Dylos monitor: What is it actually measuring? PM2.5, PM10 or PM0.5?

Experiment published on Saturday, May 3rd, 2014

The Dylos air quality monitor comes in several version: Standard and Pro: Quoted from Dylos's website, it says that " the DC1100 Pro has all the features of the standard DC1100 but with increased lower sensitivity - detecting particles down to 0.5 micron (0.5µ). The large particle size range is calibrated to 2.5 microns (2.5µ) and above".

Interestingly, the most common sizes in the Air Quality industry is 2.5 µ (refered as PM2.5) and 10 µ (refered PM10). In this case, does it really make sense for the Dylos pro to measure particules as small as 0.5 µ? Shouldn't instead measure 2.5µ and 10µ particules? Actually, maybe this 0.5 µ? claim is more a marketing argument rather than a technology argumenet, since, from the empirical data, what it measures is closer to 2.5 µ and 10 µ rather than 0.5 µ and 2.5 µ! Here is the explanation why:


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On Thursday, April 10th 2014, in the morning, the Air in Beijing was very special: It is one of those rare days where the PM10 was the dominant pollutant, reaching up to 601 µm/m3 at noon time, which is equivalant to an AQI of 497 (at 24 hours exposure). At the same time, the trend for PM2.5 was going downward, i.e improving from 130 to 70 µm/m3 (equivalant to an AQI down from 189 to 158).


Usually (empirally verified), the dominent pollutant is the PM2.5 particules, and not PM10. Therefore, that special day, in order to ensure that this higher PM10 readings were not caused by defective equipment, we checked the Dylos readings to confirm the trend. This is what you can see from those two graphs. The zone highligted in orange is Thursday 10th, from midnight to noon. The graph on the right is plotting both BAM (in blue) PM2.5 readings (in µg/m3) and Dylos (in red) ≥0.5µ particule count, while the one on the left is for the BAM PM10 and Dylos ≥2.5µ.




Comparative analysis of Beijing Chaoyang Agricultural Exhibition Hall (朝阳农展馆) BAM monitoring stations and Beijing Sanlitun Dylos monitoring station (more info about the setup.)


If the Dylos monitoring claim to detect particules down to 2.5 µ for the large particule readings, then it should be expected that this large particules reading follow the PM2.5 trend. But this is not the case. Clearly, from the graph, the Dylos readings for large particules are following the PM10 trend while readings for small particule are following the PM2.5 trend. That clearly indicates that what the Dylos monitoring is detecting is closer to 2.5 µ and 10 µ rather than what they claim to be 0.5 µ and 2.5 µ.
Multidimensional correlation graph (credits: http://bl.ocks.org/mbostock/4063663.)


To further prove the point, the multidimensional correlation graph for all 4 readings is provided below. The color code used for the plot is orange for the Thursday morning readings, and blue for the other readings. The pairs "BAM PM10" and "Dylos >2.5" as well "BAM PM2.5" and "Dylos >0.5" is obviously showing a very good linear correlation, while the pair "BAM PM2.5" and "Dylos >2.5" is showing diversion, i.e. non linearity, especially for the given Thursday.


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The conclusion for the above analysis is clear and straight forward: The claim from Dylos to detect particles down to 0.5 µ is more of a marketing argument than a technology fact. From the above data, there is no doubt that the Dylos monitor readings are closer to detect size of 2.5µ and 10µ rather than 0.5µ and 2.5µ. And, actually, this is even better like this, since those are the sizes that are used for Air Quality Standard!




The next important question is how to convert the Dylos readings into the AQI standard. This will be presented in our next experiment.


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關於空氣質量與空氣污染指數

本網站採用的污染指數和顏色與EPA是完全相同的。 EPA的指數可以從 AirNow上查到

空气质量指数空气质量指数级别(状况)及表示颜色对健康影响情况建议采取的措施
0 - 50一级(优)空气质量令人满意,基本无空气污染各类人群可正常活动
51 -100二级(良)空气质量可接受,但某些污染物可能对极少数异常敏感人群健康有较弱影响极少数异常敏感人群应减少户外活动
101-150三级(轻度污染)易感人群症状有轻度加剧,健康人群出现刺激症状儿童、老年人及心脏病、呼吸系统疾病患者应减少长时间、高强度的户外锻炼
151-200四级(中度污染)进一步加剧易感人群症状,可能对健康人群心脏、呼吸系统有影响儿童、老年人及心脏病、呼吸系统疾病患者避免长时间、高强度的户外锻炼,一般人群适量减少户外运动
201-300五级(重度污染)心脏病和肺病患者症状显著加剧,运动耐受力降低,健康人群普遍出现症状儿童、老年人及心脏病、肺病患者应停留在室内,停止户外运动,一般人群减少户外运动
300+六级(严重污染)健康人群运动耐受力降低,有明显强烈症状,提前出现某些疾病儿童、老年人和病人应停留在室内,避免体力消耗,一般人群避免户外活动
(参考详见http://zh.wikipedia.org/wiki/空气质量指数)

如果你想了解更多有關空氣質量與污染,詳見維基百科或者 AirNow

有關健康建議詳​​見北京的Richard Saint Cyr MD醫生的博客:www.myhealthbeijing.com


注意事項: 所有空氣品質數據在發佈時均未經驗證,且為了確保資料準確性,這些數據可能隨時被修改,恕不另行通知。 世界空氣品質指數專案在編制這些訊息內容時已經謹慎的運用了所有適當的技巧,在任何情況下世界空氣品質指數在任何情況下,世界空氣品質指數專案團隊或其代理人均不對因提供此數據而直接或間接引起的任何損失、傷害或損害來承擔契約、侵權或其他責任。



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