From what I read online, solar works at 10 to 25% on cloudy as compared to sunny days, so depending on the larger average number of consecutive cloudy days you would need to size up the solar panel to collect equally more and/or the rechargeable battery to buffer more collected energy. I think the purple air consumes on average about 1 watt(more power consumption details watt https://community.purpleair.com/t/q-how-much-power-does-a-purpleair-sensor-draw-and-how-much-bandwidth-data-does-it-use/847 ) Bigger panels provide faster charging and the possibility to ‘power through’ longer consecutive cloudy days or less drain on the battery, while a bigger battery can provide an energy buffer/time to draw from, but will eventually power off if not enough average energy coming in to recharge. There are national maps for ‘home’ solar power considerations https://www.nrel.gov/gis/solar-resource-maps.html and data by location for average number of cloudy days, but haven’t found much with ‘consecutive cloudy days’ which is the dependent factor the energy sizing is designed around, so end up experimenting to see what works most of the time.
If wanted to power through less frequent or rare consecutive cloud cover events of several weeks, then I might guess a 80 to 100 watt solar panel(at 25% giving the equivalent of a 20 watt panel) with 20 ampHour battery might work, but just a guess.