Most researchers simply work out the conversion by comparing two instruments located in the same place. Tasmania's BLANKET researchers report PM2.5 pollution measured by DustTraks - see
http://www.environment.tas.gov.au/file.aspx?id=7583 for pictures of the monitoring setup and a comparison graph of optical and gravitmentric PM2.5 measurements.
Broadly speaking, If your nephelometer shows scattering coefficients, 1 neph coefficient is about 22 ug/m3 of PM2.5 pollution.
If you have a DustTrack reporting PM2.5 pollution using the Arizona Dust calibration, multiplying the results by 0.42 should be close to the true PM2.5 for woodsmoke.
An earlier reference Report:
DEH, 2005. Department of Environment and Heritage. Use of Particle Counters in Woodsmoke Affected Areas:
http://www.deh.gov.au/atmosphere/airqua ... nters.html).
Hope this helps,
Regards,
Dorothy