I am the kind of person that likes things organized and always planned. During the pandemic, I took up hiking as a new hobby. I spend a lot of time indoors working in front of the computer, so I decided that it was time to find an activity in nature where I will be able to look as far away as possible in order to extend my view field and exercise my eyesight. The location I live allows me to do that hobby reality but air quality is not always optimum.
As you may know, I live in southern Spain and although traffic pollution is an issue, when I hike in the hills there is a different type of pollution that may be present, the Saharan Dust. In 2021 I have noticed that cross-border pollution events are more frequent. There are no constant warnings from local media or local authorities about these kinds of events. Newspapers or TV news will only mention them only when there is nothing else to talk about.
So when I want to go outside and practice that sport, I check not only the weather forecast but the air quality forecast from various apps and web services. In this article, I will compare three air quality services and apps that I mostly use and see if they offer an accurate forecast. The apps are eLichens Air, Plume Labs, and Windy.
The method is simple. The night before hiking, I check the AQ forecast with these three apps, and then the next day I check the actual AQ measurements from my sensors and the apps again.
Friday Night 7-May-2021 | Forecast
Saturday Morning 8-May-2021 | Real-Time
Here is the first comparison. At night of Friday 7th May, I checked the AQ for the next morning at 11am. The forecast data predicted by the Plume Labs and the eLichens Air show an index value and not a mass concentration (μg/m3) like Windy does. In most cases companies use a proprietary AQI and in order to be able to compare them I will need to “decrypt” them. I wasn’t able to find any documentation for the eLichens Air AQI.
Plume Labs gave me an AQI 21 for PM2.5 which according to their documentation corresponds to a value below 25μg/m3. eLichens Air gave me an AQI 16 for PM2.5 which I estimate it corresponds to a value below 12μg/m3. Windy estimated 8μg/m3.
The following day, I checked the measurements at the exact time like the night before plus the real-time values from my outdoor sensors. PM2.5 values were at 5μg/m3 and the eLichens Air, Plume Labs, and Windy gave me almost the same values of 9μg/m3 and 17/18 AQI which are roughly the same.
Saturday Night 8-May-2021 | Forecast
Sunday Morning 9-May-2021 | Real-Time
The second test revealed similar PM2.5 forecast values (20 AQI) between the Plume Labs and the eLichens Air and an estimated value of 14μg/m3 from Windy.
The following morning, the real-time values from the apps were far different from the in-situ values as my outdoor sensor measured 4μg/m3 and the apps gave me a value of 15 AQI and 16μg/m3.
In their defense, there are not lots of available data here because our local AQ station doesn’t share openly PM2.5 data. So they work with models that are not able to give better results. In different parts of the world where spatial coverage is better and official stations share PM2.5 values with the public, they have better results. Some apps rely on satellite imagery, but again satellites pass over Spain once a day and they have a 3.5km by 5km resolution like the sentinel-5 precursor (my favorite so far). I take them into account to make a rough estimation of the following day and of course, I check the data from my local outdoor sensors.