The 1978 edition of Garry Thomson's The Museum Environment tells you on p. 125 how to convert parts per million to micrograms per cubic meter. The formula is in a footnote:
M 1 ppm= ------ ug/m3 0.0224
where M is the molecular weight of the pollutant gas.
Timestamp: Sunday, 03-Mar-2013 21:35:42 PST
Retrieved: Tuesday, 17-Jul-2018 03:55:00 GMT