Making sense of air quality data – ozone

Ozone or O3 is expressed in parts per million (ppm) or parts per billion (ppb).

What this refers to is the concentration of ozone in the air. For instance, the U.S. Environmental Protection Agency (EPA) in 2008 assigned a health standard for ozone of 75 ppb. So, for every billion parts of air, if the concentration of ozone is greater than 75 parts per billion (or 0.075 parts for every million parts of air), the standard is exceeded. This corresponds to an Air Quality Index (AQI) of 101 and places air at the unhealthy-for-sensitive-individuals or populations level.

Nothing too complicated about this.

Next up is forecasted versus observed ozone readings or values.

Much like predicting weather, there is the forecasting of air quality. In referring to ozone, O3 may be forecasted to be at a moderate level (an AQI of between 51 and 100) on a given day in a particular location with actual observed AQI reading or value to be 49, at the upper end of the good range, in this case 0 to 50. What may have been forecasted for that day was an AQI of 51.

Even though having this type of information at one’s disposal can be both beneficial and valuable, it may, at the same time, have limited value or benefit.

Real-time, real-world application

Suppose a person resides in an area where there is unhealthy air quality regularly. Suppose also, that this same person plans outdoor exercise around air quality conditions. With limited air quality information on hand, on days when the AQI is, say, predicted to be over 100, the person in question may decide to forego all outdoor exercise altogether and wait until such time that the forecasted AQI will be 100 or less.

Well, the addition of another data tool may prove helpful in this regard.

On the San Joaquin Valley Air Pollution Control District Web site there is what is called “RAAN” or “Real-Time Air Advisory Network.” Using this tool, one is able to see detailed day-to-day monitoring data.

Take Wed., Oct. 8, 2014 as an example.

In Central Fresno, for that day, there were a range of O3 readings: On the “Air District Real-Time Outdoor Activity Risk” chart, at 1 a.m. O3 was at its lowest level: 6 ppb. In fact, the level of O3 fluctuated between 6 ppb and 19 ppb during the early morning hours between what is designated as hours zero and five. Notice though there is missing data between 5 a.m. and 8 a.m., 10 a.m. and noon as well as between 8 p.m. and midnight. But plainly evident is that between essentially noon and 2 p.m. on Oct. 8th, ozone spiked, reaching a level of 99 ppb at 2 p.m. or 24 ppb above the upper end of the healthy range. Furthermore, between noon and 1 p.m. air went from the moderate category to the unhealthy-for-sensitive-populations category and went from the unhealthy-for-sensitive-populations category to the moderate category between the hours of 3 p.m. when the observed O3 concentration was 94 ppb and 5 p.m. when the recorded concentration of O3 was 75 ppb. Meanwhile, air was unhealthy at 99 ppb at 2 p.m. and dove back into the good range at 6 p.m.

Bike-diamond-lane[1]This “RAAN” data can be referenced hourly as a means to get the most recent data available as it becomes available and not just from this one monitoring station but from a number of different monitoring sites. In response, a person may plan outdoor activities such as exercise at times of the day when ozone levels are low. Moreover, for those who may opt to walk or bike to and from work, having access to the most up-to-date data as it becomes available could indeed prove quite helpful.

Please note a disclaimer in reference to the chart in question, in essence, points out data in real-time might not be totally accurate and is furthermore unchecked. Unchecked, in this case, may mean unsubstantiated.

Even so, I view such information as useful.

Meanwhile, on the other side of the coin is information related to fine particulate matter or PM 2.5. “RAAN” data is available regarding PM 2.5 also.

Relatedly, you may be interested in knowing what kinds of air quality data and/or information is/are available in your area.

For more information, see: AirNow at

– Alan Kandel

1 thought on “Making sense of air quality data – ozone”

  1. Great note. I believe people should check air quality in the same simplicity and frequency as they check weather. What separates us today is the air quality “language” it’s still complicated (ppm, ppb, level1/2/3/4/5), not intuitive & scary. There is a great potential to innovation in that field!

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