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

  • Read or publish comments (they rely on Disqus)

    Về phương pháp đo chất lượng không khí và mức độ ô nhiễm:

    Về các thang đo chất lượng không khí

    -Giá trị chỉ số chất lượng không khí (AQI)Mức độ ảnh hưởng sức khỏe
    0 - 50TốtChất lượng không khí được xem là đạt tiêu chuẩn, và ô nhiễm không khí coi như không hoặc gây rất ít nguy hiểm
    51 -100Trung bìnhChất lượng không khí ở mức chấp nhận được; tuy nhiên, một số chất gây ô nhiễm có thể ảnh hưởng tới sức khỏe của một số ít những người nhạy cảm với không khí bị ô nhiễm.
    101-150Không tốt cho người nhạy cảmNhóm người nhạy cảm có thể chịu ảnh hưởng sức khỏe. Số đông không có nguy cơ bị tác động.
    151-200Có hại cho sức khỏeMỗi người đều có thể sẽ chịu tác động đến sức khỏe; nhóm người nhạy cảm có thể bị ảnh hưởng nghiêm trọng hơn.
    201-300Rất có hại cho sức khỏeCảnh báo nguy hại sức khỏe nghiêm trọng. Đa số mọi người đều bị ảnh hưởng.
    300+Nguy hiểmCảnh báo sức khỏe: tất cả mọi người có thể chịu tác động nghiêm trọng đến sức khỏe

    Để biết thêm về ô nhiễm và chất lượng không khí , xem wikipedia chất lượng không khí đang đề hoặc hướng dẫn về chất lượng không khí và sức khỏe của bạn .

    Để biết những lời khuyên về sức khỏe rất hữu ích của bác sĩ y khoa Richard Saint Cyr ở Bắc Kinh, xem www.myhealthbeijing.com blog.


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