2015年11月15日，兩名中國女孩戴著口罩在霧霾中眺望故宮。由于工業排放、煤炭燃燒、汽車尾氣等原因，北京以及多座中國大城市的空氣污染程度經常超過世界衛生組織制定的健康標準許多倍。

最近，中國空氣嚴重污染的消息又引起了各大媒體的關注。上周，中國的霧霾水平再創歷史新高，全國有32座城市發布了最高污染警戒級別的“紅色預警”。在此之前，僅北京就于去年12月接連發布了兩次紅色預警，中小學、工廠大面積停課停工，半數車輛限行。

有人可能難以理解中國的空氣污染問題為何如此頑固，畢竟中國近年來的治污力度不可謂不大。2013年，網絡上甚至冒出了一個新造的英語詞來形容北京的嚴重霧霾——“airpocalypse”，意為“空氣末日”。在民眾的一片焦慮中，中國政府也出臺了大手筆的全國性大氣污染防治方案。隨著老百姓每天查看空氣污染程度預報已經成為了一種習慣，“PM2.5”這個詞也走入了千家萬戶。

首先要承認，2011年以來，中國在降低污染物排放上的確取得了巨大的成功。而且這還是在國民經濟持續保持7%以上的增長且煤炭使用量保持穩定的前提之下。比如二氧化硫的總排放量（主要來源是工業和居民燃煤）從2010年的2190萬噸下跌到了2014年的1970萬噸，與2006年時的2590萬噸相比，可以看出中國的二氧化硫排放量一直在穩步下跌。這主要歸功于中國對國內發電廠采取了強制脫硫措施。而二氧化氮的排放量（主要來自燃煤和汽車尾氣）也從2011年的2400萬噸下降至2014年的2080萬噸。而這兩種氣體正是產生大氣中含硫、氮顆料（即PM2.5的主要成分）的罪魁禍首。平均來說，全國污染水平已經呈趨平或下降趨勢。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年則下降了18%。

那么，如果中國政府果真采取了積極舉措治理大氣污染，而且全國的平均空氣質量也的確有所好轉，為什么北京等大城市又接連發布了紅色預警呢？政府又應該出臺哪些措施進行深入治理？

要回答第一個問題，我們首先要明白一個事實：中國的空氣污染問題是受一系列物理和化學因素影響的，這個問題比很多人想象的要復雜得多。首先要把極端空氣污染情況與年平均污染水平區分開，雖然前者也會影響后者。盡管紅色預警偶有發生，但這與年平均污染水平下降并不矛盾，只要全年其他時間里的污染水平能夠下降就可以了。極端污染情況是一個短期事件，通常是由反常的氣象條件引起的，比如風力靜穩天氣。這種氣象條件是短暫的，但可能影響大氣的化學反應，導致污染水平激增。

在某個區域生成的“霧霾”（主要成份是PM2.5和臭氧），不僅包括來自于化石燃料燃燒等來源的直接排放，如二氧化硫、二氧化氮等，還包括這些物質與空氣中的其它污染物發生化學反應所形成的“二級”污染物，如硫酸鹽和硝酸鹽顆料物等。

更復雜的是，這些化學反應還會受氣象因素的影響。這里的氣象因素不僅包括上文提到的靜穩天氣，還包括相對濕度、云層、逆溫等因素。這些物理化學因素在每個地區都不盡一致，科學家尚且難以完全厘清，更不用說政策制定者了。PM2.5的控制在任何國家都是一個循序漸進的過程，需要幾十年才能對其產生充分的科學理解，以及出臺有效政策進行治理。因為隨著國家經濟發展方式的變化，問題的本身也在不斷變化。

舉個例子就可以說明霧霾的化學機制有多復雜，甚至能讓辛辛苦苦的治理努力在某種程度上付之東流——我們在大氣化學領域的合作專家研究表明，中國雖然成功地削減了二氧化硫的排放量，然而這對華北地區的微量顆料物總量可能毫無影響。因為二氧化硫雖然減少了，另外一種被釋放出來的污染物——氨，卻通過大氣化學反應生成了大量的硝酸氨顆粒。

中國的極端重污染天氣主要發生在冬季的北方城市，一般需要比較少見的大氣靜穩條件（低風速和弱垂直混合）才會發生，這種大氣條件又造成了污染物的累積和相互反應。清華大學的科研團隊研究表明，PM2.5的輻射反饋也會進一步增強大氣的靜穩條件——霧霾對光輻射的吸收造成了空氣溫度的升高，同時降低了地表溫度，從而進一步削弱了空氣流動，使污染物難以擴散。另外，長期的大氣監測數據也表明，東亞地區冬季季風氣候模式趨弱、冷鋒（即寒流鋒面）力度下降以及年平均風速下降等氣候因素，可能也是導致冬季華北地區極端重污染天氣多發頻發的原因之一。

那么，政府下一步應該采取哪些措施呢？上述例子表明，政府應該加大力度降低氮氧化物以及非燃燒性污染物的排放量，比如氨。通過對火電廠的整治，中國在降低氮氧化物排放上已經取得了一些進步，然而隨著中國的有車一族越來越多，要想降低汽車尾氣的排放卻并非易事。如今北京已擁有600余萬輛機動車，它們在排放尾氣的同時也造成了嚴重的交通擁堵。雖然北京的公共交通系統也在發展，但要想讓更多北京及周邊地區的居民放棄開車，或者是轉而購買電動汽車，可能還需要實行一些創新政策很多年后才會有效。

另外，人們很少意識到氨也是一種空氣污染物，因為它并非是通過燃燒排放的，而主要來自于畜牧業以及化肥的使用。如果政府要對氨排放進行控制，就需要農業部出臺一系列新政策。

當然，中國也可以通過使用非化石能源來解決本國的大氣污染和氣候變化問題。中國在新能源領域的成就早已舉世皆知了，在不到10年的時間里，中國的風能發電量已達世界第一，而且仍然在繼續大力發展風能。同時，中國的水電發電量幾乎翻了三番。目前中國在全球光伏市場上已經處于主宰地位，并且最近又成為了全球第一大太陽能發電國。上周，中國國家能源局宣布，到2020年，中國將向可再生能源領域追加投資3610億美元。然而由于中國的火電規模實在太大，加之交通運輸領域還在持續增長，這些雄心勃勃的計劃也只能將非化石燃料占總體能源消費的比例從2014年的12.5%提高至2030年的20%。更何況要想實現這個目標，中國不僅要付出高昂的成本，還要解決一些難度極高的技術挑戰，將不太穩定的風能和太陽能整合到不太靈活的由火電主導的電力系統中。

除了技術上的挑戰以外，還有一些政策方面的挑戰，比如地方政府各行其是、國有行業權力過大等等。舉個例子：雖然現有的火電廠的利用率已經出現了下降，而且非化石能源的發電能力也在不斷提高，但最近又有100多家火電廠建設項目獲得了地方政府的審批。一座新火電廠的運營壽命大約在40年左右，由于中央政府未能有效控制地方決策，這也將給未來中國能源體系向可再生能源轉型帶來阻礙。這個問題跟中國鋼鐵行業的產能過剩問題大同小異，中國的鋼鐵企業大部分都是國有的，這些企業都想通過低價策略銷售和出口鋼鐵，以保證員工就業，所以它們對政府削減過剩產能的做法都很抗拒。因此，中國能否成功降低污染物的排放量，最終要取決于政府能否開辟經濟發展的替代途徑，并且為燒煤的企業和火電廠提供一種金融方案來關停或減少產能。

霧霾問題給北京帶來的政治挑戰也是不容忽視的。北京四周都被河北省包圍。河北是一個窮省，它的工業部門主要靠煤炭提供能源。河北省的工業排放是北京的主要大氣污染物來源，但河北省基本沒有什么替代方案能保障這些高污染行業的人員就業。要解開這個死結，就必須創造性地進行改革，并對現有政策做出轉變。

上述復雜的科學原理還表明，中國必須加倍努力地研究每個地區空氣污染的化學和物理成因。目前，中國政府不僅在國內開展了大量相關研究，同時也鼓勵國內科研機構與國外科學家合作研究這一課題。中國可能還得過上幾十年才能達到西方發達國家的空氣質量，但這些都表明，中國政府已經意識到，國家必須在未來的科技發展上投入重資，以應對霧霾天氣給公民健康帶來的壓力與挑戰。（財富中文網）

本文作者Chris Nielsen是哈佛大學中國能源經濟環境項目的常務主任。何文勝（Mun Ho）是該項目的一位經濟學家，他也是華盛頓未來資源研究所的訪問學者。點此查看本文的相關參考文獻。

譯者：樸成奎

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.