I’ve taken the liberty of updating the US EPA’s AQI colors chart
A few days ago, the US embassy’s BeijingAir Twitter feed, which automatically reports Beijing’s hourly air quality, made headlines across the web by announcing that the air quality was “crazy bad.” Although the wording was quickly revised to the more politically-palatable “beyond index,” the impact was immediate. “Crazy bad” air was all the buzz of the blogosphere and at social events over the weekend; I have a feeling “crazy bad” would make it onto Beijing’s Word of the Year list, if there were such a thing. MyHealthBeijing‘s Dr. Richard St. Cyr even suggested “Crazy Bad” T-shirts.
In this post, I’ll look a little closer at the data behind the Crazy Bad incident to see what we can learn. The graph below shows hourly and daily data from the BeijingAir Twitter feed along with official air quality data from the Ministry of Environmental Protection (available here, Chinese only). For clarity, I show all data in terms of particulate matter concentration, not standardized index. (I’ve converted MEP’s reported API numbers to PM concentration; for background on the difference and methodology, see this post.) Note that MEP’s data is reported for a 24-hour period from noon to noon, which is why the daily data changes at noon each day. The break in the red corresponds to the BeijingAir Twitter feed not reporting any data for a couple of days after the Crazy Bad incident.
This graph reveals some really fascinating info:
1) The Crazy Bad spike on Thursday and Friday last week was both preceded and followed by gorgeous, wonderfully clean weekend days. On 11/15, the air quality was, by all accounts, “good.” By 11/18, though, the air pollution had steadily risen to what the US calls “hazardous“/”crazy bad,” and China calls “heavily polluted” (“重污染”). After the steep rise, the air quality improved just as dramatically; MEP’s reported PM10 numbers dropped 297 points – from 334 to 37 – from 11/21 to 11/22 alone. This demonstrates just how quickly the air quality can change in Beijing – both for better and for worse.
Why did it change so quickly? The start of the heating season on 11/15? Possibly, although that wouldn’t explain the sudden drop beginning 11/20. To be honest, the answer is probably less dramatic: weather. Most day-to-day pollution changes in Beijing are caused by changes in temperature and wind patterns. If there are a few days of static air, or light winds blowing from the southeast and trapping pollution against the mountains to the north and the west, the pollution builds up very quickly. I should find the time to post separately about this.
2) MEP’s air quality data tracked the embassy’s with reasonable consistency. This is actually encouraging; we should be grateful at least for some degree of accuracy and transparency with official data. There are some differences, but we shouldn’t expect them to track exactly. This is because the MEP data is an average of multiple sites across the city, while the embassy data is just a single point. Plus, they are measuring slightly different things.
In the past, we have seen situations where rapid and very short-term pollution spikes highlighted by the BeijingAir hourly readings were not reflected in daily MEP averages, but that doesn’t appear to have happened here.
(As I point this out, though, I should also note that I do not intend to make excuses for or to justify China’s current reporting mechanism. For the record, I would like to see at least three immediate changes to China’s air quality reporting: hourly release of data, more representative descriptions of health impacts, and some sort of real-time alert system for at-risk populations to avoid exposure.)
Lastly, a few data highlights for the numbers geeks out there (background on international standards here):
US EPA daily ambient air quality standard for PM2.5: 35 ug/m^3
Peak PM2.5 concentration reported by BeijingAir, 11/19: 557 ug/m^3
WHO recommended daily limit for PM10 exposure: 50 ug/m^3
China daily ambient air quality standard for PM10: 150 ug/m^3
Peak PM10 concentration reported by MEP, 11/19: 430 ug/m^3