Ozone AQI Scale update

Posted on August 10th 2016
Share: aqicn.org/faq/2016-08-10/ozone-aqi-scale-update

Introduction to Weather and Climate,
Univ. of Arizona (credits)

It is already several months since the US EPA updated its AQI standard for Ozone 8 hours breakpoints[1]. This is definitely a positive step towards strengthening Air Quality in the USA, since the US EPA Standard was actually among of the least conservative standard for Ozone, compared, for instance, to similar standards in India, China or Europe!

But more interestingly, AirNow recently took a step further by introducing the Ozone NowCast formula, which allows to use the weighted 8 hours breakpoint standard in place of the 1 hour breakpoint. This change, which the AirNow team did not widely communicate to the public, makes quite a change to the way Ozone AQI is reported, and this is what we will be looking at in this article.


Ozone pollution: a Global issue

Before getting into the details of the Now Cast formula, it is worth taking one step back and looking at Ozone pollution as a global issue. The total atmospheric ozone concentration simulation, done by the NOAA gives a good overview of the areas with high/low Ozone concentrations, but also shows how global climatic conditions impact trans-boundary pollution.

Real-time Ozone Concentration simulation total using GOA [2] and the GFS[3] model
Forecasted Total Atmospheric Ozone Concentration (in Dobson unit)

The above simulation shows the total atmospheric concentration, from surface up to 20 KM height, and thus contains both the good Ozone (the "Ozone layer" in the upper stratosphere) as well as the Bad Ozone in the troposphere.

When one talks about Ozone pollution, it is the tropospheric ozone, and more particularly the surface concentration which matters in order to quantify the health impact. Moreover, the tropospheric Ozone is having a diurnal cycle, with pollution peaking in the afternoon when the temperature reaches its maximum, and almost no pollution during the night. This cycle can be clearly seen in the below animation, which is based on the forecasted ground concentration.

Real-time Ozone Concentration simulation total using GOA and the SILAM[4] model
Forecasted Surface Ozone Concentration (converted to AQI)

Without surprise, Ozone pollution is still a global problem even for surface concentration (countries in the Southern hemisphere would have higher concentration during their summer).

Looking at China in particular, Ozone is actually becoming more often the primary pollutant, surpassing PM2.5 pollution. The main explaination is that all the recent efforts done for reducing air pollution in China have been really efficient and PM2.5 concentration significantly reduced [5].

Moreover, PM pollution is actually acting as a global warming cooler (i.e. decreasing the global warming by reflecting sun light). So, when PM pollution gets reduced, so are the number of blue sky days (that's good!), but as a side effect the peak daily temperature increases. And since temperature is one of the element which impacts Ozone production, increased temperature also means increased Ozone


Comparative Instant Cast and Now Cast AQI

Now that it is known that Ozone pollution tends to become the primary pollutant during the hot summery day, it is even more important to fully understand the impact of the Ozone AQI calculation on the overall AQI.

The two graphs below show the AQI for 5 different cities in China. The graph on the top shows the AQI as it is published today on aqicn.org (using the instant cast formula), and the graph below shows the AQI as it it would be using the AirNow NowCast AQI formula.

Ozone InstantCast Air Quality Index (Reference AQI from the World Air Quality Index, based on hourly readings)
Ozone NowCast Air Quality Index based on US EPA NowCast 2016 formula (please do not use as reference - this graph contains incorrect AQI values)

The most striking problem with the NowCast AQI is that it tends to overshoot and report extremely high AQIs when the actual (past hourly data) AQI is low. For instance:

  • For Beijing on Thursday August 3rd as 11PM, the Now Cast AQI is reporting an AQI for 158 (Unhealthy) while the actual AQI is 45 (Good).
  • For Shanghai on Friday 29th July at 20PM, the Now Cast AQI is reporting an AQI for 180 (Unhealthy) while the actual AQI is 50 (Good).
Similar overshoot patterns exist for the other cities. That's really bad for the Now Cast (seriously), since, when it comes to make a decision about, for instance, going out for a walk, if the published AQI does not reflect the current condition, then there is no point in calling it a "now" AQI...


Trying to fine tune the NowCast formula, as it has been done for the PM2.5 Now Cast in Asia does not work well for Ozone (see graph below for N=3 hours, and min weight=.1). The reason is that Ozone is having a diurnal cycle with peaks in the afternoon (correlated with temperature peak), so any kind of averaging would just offset the peak by few hours. That would be wrong, as what is needed is to report the Air Quality as it is now, and not as it was few hours ago.

Modified NowCast for Asia (aka NowCast Asia AQI) (please do not use as reference - this graph contains incorrect AQI values)
NowCast weight factors
You can change the NowCast paramters for the above graphs:
Wmin = and N =


Weak one-hour Ozone Breakpoints

There is however a problem with the current Instant Cast formula for Ozone which is in use on waqi.info and aqicn.org: It is that the Instant Cast breakpoints are based on the US EPA 1 hour air quality standard, which is still among the least safe Ozone standards compared to other EPAs standards (see graph on the right) for AQIs below 200.

ID: Idonesia TH: Thailand CN: China IN: India US: USA
Conversion from ppb to mg/m3 using STP US
Caution: Axis scales are not linear.

There are quite a few reasons for this low standard from the US EPA: First the USEPA focused first on improving the 8-hours standard, and that's something good - since the 8 hours standard is used for annual reports. It is also the standard used on air now (together with the now cast formula), so it made sense to focus first on this standard, and this even despite all the problems linked to the now-cast formula. Most likely, the US EPA is now working on doing the same update to the 1 hour standard, but that might take quite some time to happen, and quite a lot of lobbying.

So, while waiting the the US EPA to improve the 1-hour standard, and setup a standard which is not left behind compared to other Ozone standards in the world, the world Air Quality Index project has been looking at existing standards which could better reflect the Health Impact of Air Quality, and this without having to use the irrelevant NowCast formula based on 8-hours breakpoints.

Updated Instant Cast Ozone Breakpoints

One of the options which has been considered, and immediately dropped was to use an Instant Cast based on the 8 hours breakpoints instead of the 1 hour breakpoints (like it is actually done for PM2.5, where the AQI is reported assuming a 24-hour exposure). The reason for not using this solution for Ozone is the diurnal cycle, which makes it not straightforward to assess what an 8-hour exposure can be. For instance, if an AQI is reported to be 35 at 10AM, what does that mean for the AQI at 2PM? Will it be 80, 120, to 180? That's quite different from PM, where one can guess that the AQI is similar throughout the day, under similar visibility and humidity conditions[6].

For the other options, the analysis covered existing standards for more than 20 EPAs worldwide, the recommendations from the WHO, the preliminary findings from world-wide researchers from the TOAR project and the research papers from the from the UNEP CCA Coalition catalog. An additional constraint for selecting the standard was to use one which is officially approved. as well as one close to the median standard, as it would not over exaggerate the Health Impact, and neither underestimated it.

The best option appeared to be the CN MEP 1-hour standard, which is much more safe than the US-EPA 1-hour standard for AQI below 200 (above 200, the two standards are the same). The two graphs below show the Ozone Instant Cast AQI, based on the old breakpoints (top) and new breakpoints (bottom). Note that since the breakpoints are updated for AQI between 50 and 200 only, the only 'visible' difference on the color graphs is for Moderate, USG and Unhealthy AQIs.

Former Ozone AQI Standard, using the US EPA 1-hour breakpoints (deprecated on August, 10th 2016)

New Ozone AQI Standard, using the CN MEP 1-hour breakpoints (in use starting from August, 11th 2016)

The table below summurize the new 1-hour Ozone breakpoints used for the Instant Cast Ozone calculation. The STP US is used for converting Ozone from mg to ppb.
Index Values
Color coding
AQI CategoryInstant Cast Ozone
breakpoints (ppm)
US EPA 1-hour Ozone
breakpoints (ppm)
0 - 50
Good0 - 62.5-
51 - 100
Moderate62.5 - 101.5-
101 – 152
for Sensitive Groups
101.5 - 152.5125 - 165
151 – 200
Unhealthy152.5 - 205165 - 205
201 – 300
Very Unhealthy205 - 405205 - 405
301 – 500
Hazardous405 - 605405 - 605



This updated Ozone instant cast breakpoint are effective from August 11th 2016.

[2] GOA is our GlObal Atmosphere real-time simulation tool suite, used to create, in real-time, visualization of large amount of climatic and atmospheric data for web. more...
[3] Global Forecast System - see wikipedia.org/wiki/Global_Forecast_System
[4] System for Integrated modeLling of Atmospheric coMposition (Silam) model by the Finnish Meteorological Institute - see forecast/world/silam/
[6] There are many research papers on the correlation between PM air pollution and visibility, for instance Regression Analyses between Recent Air Quality and Visibility Changes in Megacities at Four Haze Regions in China

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

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    Về phép 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 coi là đạt tiêu chuẩn, và ô nhiễm không khí không gây ra rủi ro hoặc rất ít
    51 -100trung bìnhChất lượng không khí ở mức chấp nhận được; Tuy nhiên, một số chất gây ô nhiễm có thể gây ra ảnh hưởng đến sức khỏe của một nhóm nhỏ những người nhạy cảm với không khí ô nhiễm.
    101-150Không lành mạnh cho các nhóm người nhạy cảmNhóm người nhạy cảm có thể bị ảnh hưởng sức khỏe. Phần lớn người dân không có nguy cơ bị ảnh hưởng sức khỏe.
    151-200Có hại cho sức khỏeTất cả mọi người có thể bắt đầu bị ảnh hưởng đến sức khỏe; Những nhóm người nhạy cảm có thể bị ảnh hưởng sức khỏe 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 bị ảnh hưởng đến sức khỏe một cách nghiêm trọng

    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.

    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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