A Beginner's Guide to Air Quality Instant-Cast and Now-Cast.

Posted on March 15th 2015
Share: aqicn.org/faq/2015-03-15/air-quality-nowcast-a-beginners-guide

Typhoon and Dust on East Asia, Courtesy of NASA (attribution)

Back in March 2015, we had the chance to meet in Beijing several environmental specialists from the US State Department as well as the China Mission (the famous entity running the PM2.5 monitors in the Beijing US embassy).

Among all the topics that were addressed, there is one worth writing about, namely the "nowcast" system. This system used by the US EPA to convert the raw pollutants readings, expressed in µg/m3 or ppb, into the AQI (scale from 0 to 500). It is used for all the AQI values reported on the airnow.gov website.

The concept behind the nowcast is to compensate the "24 hours averaging", which should be used when converting concentrations to AQI. The reason for this averaging is that the AQI scale specifies that each of the Levels of Health Concern (i.e. Good, Moderate,... Unhealthy...) is valid under a 24 hours exposure[1]. For example, when seeing a 188 AQI (Unhealthy), one need to read it as "if I stay out for 24 hours, and the AQI is 188 during those 24 hours, then the health effect is Unhealthy". This is quite different from saying that "if the AQI reported now is 188, then the health effect is Unhealthy".

The problem is that the 24 hours averaging is a very bad idea, and should abolished[3] for at least those two reasons:
  • First, the dynamic of Air Pollution is such that wind than completely clean the air in less than 30 minutes! This phenomenon is frequently seen in Beijing with the strong north winds able to bring the PM2.5 AQI from more than 300 to less than 50 in less than one hour[2]. When this happens, no one wants to wait for 24 hours before knowing that the Air Quality is good (and go out for a walk to enjoy the fresh air!)

  • The second reason is when the Air Quality suddenly gets worse. One famous case is the Indonesian wildfire causing the Singapore Smog when the winds are heading to the north, under which circumstances the AQI can go from below 50 to more than 150 is just one hour. As a matter of fact, we have had many requests from asthmatic/sensitive people when Singapore was still only providing 24 hours average readings.
This is for those reasons that the US EPA introduced the nowcast system: It is an alternative conversion formula used to counter balance the need for averaging under changing Air Quality conditions. From Wikipedia, it is described as:
The NowCast is computed from the most recent 12 hours of PM monitoring data, but the NowCast weights the most recent hours of data more heavily than an ordinary 12-hour average when pollutant levels are changing. The NowCast is used in lieu of a 24-hour average PM concentration in the calculation of the AQI until an entire calendar day of hourly concentrations has been monitored."
The formula used to calculate the NowCast AQI might seem barbarian for non-scientist.

and Wmin = 1/2 and N = 12

But actually, it is quite simple when looking at this applied example for Beijing Air Quality.

Instant AQI (no averaging, AQI converted directly from the 1 hour readings):

NowCast AQI:


The result from the nowcast calculated AQI is still very pronounced in terms of averaging, at least much more than what one living in the Asian dust would expect. One of the reasons is that the weight factor, which used to determine the level of change, has a threshold set to a minimum of 0.5, while in actual fact, its value is much lower than 0.5 is many cases. The following graph shows this threshold weight factor, where red is used to denote those weigths below 0.5

NowCast weight factors:

The nowcast formula, as it is defined nowdays, is the fruit of the research based on Air Quality levels in America. While for USA it gives an accurate reporting of the current conditions, it might not be the case of Asia where the dynamic of Air Quality is much more strong. For this reason, we do believe that the nowcast formula should be adapted for each continents. For Asia, it could be defined as Wmin = 1/10 and N = 3 , corresponding to the following graph:

Modifed NowCast for Asia (aka NowCast Asia AQI):

If you want, you can also directly change the two paramters yourself: Wmin = and N =
For reference, Instant AQI:


For now, the conversion formula used on the World Air Quality Index project as well as the China Mission AQI (aka the Beijing US embassy, Shanghai US consulate, etc..) is still based on the "Instant AQI", a.k.a "InstantCast" (i.e. no averaging at all). But, based on the outcome of this "Asian Nowcast" proposal, this might be updated quite soon.

...

[1] The 24 hours duration is based on decades of epidemiological studies used to correlate the air pollution exposure and increased risk for health. There are new ongoing epidemiological studies to correlate the effect of instant exposure (i.e. exposure of around 1 hour or less), but it can take several years before their results can be converted to a new offical "instant AQI scale".
[2] Strong wind have quite often an effect on increasing the PM10 AQI due to the dust resuspension.
[3] At the time of writing , in March 2015, there were many countries still providing only 24 hours average. Nowdays, in September 2016, the only two known countries still providing only 24 hours average are Malaysia and Brunei. All other countries are providing not only unaverage (hourly) data, but also individual concentration for each pollutant.


Click here to see all the FAQ entries
  • Nitrogen Dioxyde (NO2) in our atmosphere
  • Ozone AQI Scale update
  • Kriging Interpolation




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    空気の質と大気汚染指数で

    大気品質レベルについて

    指数大気質指数の分類(米国)健康影響 / カテゴリ粒子状物質(PM10,PM2.5)
    0 - 50良い - Good通常の活動が可能なし
    51 -100並 - Moderate特に敏感な者は、長時間又は激しい屋外活動の減少を検討非常に敏感な人は、長時間または激しい活動を減らすよう検討する必要がある。
    101-150敏感なグループにとっては健康に良くない - Unhealthy for Sensitive Groups心臓・肺疾患患者、高齢者及び子供は、長時間又は激しい屋外活動を減少心疾患や肺疾患を持つ人、高齢者、子供は、長時間または激しい活動を減らす必要がある。
    151-200健康に良くない - Unhealthy上記の者は、長時間又は激しい屋外活動を中止
    すべての者は、長時間又は激しい屋外活動を減少
    心疾患や肺疾患を持つ人、高齢者、子供は、長時間または激しい活動を中止する必要がある。それ以外の人でも、長時間または激しい活動を減らす必要がある。
    201-300極めて健康に良くない - Very Unhealthy上記の者は、すべての屋外活動を中止
    すべての者は、長時間又は激しい屋外活動を中止
    心疾患や肺疾患を持つ人、高齢者、子供は、全ての屋外活動を中止する必要がある。それ以外の人でも、長時間または激しい活動を中止する必要がある。
    300+危険 - Hazardous上記の者は、屋内に留まり、体力消耗を避ける
    すべての者は、屋外活動を中止
    全ての人が屋外活動を中止する必要がある。特に、心疾患や肺疾患を持つ人、高齢者、子供は、屋内に留まって激しい活動を避け静かに過ごす必要がある。
    (Reference: see wikipedia,and cn.emb-japan.go.jp/)

    あなたは空気の質や汚染についての詳細をお知りになりたい場合は、WikipediaAirNowを参照してください。

    北京の医者Richard Saint Cyrの非常に便利な健康アドバイスについては、 www.myhealthbeijing.com をチェックしてください。


    Usage Notice: All the Air Quality data are unvalidated at the time of publication, and due to quality assurance these data may be amended, without notice, at any time. The World Air Quality Index project has exercised all reasonable skill and care in compiling the contents of this information and under no circumstances will the World Air Quality Index project team or its agents be liable in contract, tort or otherwise for any loss, injury or damage arising directly or indirectly from the supply of this data.



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