CNN has an agreement with Copernicus to provide air quality data. The content below is provided by Copernicus, but CNN's weather team has verified the information and methodology.
Air quality index
The air quality forecasts presented in the CNN bulletins are provided by the Copernicus Atmosphere Monitoring Service (CAMS). As part of the European Union's flagship program Copernicus, CAMS delivers information about air quality and atmospheric composition globally based on both satellite and non-satellite observations in combination with forecast models.
Precisely, this is done using a version of the highly successful numerical weather prediction system of the European Centre for Medium Range Weather Forecasts (ECMWF), which comprehensively accounts for emissions and atmospheric composition processes.
Information and data are unrestricted and free of charge, with the aim to support policymakers, business and citizens with enhanced atmospheric environmental information. CAMS is implemented by the European Centre for Medium Range Weather Forecasts (ECMWF), with the support of over 30 entities distributed in Europe. Follow these links to have more information on Copernicus, CAMS and ECMWF.
Air quality information provided by CAMS for CNN is using a bespoke Air Quality Index (CNN-AQI) which is inspired by the European Air Quality Index defined by the European Environment Agency (EEA).
The CNN-AQI currently takes account of four main pollutants: O3 (ozone), NO2 (nitrogen dioxide), PM2.5 and PM10 (fine particulate matter with a diameter smaller than 2.5 micrometers and 10 micrometers, respectively). At present, the CNN-AQI excludes SO2. The reason is that it has not been possible so far to acquire enough observations of SO2 worldwide in order to perform the quality assurance of the CAMS forecasts for this pollutant and properly assess their quality.*
For each pollutant, the value of the index ranges from 1 ("good") to 5 ("very poor"). The CNN-AQI is computed for the different pollutants separately according to the concentrations (instantaneous or on average for the day, depending on the pollutant and following the recommendation of the EEA): the higher the concentrations, the higher the index.
The CNN-AQI is given as an integer value, corresponding to five concentrations bands that are specific to each pollutant as defined in the table below.
The overall hourly European Air Quality index is simply defined as the highest value of the four individual pollutants indexes computed for the same hour. For instance, if the indices relative to O3, NO2, SO2, PM2.5 and PM10 are 1,3,1,2,2 respectively, the overall index will be 3. The overall daily European Air Quality index is the highest value of the overall hourly European Air Quality index in the corresponding day.
Information about the pollutants and their main effects on health can be found on EEA's website. This information is based on latest scientific information compiled by the World Health Organization. In the case that the CNN-AQI is "poor" (4) or "very poor" (5), please also check the observations and/or forecasts of your national or local agency in charge of air quality. They can also provide advice about what you should do to limit your exposure (e.g. avoid exercising outside) and to help mitigate the situation (e.g. take public transport).
These bulletins are forecasts. Just like a weather forecast, this information relies on advanced mathematical models that replicate the laws of physics in combination with past observations. As a result, there is a chance that a forecast is inaccurate. However, careful evaluation against independent surface observations has indicated that forecasts were correct within 1 index value (perfect or over/under-estimated by 1) in the vast majority of cases. You can find more information about how the CAMS air quality forecasts work in this video.
Forecasts presented on CNN are defined as mean values for areas of 40 km by 40 km, which means that they cannot represent local effects (e.g. a road with heavy traffic within a few hundred meters). Within large cities, the values provided by CAMS are representative of the "urban background" concentrations, corresponding to areas in the city that are not directly affected by local sources, such as car traffic (e.g. the middle of a large park, or a quiet residential area).
Areas affected by local sources are expected to have higher concentrations of NO2, PM2.5, PM10 (and SO2) and lower concentrations of O3. For local information within cities and especially in case of the forecasts indicating "poor" (4) or "very poor" (5) air quality, please check the measurements and/or forecasts from your national or local agency in charge of air quality.
* Please note that, as an exception, the Air Quality Index calculated by CAMS currently does not include ozone for the Los Angeles area due to quality issues with the CAMS forecasts in this specific zone, which are currently under investigation. For Los Angeles, only NO2, PM2.5 and PM10 are taken into account in the CNN-AQI.