霧霾之下京滬居民戶外活動如何擇時

With its notoriously smoggy skies, Beijing seems to launch a new initiative to reduce air pollution almost every week.

在北京灰濛濛的天空已經“聞名遐邇”之際，該城市似乎每週都要推出一項減少空氣污染的新措施。

Last week, local authorities said they wanted to build wind tunnels to funnel wind through the streets and blow away the bad air. This week, the state-run People's Daily reported plans to spend $7.7 billion in the next five years to reduce air pollution. That spending will go toward measures that reduce air pollution from coal-fired power plants, vehicles, and industrial facilities. Such measures include building waste-gas treatment facilities, installing emissions-reduction technology at factories and power plants and using cleaner alternative fuels, it said.

當地有關部門上週稱，他們想建立風道，引導風從大街小巷通過並吹散霧霾。官方媒體《人民日報》本週則報道稱，北京市計劃5年內斥資77億美元以減少大氣污染；這些資金將用於燃煤、機動車排放、工業等方面的污染防治措施，具體包括建設廢氣處理設施、對工廠和電廠進行低排放術改造和採用清潔燃料替代燃煤。

It's an uphill battle, with some scientists concluding that China's air quality is continuing to get worse despite such efforts. Environmental authorities recently said Beijing's air quality won't drop to healthy levels until 2030. Other cities across the country face similar challenges.

這是一場苦戰，一些科學家得出結論稱，儘管採取了這類措施，但中國的空氣質量將繼續惡化。環保部門最近稱，北京的空氣質量達標要等到2030年。中國其他城市也面臨着類似的挑戰。

That's grim news. But if you can't avoid the air pollution, at least you can try and avoid the times when it's at its worst. Last week, China Real Time did the math to figure out the best months for air quality in Beijing and Shanghai. This week, we've crunched more numbers to determine the worst (and best) hours for residents to be outdoors in both cities.

這是個不幸的消息。但如果你無法避免空氣污染，至少你可以嘗試躲開污染最嚴重的時期。上週，《中國實時報》欄目(China Real Time)算出了北京和上海空氣質量最佳的月份。本週，我們調用了更多數據，確定了兩個城市居民戶外活動的最糟（以及最好）的時段。

The results show that Beijing's air improves during the day, but gets worse at night. In Beijing, air quality is at its best between noon and 4 p.m., when the average concentration of PM2.5 -- particles measuring less than 2.5 microns in length -- falls to around 87 micrograms per cubic meter. By contrast, between 7 p.m. and 3 a.m., PM2.5 levels rise to as high as 113 micrograms per cubic meter.

計算結果顯示，北京的空氣質量在白天會好轉，晚上會惡化。在北京，空氣質量最好的時間是中午到下午4點，平均PM2.5（直徑小於2.5微米的顆粒物）濃度降至每立方米87微克。與之相比的是，下午7點到凌晨3點，PM2.5水平升至每立方米113微克之高。

Shanghai's air gets worse after rush hour. The city's air is cleanest during the early morning and late afternoon, when PM2.5 levels fall to 53 micrograms per cubic meter or lower. After the morning and evening rush hour, PM2.5 levels rise to as high as 59 micrograms per cubic meter.

上海空氣質量在交通高峯期過後會下降。這個城市的空氣是在大早上和臨近傍晚最乾淨，PM2.5水平降至不超過每立方53微克。在早晚高峯過後，PM2.5水平會升至每立方米59微克。

Why does Beijing's air quality get worse at night? Antony Chen, an associate research professor at the Desert Research Institute in Nevada, notes that Beijing's air quality is mainly influenced by the difference in temperature between night and day.

北京的空氣質量爲什麼會在夜間變差？內華達州沙漠研究所(Desert Research Institute)的副教授Antony Chen指出，北京的空氣質量主要受到晝夜溫差的影響。

Mr. Chen says PM2.5 gets trapped in the cooler air near the surface in the evenings, which accounts for the higher concentrations at night. When the sun comes out, the air heats up and rises, which leads to lower concentrations in the day, he says.

Chen說，傍晚的時候，接近地表的空氣溫度較低，PM2.5在這樣的空氣裏無法擴散，這就是爲什麼在夜裏PM2.5的濃度會更高。他說，等到太陽一出來，空氣溫度上升後上行，所以白天PM2.5的濃度較低。

'The warm layer of air wants to go up, and the cold air wants to go down, and there basically becomes circulation, movement and turbulence,' Mr. Chen says.

Chen說，暖空氣會往上走，冷空氣會向下走，由此形成對流、空氣運動和湍流。

By contrast, Shanghai's temperature is more constant, thanks to its proximity to the ocean, says Mr. Chen. With fewer temperature swings during the day, he says, Shanghai's air pollution patterns are more impacted by factors such as vehicle emissions.

Chen說，相比之下，上海的晝夜溫差因爲近海的緣故變化較小。他說，由於白天上海的溫度變化更小，所以當地空氣污染模式受機動車尾氣排放等因素的影響更大。

Based on data from the U.S. State Department, the average PM2.5 concentration over a 24-hour period is about 99 micrograms per cubic meter in Beijing and 56 micrograms per cubic meter in Shanghai. That means Shanghai's air is on average about 40% better than Beijing's. (The U.S. State Department cautions that such data is not complete, as the readings come from a single monitoring station that was installed in Beijing in 2008 and in Shanghai in 2011.)

根據美國國務院的數據，北京24小時PM2.5平均濃度大約爲每立方米99微克，上海是56微克。這表明上海的空氣質量平均而言比北京好40%。（美國國務院稱，上述數據並不全面，因爲數據只來自2008年在北京設立、2011年在上海設立的單一監測站。）

But no matter the time of day, neither city has anything approaching clean air. Air quality in both Beijing and Shanghai falls far short of the U.S. Environmental Protection Agency's recommended PM2.5 limit of 35 micrograms per cubic meter for a 24-hour period. High concentrations of PM2.5 increases risks for cardiovascular and respiratory diseases, as well as lung cancer, according to the World Health Organization.

但無論是一天中的哪個時段，這兩個城市的空氣都算不上乾淨。美國國家環境保護局(Environmental Protection Agency)建議的24小時PM2.5均值上限爲每立方米35微克，北京和上海的空氣質量都差得遠。根據世界衛生組織(World Health Organization)的說法，高濃度PM2.5會增加罹患心血管、呼吸系統疾病以及肺癌的風險。