【英语中国】细数五年来北京的雾霾天数 Beijing’s Bad-Air Days, Finally Counted

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2014-4-14 12:58

小艾摘要: Thanks to the U.S. State Department, Beijing residents finally have the answer to one of the city's greatest mysteries: How often is the air polluted?Well, a lot, a resident would say. But how many is ...
Beijing's Bad-Air Days, Finally Counted
Thanks to the U.S. State Department, Beijing residents finally have the answer to one of the city's greatest mysteries: How often is the air polluted?

Well, a lot, a resident would say. But how many is a lot? Heavy pollution levels can stretch for weeks, leaving locals to grouse over hot pot about how the smog seems to last forever. Then a crisp, blue-sky day comes with the western wind, and it's all forgotten.

China Real Time crunched the recent release of historical data on Beijing pollution from the U.S. State Department, which it gathered from a monitor installed at the U.S. Embassy there in 2008. First we crunched those hourly readings into daily averages. Then we converted those readings to match a widely used air-quality index and compared it to U.S. standards for air quality, which tell the public whether the air quality is good -- or not good -- for a person's health.

The results aren't pretty. Based on data collected for 2,028 days between April 2008 and March 2014, only 25 days were considered 'good' by U.S. standards.

Good, on the index, is a reading of 50 or below. The U.S. Environmental Protection Agency says readings of below 100 on the index are 'satisfactory.' But by that U.S. standard, Beijing isn't satisfactory, either.

Beijing's air-quality index has averaged above 100 for 1,632 days -- or about fourth-fifths of the time -- since April 2008. Based on Chinese standards, the air-quality index has averaged above 100 for 1,105 days -- or a bit more than half of the time.

There are caveats of course. The data aren't scientific because they stem from only one air-monitoring station in one location in the city. Chinese officials have many more monitors throughout the city and country, but they beefed up monitoring only relatively recently.

Still, the data offer a guide to how frequently the Chinese capital's skies get dirty -- as well as a useful comparison between Chinese and U.S. standards.

China has its own standard for what constitutes a 'good' or a 'hazardous' day. When China Real Time held the data to the Chinese standard, it found 380 'good' days over the same period.

The wide discrepancy is because the U.S. has tighter standards for concentrations of PM2.5 -- tiny particulate matter measuring less than 2.5 microns in length that are harmful to human health. The U.S. limit for a 'good' day is a PM2.5 concentration of no more than 12 micrograms per cubic meter. By comparison, China's limit for a 'good' day is 34 micrograms per cubic meter.

The biggest discrepancy between the U.S. and China are days that are 'unhealthy for sensitive groups.' Based on U.S. standards, 979 days -- roughly half the days surveyed -- were in this category. Based on Chinese standards, there were only 245 days in this category.

Sensitive groups are people with respiratory or heart disease, the elderly and children, according to the EPA. Health effects on them include the increasing likelihood of respiratory symptoms, aggravation of heart or lung disease and premature mortality, the EPA says.

The U.S. and China both agree on days that were 'very unhealthy' and 'hazardous.' Of the more than 2,000 days surveyed, about 311 days were 'very unhealthy' and about 94 days were 'hazardous.'

Reuters
北京空气污染的天数到底有多少?这个北京居民心中最感困惑的问题,在美国国务院的帮助下终于有了答案。

很多人会说,很多。但多少才算得上很多呢?北京的重度污染有时会持续好几周,令当地人茶余饭后无不慨叹雾霾永无消散之日。可接着,一阵西风吹出了一片湛蓝的天空,人们对雾霾天的记忆于是也随风而逝。

“中国实时报”栏目对美国国务院近期发布的北京空气污染历史数据进行了分析,该数据收集自北京美国驻华大使馆2008年安装的一个空气监测仪。首先我们将每小时数据折算成日均值,之后根据一套广泛使用的空气质量指数(AQI)体系对数据进行换算,并把换算后的AQI数值与美国空气质量标准进行比对。这一标准可令公众了解空气质量对健康有益还是有害。

结果不容乐观。根据2008年4月至2014年3月收集到的2,028天数据,只有25天的空气质量在美国标准下可被视为“良好”。

“良好”即AQI小于或等于50。美国环境保护署(U.S. Environmental Protection Agency)称,AQI小于或等于100时空气质量“令人满意”。但以该标准来看,北京的空气也并不令人满意。

自2008年4月以来,北京有1,632天的AQI日均值在100以上,占全部天数的五分之四。根据中国标准,北京有1,105天的AQI在100以上,略高于总天数的一半。

当然要说明的是,相关数据并不非常科学,因为这些数据全部来自设置在北京一个地点的一个空气监测站。北京市和全国各地设有更多的监测站点,但中国官员只是在不久之前才加大了监测力度。

不过,相关数据为衡量中国首都天空的变脏频率提供了参考,同时也为中美标准提供了有益的比较。

对于什么样的空气质量算是“良好”或“有毒害”,中国有自己的标准,“中国实时报”将相关数据与中国标准进行了比较,发现在同一个时期内,空气质量“良好”的天数为380天。

出现此种差异的原因是,美国标准对PM2.5的浓度规定更加严格。PM2.5是指长度小于2.5微米的颗粒物,对人体健康有害。美国对空气质量“良好”的规定是,PM2.5的浓度不高于每立方米12微克。相比之下,中国对“良好”的规定是PM2.5的浓度不高于每立方米34微克。

中美标准之间最大的差异是“对敏感人群不健康”的天数。按照美国的标准,天数为979天,大致相当于所调查天数的一半,而按照中国的标准,则只有245天。

按照美国环境保护署的说法,敏感人群指的是有呼吸系统疾病或心脏病的人,以及老人和小孩。美国环境保护署说,对这些人的健康影响包括出现呼吸系统症状的可能性增大、心脏病或肺病程度加重、以及过早死亡。

美国和中国数据相符的是“非常不健康”和“有毒害”的天数。在调查过的2,000多天中,约311天属于“非常不健康”,约94天属于“有毒害”。

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