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

Posted on March 15th 2015
(re-edited on January 12th 2020)
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:
Modified NowCast for Asia (aka NowCast Asia AQI):

If you want, you can also directly change the two parameters 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 re-suspension.
[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 underage (hourly) data, but also individual concentration for each pollutant.

Click here to see all the FAQ entries
  • AQI Scale: What do the colors and numbers mean?
  • Using Statistical Distances for Real-time Sensor Networks Validation
  • Nitrogen Dioxyde (NO2) in our atmosphere

    • About the Air Quality and Pollution Measurement:

    About the Air Quality Levels

    AQIAir Pollution LevelHealth ImplicationsCautionary Statement (for PM2.5)
    0 - 50GoodAir quality is considered satisfactory, and air pollution poses little or no riskNone
    51 -100ModerateAir quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.
    101-150Unhealthy for Sensitive GroupsMembers of sensitive groups may experience health effects. The general public is not likely to be affected.Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.
    151-200UnhealthyEveryone may begin to experience health effects; members of sensitive groups may experience more serious health effectsActive children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion
    201-300Very UnhealthyHealth warnings of emergency conditions. The entire population is more likely to be affected.Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.
    300+HazardousHealth alert: everyone may experience more serious health effectsEveryone should avoid all outdoor exertion

    To know more about Air Quality and Pollution, check the wikipedia Air Quality topic or the airnow guide to Air Quality and Your Health.

    For very useful health advices of Beijing Doctor Richard Saint Cyr MD, check www.myhealthbeijing.com blog.


    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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