Chinese air quality measurements lower than US

There are two pollution measurements (fine particulates less than 2.5 µm in diameter, PM 2.5) published in Beijing, one from the US Embassy and one official Chinese measurement. The US measurement has caused quite some controversy, and Chinese authorities claim that its illegal. Still, the US measurement has been trusted by both expats and Chinese for many years, while the Chinese only started publishing their PM 2.5 measurements this January. Just out of curiosity I have systematically sampled the two, and found that even though both report on average unhealthy pollution levels the Chinese measurement is significantly lower than the US, meaning that the Chinese report lower pollution levels or better air quality. When comparing the two it should be noted that the Chinese measurement is apparently somewhere in western Beijing, whereas the US measurement is performed at their embassy in Chaoyang.

Method: From April 17th 2012 at 19:00 to May 16th at 22:00 I recorded both US and Chinese measurements every hour. The Chinese measurement is only available for 24 hours, and I missed recording them at 10 instances. There was also 19 instances where there was no US measurement and 5 instances where there was no Chinese measurement. This left me with a sample size of 667 measurement pairs.

The US Embassy publishes both PM 2.5  and air quality index (AQI). The AQI is a handy measure for the public to understand the severity of the pollution, but the conversion from PM 2.5 to AQI is nonlinear, hence for statistical analysis one should use the PM 2.5 measurements.The US embassy PM 2.5 measurement is in µg/m3 and the Chinese in mg/m3, and I’ve been converting to µg/m3 throughout. Just for reference I have converted some results to AQI.

For statistical analysis I’ve been using a paired t-Test and linear regression. I have not performed any analysis of outliers but included all measurements.

Results: Descriptive statistics:
US PM 2.5 (µg/m3): max: 312, min: 4, average: 113.9
Chinese PM 2.5 (µg/m3) : max: 264, min: 3, average: 77.5
Converting these results to AQI, this means that the US measurement is on average 179 and the Chinese is 158, both falling in the AQI category “Unhealthy”. The maximum levels reported by the two corresponds to AQI 362 and 314 respectively. All AQI levels above 300 are considered “Hazardous”.

According to the t-test the correlation between the US and Chinese PM 2.5 measurements is suprisingly low, only 0.57, indicating that there is in fact a very poor relationship between the two. By reading one one cannot really guess what the other one might be and one cannot substitute the other.

Histogram of US vs Chinese AQI measurements

Histogram of US and Chinese AQI measurements in Beijing. These figures are the AQI values, and one can clearly see that the Chinese measurements are skewed towards lower AQI values than the US. ‘Sensitive’ refers to the category ‘Unhealthy for sensitive groups’ and ‘V unhealthy’ is the ‘Very unhealthy groups.

Scatter plot of US and Chinese PM 2.5 measurements

Scatter plot of US and Chinese PM 2.5 measurements (not AQI values) with 95% confidence intervals. This plot can be interpreted as that if the Chinese PM 2.5 measurement is for instance 100 µg/m3, one can be 95% sure that the US measurement will be between approximately 50 and 220 µg/m3. In other words the one is not a good indicator of what the other might be.

Please note: I have no knowledge of the technology behind pollution measurements, hence I am not able to make any discussion of the reasons for the discrepancies between the US and Chinese measurements. I am also not able to judge which one is the “better” or more reliable one. It is also important to note that they are located quite far from each other. I have just done this comparison because I thought it was fun, thus I make no scientific claims.

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