Why are the CAMS forecasts sometimes showing poor air quality (high values of air quality index), while we are in lockdown?

The quality of the air at any location is determined by many factors and the effects of restrictions and lockdown measures are more complex than it may seem.

Firstly, levels of atmospheric pollution are driven by emissions from different sources. CAMS uses inventories of emission data that are derived from official nationally-reported activity statistics for various sectors (e.g., traffic, industry, agriculture, and shipping). This is currently the most accurate way of including emission estimates in air quality forecast models. However, these activity statistics are not immediately available and the emissions used in the CAMS operational forecasts are therefore representative of a business-as-usual situation (see also the information on the CAMS Scientific User Forum). In the current COVID19 situation with various levels of lockdown, emissions from various sectors are reduced and this will have an impact on the quality of the air we breathe. However, only some sectors are affected, and total emissions are therefore by no means reduced to zero (see also “How are COVID-19 restrictions and lockdown measures expected to affect air quality?”). Also, emission reduction measures in countries around the world vary significantly and are therefore difficult to estimate accurately. This is an activity that CAMS is currently working on in collaboration with other entities in Europe and worldwide. Therefore, while emissions have been reduced in many parts of the world, the CAMS forecasts still use the carefully estimated business-as-usual emissions as the best estimate. This can lead to an overestimation of especially nitrogen dioxide (NO2), but other pollutants are less affected. Also, some (natural) emission sources, such as wildfires and desert dust, are essentially unaffected and can generate pollution events affecting local or even regional scales.

Secondly, a significant amount of the day-to-day variability of air quality is caused by the weather. Changing winds will transport pollutants in different directions and also the vertical mixing of pollutants is directly affected by the weather. This makes it very difficult to draw conclusions about the impact of reduced emissions on the levels of pollutants in the atmosphere from forecasts or observations without doing a more detailed study. For instance, if weather conditions are very stable (no wind and little vertical mixing), levels of pollutants can still build up significantly despite any reductions in the emissions.

Thirdly, the often-used Air Quality Index (AQI) is a reflection of the concentrations of the main pollutants. While there are different definitions of AQI, CAMS currently uses one that represents levels of ozone (O3), nitrogen dioxide (NO2) and particulate matter (PM). AQI values are determined by the pollutants for which levels are highest, as is for instance explained for the CAMS forecasts on CNN. This is important, because a high value of the AQI can point to high levels of any of these pollutants. For instance, low levels of NO2 due to reduced traffic can cause high levels of O3 through non-linear chemical reactions in the presence of strong sunlight. This would still result in high AQI values, even though emission restrictions are in place.

Finally, because air quality forecasts will always have some uncertainties, CAMS routinely uses independent observations to verify the forecasts. While observations cannot predict the future evolution, they are the most direct measure of pollutant levels and therefore critical for monitoring rapidly changing conditions. The comparison between forecasts and observations has to be done carefully, though, as is explained in our explanatory video. To benefit from both worlds, CAMS also generates daily analyses both for the global and the European domain. These analyses merge the information from the previous CAMS forecast with the latest observations to provide a full 3-dimensional description of the pollutants in the atmosphere. This is the most comprehensive way to monitor the impact of emission reductions on air quality.

More information related to the current COVID19 situation can be found on the CAMS website, which is updated daily.