Hi,
Assuming your PM2.5 data is ending up in the database field pm2_5 then you can plot that in your Belchertown skin by adding to
/etc/weewx/skins/Belchertown/graphs.conf
this stanza...
[[chart5]]
title = PM 2.5
[[[pm2_5]]]
name = PM 2.5
color = "#ffc83f"
y_label = "ug/m3"
You will need to change [[chart5]]
to what ever suites the chart position in your graphs.conf.
This is taken directly from my weather station that you can see here...
http://metoffice.skybase.net/weewx/belchertown/
cheers
Tim