差异是影响SOA产率的关键原因.同时, 低前体物体积分数条件下光氧化生成的SOA氧化程度也更高, 更接近外场观测中SOA的化学组成[25].因此, 应在足够低的前体物体积分数下开展SOA生成的烟雾箱模拟研究, 以获得接近实际大气环境水平下的SOA产率参数, 为空气质量模型提供更具有环境意义的产率参数.此外, 低前体物体积分数和氧化剂条件下反应达到采样分析要求所需的反应时间更长, 导致在线采样分析周期变长, 这一方面要求烟雾箱反应器具有更大的体积以满足长时间采样分析的需要, 另一方面也对检测设备的灵敏度提出了更高的要求.因此, 更加准确地揭示复合污染条件下SOA的形成, 亟需研发灵敏度更高的检测设备, 建设体积更大、背景更低和反应条件更可控的多功能烟雾箱.
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Mo为生成的SOA浓度; 图内的散点为文献[25]中的实验结果, 实线为根据实验结果拟合所得的产率曲线 图 3 不同体积分数条件下甲苯光氧化的SOA产率曲线 Fig. 3 SOA yield curves for toluene photo-oxidation at different precursor concentrations |
(1) 芳香烃含量的影响机动车排放源清单结果表明, 机动车蒸发排放已成为我国VOCs排放不容忽视的人为源[98].芳香烃是汽油燃料的重要成分, 其含量约占30%[99].有研究表明, 随着汽油中芳香烃含量的增加, 一次颗粒物(POA)和多环芳香烃(PAHs)的排放均增加[100, 101], 相应的SOA生成潜势也更大[102], 说明芳香烃是影响有机颗粒物排放的关键因素.Peng等[65]研究发现, 随着汽油中芳香烃含量的增加(29% ~37%), 尾气的SOA产率可增加约3~6倍.Chen等[90]针对汽油蒸气的SOA研究结果表明, 随着芳香烃含量的增加(22.8% ~50.5%), 汽油蒸气SOA产率增加约4~7倍.同时, 利用典型芳香烃的消耗和相应的SOA产率无法完全解释汽油蒸气光反应体系中SOA的生成, 这与汽油蒸气中的S/IVOCs对SOA的贡献有关[103, 104]; 并且随着芳香烃含量的增加, 对SOA的解析程度明显降低, 推测是由于共存芳香烃对SOA的生成具有协同促进作用, 且随着芳香烃含量的增加, 这种协同作用更明显.此外, 随着芳香烃含量的增加, SOA中源于羧酸的CO2+碎片等高含氧碎片强度均明显增加, 表明更易氧化生成高氧化态的物种, 有利于经过气-粒分配贡献SOA的生成, 使得SOA的氧化态增加, 这说明芳香烃含量也是影响SOA的组成和氧化程度的重要原因.根据芳香烃含量与SOA产率之间的关系以及我国机动车蒸发排放源清单[98], 估算机动车蒸发源排放的VOCs对SOA的贡献约为0.27 Tg ·a-1, 占全球范围内人为源对SOA贡献(2.62 Tg ·a-1)[105]的10%, 因此, 降低汽油中的芳香烃含量将有利于缓解我国大气复合污染程度.
(2) 无机气体的影响SO2和NH3等无机气态污染物是影响SOA生成的重要因素[106~108].有研究表明, SO2主要通过生成气态H2SO4来增加颗粒相的酸度[41, 109], 促进气态有机物种的反应性摄取或促进低聚物的形成[42, 110, 111], 进而通过酸催化非均相作用机制影响SOA的生成[19, 112].Lu等[113]针对酸性气溶胶种子对间-二甲苯SOA生成的影响研究也表明, 其表面发生的酸催化非均相反应可以显著促进SOA的生成.同时, SO2存在还会促进低挥发性含硫有机物(OS)的形成, 进一步促进SOA的生成[63, 91, 92, 106].对于NH3而言, 一方面, NH3摄取后可以与小分子有机羧酸和羰基化合物反应, 生成含多个氧原子的咪唑类产物[63]、羧酸盐和有机硝酸盐[93, 114]等; 另一方面, NH4+离子也可以通过Bronsted酸途径或亚胺离子途径与羰基化合物(如乙二醛)生成含氮有机物(ON)和低聚物, 进而促进SOA的生成, 并影响SOA的化学组成[91, 106].Huang等[115]研究也发现, 在(NH4)2SO4种子共存的均三甲苯光反应体系中, NH4+离子可与中间产物甲基乙二醛反应生成咪唑类化合物.有研究表明, NH3可以促进芳香烃(如甲苯和二甲苯)光氧化体系中的新粒子生成(NPF)和颗粒物增长, 进而促进SOA的生成[40, 116, 117].Chen等[63]基于汽油蒸气SOA生成的研究结果表明, 在我国多污染物(NOx、SO2和NH3等)共存的复合污染条件下, 机动车蒸发源排放的VOCs对SOA的贡献将显著增加, 估算约为0.49 Tg ·a-1, 对全球范围内人为源SOA的贡献(2.62 Tg ·a-1)[105]将提高至19%, 因此, 我国机动车蒸发排放源是二次气溶胶不容忽视的来源, 应引起足够的重视并采取相应的控制措施.
2.2.2 生物质燃烧VOCs生成SOA(1) 种子气溶胶的影响生物质燃烧排放是大气中气态污染物和细颗粒物的重要来源之一[118], 对区域空气质量和全球气候变化的影响不容忽视[119].甲氧基苯酚类VOCs作为生物质燃烧排放的有效示踪物, 主要源于生物质燃烧过程中木质素热裂解的排放, 主要由邻甲氧基苯酚(C7H8O2)、二甲氧基苯酚(C8H10O3)及其衍生物组成.有研究表明, 邻甲氧基苯酚和二甲氧基苯酚与· OH光化学反应具有显著的SOA生成潜势, SOA产率分别为3% ~87%和10% ~36%[120, 121].Liu等[53]研究表明, 在NOx共存的光化学反应体系中邻甲氧基苯酚也具有显著的SOA生成潜势, SOA产率为9.5% ~26.4%.NaCl和(NH4)2SO4等无机种子气溶胶均有利于邻甲氧基苯酚光氧化SOA的生成, 这是由于种子气溶胶可以为半挥发性中间产物的凝结和分配提供表面[30, 122].相比(NH4)2SO4, NaCl种子颗粒对SOA生成的促进作用更加明显, 表明种子气溶胶的不同组分还可以通过影响中间产物的后续反应过程进一步影响SOA的生成.NaCl较强的吸湿性有利于小分子高级氧化产物(如小分子有机酸)的摄取, 进而有助于非均相反应的发生.Huang等[115]针对不同无机种子气溶胶[(NH4)2SO4、NaNO3和CaCl2]对均三甲苯SOA生成的影响研究发现, 在(NH4)2SO4体系中, NH4+离子可与中间产物甲基乙二醛反应生成咪唑类化合物; 在NaNO3种子存在下, 酸性NO3-水溶液在紫外线照射下可产生NO2 ·, 进而与酚类化合物反应生成芳香族含氮有机化合物; 在吸湿性最强的CaCl2体系中, 气溶胶表面较高的含水量有利于气相有机酸的凝结和H+的生成, 进而通过酸催化非均相反应生成高分子量化合物.Liu等[53]进一步研究表明, SO2和种子气溶胶共存时对SOA的生成具有协同促进作用, 有机硫酸酯、高级氧化产物的气-粒分配和酸催化过程是促进SOA生成的重要原因.并且随着SO2浓度的增加, SOA中低分子量物种的信号强度显著增加, 远高于高分子量物种(m/z>300)的信号强度, 表明在SO2和种子气溶胶共存时, 有机物官能化反应较低聚反应占主导地位, 进而使得SOA的氧化态增加[53].
(2) NOx的影响丁子香酚(4-烯丙基-2-甲氧基苯酚, C10H12O2)是一种代表性的具有不饱和支链的甲氧基苯酚类VOCs, 它在实际大气中的浓度与其它甲氧基苯酚类VOCs的浓度相当[123].Liu等[94]指出丁子香酚与· OH光化学反应具有显著的SOA生成潜势, SOA产率(11% ~31%)随着丁子香酚初始浓度的增加而增加.有研究还指出, 在NO2共存的复合污染条件下可以显著促进其SOA生成, 并且随着NO2浓度的增加, SOA产率逐渐提高[94].相比于其他芳香族前体物形成SOA基本不受低体积分数NO2(×10-9级)的影响, 在苯酚类前体物的氧化过程中, 0.5×10-9的NO2足以与O2和苯氧自由基的反应竞争[124].因此, NO2对SOA产率增加的影响与特定的前体物种类有关, 比如苯酚类或甲氧基苯酚类.在NOx存在的条件下, 苯酚类前体物与· OH的反应产物以硝基取代产物为主[120].因此, 随着NO2浓度的增加, 丁子香酚SOA中的N/C(0.032~0.041)明显增加, NO+/NO2+(3.98~6.09)也明显增加, 这进一步表明NO2通过加成到苯氧自由基参与了丁子香酚与· OH的反应过程, 并生成了含氮有机物(ON), 通过估算其贡献约为25.6% ~82.1%[94].而在苯、甲苯和间-二甲苯等芳香烃VOCs的光氧化过程中, NOx主要通过改变反应途径和反应体系氧化性来影响SOA的生成.在低NOx浓度条件下, HO2 ·+RO2 ·反应占主导, 生成的产物挥发性更低, 更易经过气-粒分配贡献SOA的生成, 使其具有更高的SOA产率; 而NOx+RO2 ·反应生成的产物挥发性较高, 不利于SOA的生成[26, 27].Wang等[125]研究表明, 在不同NOx浓度范围内, RO2 ·的反应途径不同, 在低NOx条件下, 主要生成氢过氧化物、醇和羰基类产物, 而在高NOx条件下, 以RO2 ·与NO和NO2反应生成有机硝酸酯为主[67, 126].NOx也可以通过化学反应· OH+NO2 HNO3和HO2 ·+NO · OH+NO2来改变反应体系中· OH的浓度, 进而影响SOA的生成[127~129].这些说明NOx对SOA生成的影响与前体物的种类和官能团密切有关, NOx对苯酚类或甲氧基苯酚类化合物的SOA生成具有显著的促进作用.
2.2.3 含氧VOCs生成SOA含氧VOCs(OVOCs)作为大气中一类重要的VOCs, 除了自然源和人为源的直接排放外, VOCs(如芳香烃、萜烯类和角鲨烯等)的大气氧化过程也是其重要来源[130].Zhang等[31]研究表明, 以丁基乙烯基醚为代表的OVOCs具有较高的O3氧化SOA产率, 在SO2存在的复合污染条件下可以显著促进SOA的生成, 与H2SO4的酸催化作用机制有关[31, 75, 131].同时, RH也会对其SOA生成过程产生影响.有研究发现, 在SO2共存的复合污染条件下, RH对丁基乙烯基醚SOA的生成表现出不同的作用, 这与在不同RH范围内H2 O和SO2与Criegee中间体(sCI)的竞争性反应有关[31].在较低RH(1% ~10%)范围内, SO2与sCI的反应速率常数比H2 O与sCI的反应速率常数高约4个数量级, 占主导地位, RH的增加可以促进H2 O与SO3反应生成H2SO4, 进而促进SOA的生成; 在中等RH(10% ~42%)范围内, RH的增加会使H2 O竞争性消耗sCI, 动力学模型定量结果表明超过95%的sCI会被H2 O反应消耗, 不利于H2SO4的生成, 进而对SOA的生成产生抑制作用; 随着RH的进一步增加(42% ~64%), H2 O竞争性消耗sCI不利于H2SO4的生成, 进而对SOA的生成产生一定的抑制作用.在此条件下, 未被sCI消耗的SO2与有机过氧化物反应生成的硫酸盐可以为SOA的生成提供更多的分配界面, 进而在一定程度上促进SOA的生成.也有研究表明, 高湿度下气溶胶水可以改变参与分配的物种在颗粒相中的活度, 从而有利于中间产物的气-粒分配, 进而促进SOA的生成[74].
RH对不同前体物SOA生成的影响取决于前体物的分子结构和SOA形成过程中与水有关的形成机制[28, 31, 132, 133].Jia等[28]研究发现, RH对人为源和天然源VOCs生成SOA的过程表现出相反的作用效果.高RH可以通过促进对乙二醛等的反应性摄取来促进芳香烃VOCs的SOA生成[134].但是, 对异戊二烯-NOx体系[28]和间-二甲苯-· OH体系[135]的研究发现, 较高的RH可以抑制体系中SOA的形成, 这主要是由于较高的RH极大抑制了缩合反应和低聚物的形成过程[110], 并进一步影响了高含氧有机分子(HOMs)的颗粒相反应[135].Chen等[74]研究表明, RH可以通过影响· OH浓度、气相中间产物的分配过程和液相氧化过程共同影响SOA的生成和化学组成.在RH为2% ~50%, 随着RH的增加, · OH浓度逐渐升高, 促进多代气相产物的生成, 进而使得高氧化态SOA的浓度及其占比均显著增加, SOA的氧化态增加; 在RH为50% ~90%, RH的增加会促进气相中间产物的分配, 同时, RH的增加也会促进· OH的摄取, 使得液相氧化过程加强.以上研究促进了对SOA生成过程中RH影响机制的深入认识.
2.2.4 实际大气VOCs生成SOA相比于单一组分VOC或者已知组分的混合VOCs, 实际大气中VOCs组分更为复杂, 形成SOA的机制和贡献仍需进一步明确.目前, 有研究基于外场观测来阐明和量化实际大气VOCs的SOA转化过程和生成潜势, 主要利用氧化流动管反应器(OFR), 通过测定反应后有机气溶胶(OA)的浓度并利用正交矩阵因子分解法(PMF)解析出SOA的量, 结合实测的各个VOCs的浓度, 来阐明实际大气VOCs的SOA生成机制、生成潜势、变化规律及影响因素和对PM2.5的潜在贡献等[81~83, 136, 137].实际大气的SOA生成潜势主要取决于大气中前体物VOCs的物种及浓度.有研究表明, 芳香烃是SOA生成潜势的主要贡献者[137, 138].Liu等[82]研究发现, VOCs浓度水平的差别也会造成SOA生成潜势的不同, 相比美国洛杉矶等发达地区, 北京城区由于具有更高的VOCs浓度, 其SOA生成潜势也更高[82, 139].同时有研究发现, · OH暴露量也会显著影响SOA生成潜势.前述影响SOA生成的各环境因素也都会影响实际大气VOCs的SOA生成潜势.除此之外, Liu等[83]研究发现, 大气氧化能力可以促进VOCs的二次转化, 从而间接影响实际大气VOCs的SOA生成潜势, 在污染程度更低时, 大气氧化性增强, 更高比例的VOCs在传输过程中被氧化为SOA, 导致进一步生成SOA的潜势降低.Palm等[136, 140]研究实际大气VOCs的SOA生成潜势发现, 通过实测的VOCs估算的SOA生成量与实测的SOA生成潜势之间存在较大偏差, 模型结果表明尚无法定量的S/IVOCs可以较好地解释此偏差, 进一步说明实际大气VOCs二次转化为SOA过程的复杂性和对细颗粒物污染贡献的不确定性, 亟待进一步深入研究.
3 讨论我国大气污染具有高度复合的污染特征.在高度大气复合污染条件下, 由于污染物之间的协同作用以及大气颗粒物界面过程的加强, 造成我国大气氧化能力增强和二次颗粒物呈现暴发性增长, 使得单一污染物的大气环境容量下降和我国中东部灰霾污染事件频发[141].有研究表明, 复合污染条件下不仅二次无机颗粒物的形成会出现暴发性增长[142~146], 同时, SOA的生成也会发生明显改变.通过复合污染条件下SOA形成的研究, 初步揭示了不同环境因素对SOA形成的影响机制(图 4), 有助于对SOA来源贡献及其环境气候效应的深入认识.
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图 4 各因素对SOA生成的影响机制概念 Fig. 4 Conceptual diagram of the influence mechanism of various factors on SOA formation |
当前, 我国大气污染形势依然严峻, PM2.5浓度仍远高于世界卫生组织指导值[147, 148], 进一步降低大气细颗粒物污染的难度加大.同时, 我国O3污染问题不断凸显, 面临PM2.5和O3双高的污染态势.PM2.5和O3协同控制成为当前我国大气污染防治的紧迫任务和重大挑战.VOCs光化学反应过程不仅决定大气中SOA的生成, 对于O3形成过程中的NOx循环也有重要作用.因此, 开展复合污染条件下VOCs光化学反应过程研究, 是阐明PM2.5和O3耦合关系的重要途径.然而, 复合污染条件下SOA生成机制的研究仍然存在较多未解决的科学问题, 亟需进一步开展更加深入的研究.
4 结论与展望(1) 本文系统地介绍了我国复合污染条件下典型人为源VOCs的SOA生成过程中各因素的影响机制, 结果表明, NOx、SO2和NH3等共存无机气体有利于SOA中含硫(OS)和含氮有机物(ON)的生成, 其中的酸碱催化过程也会促进SOA的生成; 种子气溶胶可以促进高氧化态有机物的分配和摄取, 进而促进SOA的生成, 并使SOA的氧化程度增加; 高相对湿度则会通过促进液相氧化过程来协同促进SOA的生成和氧化程度的提高.
(2) 我国大气复合污染条件下, 高浓度的颗粒物、无机气态污染物和VOCs共存, 导致复杂的有机-无机耦合效应, 这些共存的污染物会如何影响二次气溶胶和O3的生成途径以及大气氧化性, 亟需进一步深入研究.
(3) 亟需阐明界面反应过程对关键氧化剂和中间体的吸附淬灭和催化转化作用, 揭示二次污染形成过程中关键氧化剂的转化机制和颗粒物界面对污染物氧化过程的催化作用, 为模式评估界面反应对我国二次污染形成的贡献提供接近真实大气环境下的参数化方案.
(4) 目前对于SOA的吸湿性、消光性质以及毒性等理化性质的认识仍然不足, 各化学组分和老化过程对SOA理化性质的影响缺乏定量表征和系统性研究, 亟需进一步总结和完善规律性研究结论, 为缓解二次污染和健康风险提供科学依据.
(5) 亟需利用机器学习等手段将实验室模拟和外场观测中获得的关键化学过程和关键影响因素进行多参数回归分析, 相互校验, 构建更精确的参数化方案并应用到空气质量模型, 为量化大气污染与前体物减排的非线性响应关系提供科学依据, 并最终服务于大气复合污染防治策略的制定.
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