Particle Mode uses model data from the U.S. National Centers for Environment Prediction (NCEP). It’s a large dataset, consisting of terabytes of data, and occasionally their services go offline. Most likely their servers are experiencing issues and the particles will be available the next time you launch Seasonality.

Check to make sure you can visit https://nomads.ncep.noaa.gov/dods/gfs_0p25 in a web browser. You should see a list of dates/times the model is available.