BEIJING, CHINA - NOVEMBER 15: Chinese women wear masks as haze from smog caused by air pollution hangs over the Forbidden City on November 15, 2015 in Beijing, China. As a result of industry, the use of coal, and automobile emissions, the air quality in China's capital and other major cities is often many times worse than standards set by the World Health Organization. (Photo by Kevin Frayer/Getty Images) Kevin Frayer Getty Images

Commentary

China’s problems with severe air pollution are back in the news. Last week, smog levels in China reached historic levels; as many as 32 cities were under “red alert,” the country’s most severe pollution warning. This followed two other red alerts in Beijing in December, which resulted in closures of schools and factories; half of the capital’s cars were banned from roads.

The persistence of China’s air pollution may puzzle some, given the country’s campaign to curb pollution in recent years. In 2013, a new English word, “airpocalypse,” emerged after severe smog in Beijing prompted the government to enact a massive national air pollution control plan amid public anxiety. “PM2.5,” a term for fine particulates, also entered the public lexicon as citizens monitored the daily reports of pollution levels.

Indeed, China has had substantial successes in reducing pollution emissions since 2011, even as its economy has grown more than 7% per year and coal use has stabilized. For example, total sulfur dioxide (SO2) emissions from all sources (mainly industrial and residential coal combustion) fell from 21.9 million tons in 2010 to 19.7 million in 2014, continuing a downward trend from 25.9 million in 2006 initiated by mandated desulfurization of emissions from electric power plants. Total emissions of nitrogen oxides (NOX), chiefly from the same coal-burning sources as well as fuel use by vehicles, fell from 24 million tons in 2011 to 20.8 million in 2014. These two gases form sulfate and nitrate particles in the atmosphere, key components of PM2.5. On average, national pollution levels have been stable or falling as a result. In Beijing, the annual average PM2.5 level was 73 micrograms per cubic meter in 2016, down 9.9% from 2015 and 18% from 2013.

If China’s government has acted aggressively and average levels have improved, why then do Beijing and other cities continue to experience red-alert pollution events? What more can the government do to reduce them?

The answer to the first question begins with the fact that China’s air pollution is influenced by a wide variety of physical and chemical factors; the problems are a lot more complex than most realize. One should first differentiate severe air pollution episodes and annual average pollution levels, although the former contributes to the latter. Annual average pollution levels can decline in spite of occasional red-alert spikes, as long as levels fall sufficiently over the rest of the year. A pollution episode is a short-term event, generally initiated by unusual meteorological conditions—such as low wind speeds—that are themselves transient and may affect chemical processes in the atmosphere that contribute to the spike.

Smog—chiefly PM2.5 and ozone—in a given location results not only from “primary” pollutants, meaning those emitted directly from fossil fuels and other sources, including SO2, NOX, and some forms of PM2.5. It also results from chemical reactions of these and other pollutants in the air, creating new “secondary” pollutants, including sulfate and nitrate particles.

Complicating things further, this chemistry is influenced by meteorology, not just wind speeds but also relative humidity, cloudiness, temperature inversions, and more. The complex chemical and physical pathways unique to each region are challenging for scientists to untangle, let alone for policy makers to address. PM2.5 control progresses incrementally in all countries, taking decades to build the requisite scientific understanding and to develop and enforce effective policies, as the problems themselves evolve due to changes in the underlying economy.

As an example of how the chemical complexity can confound hard-won efforts at pollution control, research by our collaborators in atmospheric chemistry suggests that China’s successful reduction of SO2 emissions may have had no effect on fine particles overall in North China. This is because the reduced SO2 may simply free another pollutant, ammonia, to react instead with abundant NOX creating ammonium nitrate particles.

China’s worst air pollution episodes occur in northern cities in the winter, and generally require unusually stable atmospheric conditions (low wind speeds and weak vertical mixing), which allow pollutants to accumulate and react with each other. Research by colleagues at Tsinghua University suggests a PM2.5-radiation feedback might be reinforcing these conditions; absorption of solar radiation by airborne haze effectively warms the atmosphere aloft and reduces warming of the surface, further reducing air movement and trapping pollutants. Moreover, long-term meteorological data now suggest a weakening winter monsoon climate pattern in East Asia, diminishing strength of cold fronts, and declining annual average wind speeds. It is possible that climate change is playing a role in the persistent severity of episodic wintertime pollution events in North China.

As to what more the government can do, the example above shows that greater efforts may be needed to reduce emissions of NOX and also non-combustion pollutants, including ammonia. Beijing is making progress in reducing NOX from coal-fired plants, which is relatively straightforward if costly, but it is more difficult to do so from the transportation sector as more people drive. There are 6 million vehicles in Beijing today spewing fumes while crawling along congested roads. The transit system is expanding, but it will take many years of innovative policy to get more people in the city and surrounding areas to leave their cars or switch to electric vehicles.

What’s more, ammonia is rarely even recognized as an air pollutant because it results not from combustion but livestock farming and fertilizer use, and its control would require new types of policies developed with the Ministry of Agriculture.

China of course can address local air pollution and climate change by shifting to non-fossil energy sources. The government’s efforts to expand renewables are now well known: in less than 10 years it built the world’s largest wind power capacity, and continues to expand it. China is also nearly tripling its hydropower capacity; it now dominates the world market in photovoltaic cell production and has just become the world’s largest generator of solar power. Last week, China’s National Energy Administration announced investment of another $361 billion into renewable power generation by 2020. However, given the large base of coal-fired electricity and growing transport sector, these ambitious plans will only raise the non-fossil fuel share of total energy consumption to 20% by 2030 from 12.5% in 2014. Achieving this goal will not only be expensive but also involves difficult technical challenges of integrating inherently variable wind and solar power into an inflexible coal-dominated system.

These technical challenges are compounded by political ones that arise from decentralized decision-making in China and the bureaucratic power of state-owned industries. A prime example is a glut of more than 100 new coal-fired power plants now approved for construction by local governments, despite falling utilization rates at existing plants, and growing non-fossil generating capacities. Given 40-year lifetimes of new plants, this failure of the central government to control local decision-making may further entrench resistance to a shift to renewables for decades into the future. This resembles the excess capacity in the steel-making industry: iron and steel enterprises, which are mostly state-owned, are willing to sell and export at low prices to maintain employment, and are resisting efforts to curtail production. The ability to reduce pollution emissions thus depends crucially on the ability of the political system to find alternative sources of economic development and provide a financial path for coal-fired industrial enterprises and power plants to close or reduce output.

In the case of Beijing, the political challenges are profound. The city is surrounded by Hebei, a poor province with a large industrial sector fueled by coal. Its emissions are major sources of pollution in the capital, but Hebei’s government has little alternative to maintaining employment in these industries. Untangling this knot will require imaginative reform and transfer policies.

The complex science noted above also indicates that efforts must be redoubled to build knowledge on the chemical and physical processes driving air pollution in each region. The government is attempting to do so by underwriting domestic research and encouraging partnerships with scientists abroad. China may still be decades away from the air pollution levels of western countries, but it is telling that its government recognizes it must invest heavily in scientific advance for the future as it also grapples with the challenges and tensions of protecting the environmental well-being of its citizens today.

Chris Nielsen is executive director of the Harvard-China Project on Energy, Economy and Environment, Mun Ho is an economist there. Ho is also a Visiting Scholar at Resources for the Future, Washington DC. Click here to see full citation of research and papers.

2015年11月15日，两名中国女孩戴着口罩在雾霾中眺望故宫。由于工业排放、煤炭燃烧、汽车尾气等原因，北京以及多座中国大城市的空气污染程度经常超过世界卫生组织制定的健康标准许多倍。

最近，中国空气严重污染的消息又引起了各大媒体的关注。上周，中国的雾霾水平再创历史新高，全国有32座城市发布了最高污染警戒级别的“红色预警”。在此之前，仅北京就于去年12月接连发布了两次红色预警，中小学、工厂大面积停课停工，半数车辆限行。

有人可能难以理解中国的空气污染问题为何如此顽固，毕竟中国近年来的治污力度不可谓不大。2013年，网络上甚至冒出了一个新造的英语词来形容北京的严重雾霾——“airpocalypse”，意为“空气末日”。在民众的一片焦虑中，中国政府也出台了大手笔的全国性大气污染防治方案。随着老百姓每天查看空气污染程度预报已经成为了一种习惯，“PM2.5”这个词也走入了千家万户。

首先要承认，2011年以来，中国在降低污染物排放上的确取得了巨大的成功。而且这还是在国民经济持续保持7%以上的增长且煤炭使用量保持稳定的前提之下。比如二氧化硫的总排放量（主要来源是工业和居民燃煤）从2010年的2190万吨下跌到了2014年的1970万吨，与2006年时的2590万吨相比，可以看出中国的二氧化硫排放量一直在稳步下跌。这主要归功于中国对国内发电厂采取了强制脱硫措施。而二氧化氮的排放量（主要来自燃煤和汽车尾气）也从2011年的2400万吨下降至2014年的2080万吨。而这两种气体正是产生大气中含硫、氮颗料（即PM2.5的主要成分）的罪魁祸首。平均来说，全国污染水平已经呈趋平或下降趋势。在北京，2016年PM2.5的平均水平是每立方米73微克，比2015年下降了9.9%，相比2013年则下降了18%。

那么，如果中国政府果真采取了积极举措治理大气污染，而且全国的平均空气质量也的确有所好转，为什么北京等大城市又接连发布了红色预警呢？政府又应该出台哪些措施进行深入治理？

要回答第一个问题，我们首先要明白一个事实：中国的空气污染问题是受一系列物理和化学因素影响的，这个问题比很多人想象的要复杂得多。首先要把极端空气污染情况与年平均污染水平区分开，虽然前者也会影响后者。尽管红色预警偶有发生，但这与年平均污染水平下降并不矛盾，只要全年其他时间里的污染水平能够下降就可以了。极端污染情况是一个短期事件，通常是由反常的气象条件引起的，比如风力静稳天气。这种气象条件是短暂的，但可能影响大气的化学反应，导致污染水平激增。

在某个区域生成的“雾霾”（主要成份是PM2.5和臭氧），不仅包括来自于化石燃料燃烧等来源的直接排放，如二氧化硫、二氧化氮等，还包括这些物质与空气中的其它污染物发生化学反应所形成的“二级”污染物，如硫酸盐和硝酸盐颗料物等。

更复杂的是，这些化学反应还会受气象因素的影响。这里的气象因素不仅包括上文提到的静稳天气，还包括相对湿度、云层、逆温等因素。这些物理化学因素在每个地区都不尽一致，科学家尚且难以完全厘清，更不用说政策制定者了。PM2.5的控制在任何国家都是一个循序渐进的过程，需要几十年才能对其产生充分的科学理解，以及出台有效政策进行治理。因为随着国家经济发展方式的变化，问题的本身也在不断变化。

举个例子就可以说明雾霾的化学机制有多复杂，甚至能让辛辛苦苦的治理努力在某种程度上付之东流——我们在大气化学领域的合作专家研究表明，中国虽然成功地削减了二氧化硫的排放量，然而这对华北地区的微量颗料物总量可能毫无影响。因为二氧化硫虽然减少了，另外一种被释放出来的污染物——氨，却通过大气化学反应生成了大量的硝酸氨颗粒。

中国的极端重污染天气主要发生在冬季的北方城市，一般需要比较少见的大气静稳条件（低风速和弱垂直混合）才会发生，这种大气条件又造成了污染物的累积和相互反应。清华大学的科研团队研究表明，PM2.5的辐射反馈也会进一步增强大气的静稳条件——雾霾对光辐射的吸收造成了空气温度的升高，同时降低了地表温度，从而进一步削弱了空气流动，使污染物难以扩散。另外，长期的大气监测数据也表明，东亚地区冬季季风气候模式趋弱、冷锋（即寒流锋面）力度下降以及年平均风速下降等气候因素，可能也是导致冬季华北地区极端重污染天气多发频发的原因之一。

那么，政府下一步应该采取哪些措施呢？上述例子表明，政府应该加大力度降低氮氧化物以及非燃烧性污染物的排放量，比如氨。通过对火电厂的整治，中国在降低氮氧化物排放上已经取得了一些进步，然而随着中国的有车一族越来越多，要想降低汽车尾气的排放却并非易事。如今北京已拥有600余万辆机动车，它们在排放尾气的同时也造成了严重的交通拥堵。虽然北京的公共交通系统也在发展，但要想让更多北京及周边地区的居民放弃开车，或者是转而购买电动汽车，可能还需要实行一些创新政策很多年后才会有效。

另外，人们很少意识到氨也是一种空气污染物，因为它并非是通过燃烧排放的，而主要来自于畜牧业以及化肥的使用。如果政府要对氨排放进行控制，就需要农业部出台一系列新政策。

当然，中国也可以通过使用非化石能源来解决本国的大气污染和气候变化问题。中国在新能源领域的成就早已举世皆知了，在不到10年的时间里，中国的风能发电量已达世界第一，而且仍然在继续大力发展风能。同时，中国的水电发电量几乎翻了三番。目前中国在全球光伏市场上已经处于主宰地位，并且最近又成为了全球第一大太阳能发电国。上周，中国国家能源局宣布，到2020年，中国将向可再生能源领域追加投资3610亿美元。然而由于中国的火电规模实在太大，加之交通运输领域还在持续增长，这些雄心勃勃的计划也只能将非化石燃料占总体能源消费的比例从2014年的12.5%提高至2030年的20%。更何况要想实现这个目标，中国不仅要付出高昂的成本，还要解决一些难度极高的技术挑战，将不太稳定的风能和太阳能整合到不太灵活的由火电主导的电力系统中。

除了技术上的挑战以外，还有一些政策方面的挑战，比如地方政府各行其是、国有行业权力过大等等。举个例子：虽然现有的火电厂的利用率已经出现了下降，而且非化石能源的发电能力也在不断提高，但最近又有100多家火电厂建设项目获得了地方政府的审批。一座新火电厂的运营寿命大约在40年左右，由于中央政府未能有效控制地方决策，这也将给未来中国能源体系向可再生能源转型带来阻碍。这个问题跟中国钢铁行业的产能过剩问题大同小异，中国的钢铁企业大部分都是国有的，这些企业都想通过低价策略销售和出口钢铁，以保证员工就业，所以它们对政府削减过剩产能的做法都很抗拒。因此，中国能否成功降低污染物的排放量，最终要取决于政府能否开辟经济发展的替代途径，并且为烧煤的企业和火电厂提供一种金融方案来关停或减少产能。

雾霾问题给北京带来的政治挑战也是不容忽视的。北京四周都被河北省包围。河北是一个穷省，它的工业部门主要靠煤炭提供能源。河北省的工业排放是北京的主要大气污染物来源，但河北省基本没有什么替代方案能保障这些高污染行业的人员就业。要解开这个死结，就必须创造性地进行改革，并对现有政策做出转变。

上述复杂的科学原理还表明，中国必须加倍努力地研究每个地区空气污染的化学和物理成因。目前，中国政府不仅在国内开展了大量相关研究，同时也鼓励国内科研机构与国外科学家合作研究这一课题。中国可能还得过上几十年才能达到西方发达国家的空气质量，但这些都表明，中国政府已经意识到，国家必须在未来的科技发展上投入重资，以应对雾霾天气给公民健康带来的压力与挑战。（财富中文网）

本文作者Chris Nielsen是哈佛大学中国能源经济环境项目的常务主任。何文胜（Mun Ho）是该项目的一位经济学家，他也是华盛顿未来资源研究所的访问学者。点此查看本文的相关参考文献。

译者：朴成奎