来自芬兰赫尔辛基大学大气物理学教授MarkkuKulmala在nature杂志上撰写评论，讨论中国空气污染的复杂性，并从纯学术角度提出清理空气污染的建议及需要解决的科学问题与途径。

大气化学：中国的窒息性空气混合物(cocktail)

2015年10月21日

MarkkuKulmala认为要想清洁城市户外和室内空气，需要对空气中污染物间极复杂的化学反应有深刻地认识。

污浊的空气全球范围内威胁者数十亿居民的健康。中国的大都市最为严重，其空气污染物含量是欧洲或北美高出10-100倍，有时甚至高出1000倍。估计中国每年有250万人死于因户内或或外空气污染所引发的健康效应。

提高空气质量所做的努力针对的却只是这座冰山的一角。例如北京这样的城市，常规只测定特定诸如10微米(PM10)和2.5微米(PM2.5)直径颗粒物和包括二氧化硫(SO2)，氮氧化合物(NOx)，一氧化碳(CO)和臭氧等少数几种有害气体。城市大气却是一个复杂的化合物混合体，这些物质间鲜为人知的相互作用和反馈(feedbacks)可能加剧了其带来的健康问题。因条件变化，消减某种污染物的努力可能给其它化合物带来负面效应。

对中国污染的城市而来，对大气化学的挑战是前所未有的。更严重的污染、重工业和现代商品制造业，以及气候条件等因素导致北京雾霾显著不同于50-100年前影响伦敦和其它欧洲城市的黄色浓雾(pea soupers)。很多大气过程是非线性的—意味着原因与结果之间并不成比例关系。因此我们并不知道，也不能预测哪些有害化学物因此而生成。在中国，我同事和我无论何时使用新型仪器进行测定，我们总能获得意料之外的结果。室内空气质量也同样如此。

只有在广泛充分检测，并将大气化学考虑进来来建模的基础上，才能设法对中国城市大气找到合理的净化举措。为了指导行动决议，我们需要知道存在有哪些有害污染物，它们如何相互作用生成次生污染(secondary pollution)。

在这里我列出十年内让中国城市空气质量达到欧洲标准的路线图。向更清洁空气的方向行动，同时改善健康(见“深度净化”部分)，将减少温室气体和黑炭(煤烟)浓度，加强淡水质量和食物供应。

毒性混合物

中国空气污染因工业，能源产生和交通排放增长而变得更加恶化。全球30-35%的SO2, NOx, CO和颗粒物，40%的直径20-100纳米范围的细粒子(particle numbers, PN) (见go.nature.com/uw3jx6)排放来自中国。全球温室气体排放中,二氧化碳的29%份额，甲烷的近20%份额来自中国。政府正在努力减少这些污染的排放量。

但是没有很好的理解污染物间化学反应链和物理相互作用，要减少诸如臭氧和有机气溶胶(organic aerosols)等次生污染物是不可能的。次生污染物的生成和消散依赖于气温，湿度，风速和城市大气中其它化合物和粒子。这些都是我们所不掌握的。因很多物理和化学过程——例如表面化学，氧化，聚集(clustering)和动态效应——可能同时发生，其结果难于预测。

试图控制单一污染物会增加其它物质的浓度。例如在南京的测量显示，在夏季减少NOx排放能引起臭氧量十倍增高。减少雾霾能增加光照水平和温度，改变降水和降雪布局(pattern)。

在洁净的空气条件下不出现的化学反应，在中国城市环境中惊奇地发生。例如，在上海，南京和北京，小空气分子系列(1–3纳米)的浓度较欧洲城市高出数十倍。与现有预测模型相比，次级气溶胶(含硫，硝化合物和有机物)在上海和南京更易生成，未知的化学反应途径和物理过程导致新型氧化物或气溶胶表面特性的改变，限制它们形成可凝蒸气(condensable vapours)。

室内空气质量也是一个问题。城市居民超过90%的时间呆在室内，尤其在户外空气质量差时。做饭，吸烟，采暖和家具释放PM, PN, CO,挥发性有机物(VOCs)和NOx,，增加了户外已有污染物的类型。我们对中国家庭室内空气污染的情况所知甚少，但我们的研究组的初步发现表明一些污染物浓度高于户外(尤其是PN和VOCs)，高出欧洲家庭室内空气100-1000倍，同事和我估计户内空气污染导致的死亡对污染相关死亡的贡献与户外雾霾所致死亡相当(户外雾霾所致死亡约每年130万)。

另外，对室内化合物次生反应的生成物所致甚少，换气装置能过滤掉大颗粒，但却不能过滤掉VOCs, NOx和SO2等气体，这些物质继续形成超细硫化物，硝化物和有机粒子。经过滤后，因更少的颗粒表面用于浓缩，由这些成分产生次生气溶胶的风险增加。当户外空气变得清洁时，室内空气质量可能反而更差。

如何洁净空气

减少污染的努力依旧重要。但要达到中国中央政府的目标—让城市空气质量达到美国和欧洲国家的标准，需要更多努力：同时追踪(tracking)所有健康相关空气污染物、它们的反馈和相互作用—至少需要十年以上时间。当净化措施实施时，我们需要了解这些混合物水平及它们的毒性变化。仅靠短期的行动是不够的。

在全球大气化学家的帮助下，中央和地方政府，研究机构和大学需要合作做如下事情：

首先，建立和资助旗舰站(flagship stations)的网络来检测：通量(fluxes)，相互作用和反馈，以及更普通的大气质量和气象数据。对一个大城市而而言，大约5-8个这样的旗舰站(每个花费在700万到1100万美元间)将足够用。使用车载或飞机移动观测平台，地面、卫星遥感观测大气柱和雾霾室(chambers)进行观测。通过历史数据确认主要污染源。

第二，必须选择代表性居住地和办公建筑物对室内空气质量进行测定和监测。

第三，空气化学家需在多种污染物高浓度存在下，模拟次生污染物产生途径和反馈机制。这些模拟结果需与观测结果相比较。

第四，确立空气污染与死亡及其它健康效应间的关系。与健康最相关的污染物和污染源必须首先确认和纾解(mitigated)。建立数据库确定其健康影响。

第五，长期、持续性工程化举措，例如提高工业过程和材料流(material flows)，用以保持较低空气污染水平。这需要建立跨中国官方和研究机构使用空气质量评估数据的能力：用于决策，用于立法和制定清洁空气的行动计划。

只有理解空气化学才可能净化中国污染的大气。

原文链接：

http://www.nature.com/news/atmospheric-chemistry-china-s-choking-cocktail-1.18586

Atmosphericchemistry: China’s choking cocktail

MarkkuKulmala

21October 2015

Cleaningup city and indoor air will require a deeper understanding of the unprecedentedchemical reactions between pollutants, says Markku Kulmala.

Dirtyair threatens the health of billions of city dwellers around the world. China'smegacities are among the worst, with concentrations of airborne pollutants10–100 times higher than those in Europe or North America, and occasionallyeven 1,000 times higher. An estimated 2.5 million people in China die each yearfrom the health effects of indoor and outdoor air pollution1, 2.

Effortsto improve air quality are targeting only the tip of the iceberg. Cities suchas Beijing routinely measure levels of particulate matter measuring 10micrometres (PM10) and 2.5 micrometres (PM2.5) in size, as well as a few gasessuch as sulfur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO) andozone. But urban air is a complex cocktail of chemicals whose poorly understoodinteractions and feedbacks may exacerbate health problems. Efforts to reduceone pollutant can have perverse effects on others as conditions change.

Thechemistry of China's polluted urban air is unprecedented. Higher populations,heavier industries and modern goods manufacturing, as well as the climaticconditions, make Beijing's smogs markedly different from the 'pea soupers' thatafflicted London and other European cities 50 to 100 years ago. Manyatmospheric processes are nonlinear — meaning that the relationship betweencause and effect is not proportional. So we do not know and cannot predictwhich harmful compounds are being formed. Whenever my colleagues and I makemeasurements with new instruments in China, we find unexpected results. Indoorair quality is equally affected.

Thenation's urban air can begin to be cleaned only if a comprehensive monitoringand modelling approach takes atmospheric chemistry into account. To guidedecisions, we need to know which hazardous pollutants are present and how theyinteract to generate secondary pollution.

HereI outline a road map for bringing China's cities up to European air-qualitystandards within a decade. Actions towards cleaner air, as well as improvinghealth (see 'Deep clean'), will reduce greenhouse-gas and black-carbon (soot)concentrations and enhance freshwater quality3 and food supply.

Toxicmix

China'sair pollution has worsened as emissions from industry, energy production andtraffic have grown. China is responsible for 30–35% of the global SO2, NOx, COand particulate emissions, and 40% of global particle numbers (PN) in the 20–1,000-nanometresize range (see go.nature.com/uw3jx6). The nation's share of globalgreenhouse-gas emissions is 29% for carbon dioxide and almost 20% for methane.Government efforts are under way to reduce the emissions of all these.

Butit is impossible to reduce secondary pollutants such as ozone and organicaerosols without a deep understanding of the chains of chemical reactions andphysical processes that pollutants undergo. The formation and decay ofsecondary pollutants depend on temperature, humidity and wind speed as well asother chemicals and particles in the urban atmosphere. There is so much that wedo not know. Results are hard to predict because many physical and chemicalprocesses — such as surface chemistry, oxidation, clustering and dynamicaleffects — happen simultaneously4.

Attemptsto control one pollutant can increase the concentration of others. Measurementsin Nanjing, for example, show5 that reducing NOx emission could cause a tenfoldincrease in summer ozone concentrations. Reducing smog increases sunshinelevels and temperatures, and alters rain and snowfall patterns6.

Surprisingreactions are going on above Chinese cities that do not occur in cleaner air.For example, small atmospheric molecular clusters (measuring 1–3 nanometres) aretens of times more concentrated in Shanghai, Nanjing and Beijing than inEuropean cities. Secondary aerosols (containing sulfates, nitrogen compoundsand organics) form more readily in Shanghai and Nanjing than existing modelspredicts7. Unknown chemical pathways and physical processes must be occurring8that could create new types of oxidant or change the surface properties ofaerosols, limiting their ability to take up condensable vapours.

Indoorair quality is also a problem. City dwellers spend more than 90% of their timeindoors, particularly if outdoor air quality is poor. Cooking, smoking, heatingand furnishings release PM, PN, CO, volatile organic compounds (VOCs) and NOx,adding to pollutants drawn in from outside. Little is known about the pollutionlevels in Chinese homes, but my team's preliminary findings indicate thatconcentrations of some pollutants are higher than those outdoors (especiallyfor PN and VOCs) and 100–1,000 times higher than inside European homes. Mycolleagues and I estimate that indoor air pollution contributes almost as muchas outdoor smog (which causes more than 1.3 million deaths per year) to thepollution-related death toll.

Again,little is known about the secondary production of chemicals indoors.Ventilation systems filter large particles but not gases such as VOCs, NOx andSO2, which can go on to form ultrafine sulfates, nitrates and organicparticles. The risk of secondary aerosol production from these gases rises infiltered air because there are fewer grains on which to condense. As outdoorair gets cleaner, indoor air quality might even worsen.

Clearingthe air

Itis important to continue efforts to cut pollution. But meeting the Chinesecentral government's goal of improving urban air quality to levels typical ofthe United States and Europe requires more: simultaneous tracking of all airpollutants relevant to health and their feedbacks and interactions — over atleast a decade. We need to understand how the mixture and its toxicity changesas air quality measures are implemented. Short campaigns are not sufficient.

Centraland regional governments, research institutes and universities shouldcollaborate to do the following, with help from atmospheric chemists fromaround the world.

First,establish and fund a network of 'flagship' stations9 to monitor:concentrations, fluxes, interactions and feedbacks as well as more general airquality and meteorology data. Around 5–8 such stations (costing between US$7million and $11 million each) would suffice for a major city. These should becomplemented by mobile measurement platforms on cars and aeroplanes, remotesensing of air columns from the ground, satellite observations and smogchambers. Major sources of pollutants can be identified using historic data.

Second,indoor air-quality measurements and monitoring must be conducted concurrentlyin a representative selection of residential and office buildings.

Third,atmospheric chemists must model secondary-pollutant production pathways andfeedback mechanisms under high concentrations of various pollutants. Thesemodels must then be compared with observations.

Fourth,the links between air pollutants and mortality and other health effects need tobe established. That way the most health-relevant pollutants and their sourcescan be identified and mitigated first. A database should be developed to trackhealth impacts.

Fifth,long-term sustainable engineering solutions such as improving processes andmaterial flows in industry must be implemented to maintain low levels of airpollution. This will require capacity building across the Chinese authoritiesand institutes on using air-quality assessment data in decision-making, indeveloping legislative tools and in clean-air action plans.

Onlyby understanding atmospheric chemistry will China clean its air.

Nature526, 497–499 (22 October 2015) doi:10.1038/526497a

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