Weather is certainly one factor contributing to this year's higher pm2.5 - last year Chiang Mai experienced higher than usual rainfall during January, which helped improve early season air quality. However, it is unlikely to be the only reason for the increase we have seen this year.
We do know the burn ban started later this year (March 1 vs. February 21), however our data does not show any significant reduction in pm2.5 levels during the ban. In fact most of the highest pollution days occur while the burn ban is in place.
The honest answer is we still don't really know all of the underlying factors contributing to the smoke problem. Building a more extensive network of pollution sensors can help, and in the future we should be able to combine this with satellite data to get a more complete picture.
We'll report back at the end of the smoke season for a definitive 2018 vs. 2017 number.
How did we calculate this number?
The data used for this calculation comes from the two government stations which report pm2.5 data - City Hall and Yuppaj.
We took daily 24hour average pm2.5 values for each station and averaged them, so for each day we get an average pm2.5 value. The "Cumulative pm2.5 days" shown on the y-axis of the chart is simply the sum of each day's pm2.5 value added over time. This let's us determine the total accumulated pollution during the season, which is what's important for your lungs.