let me try in another solution
using mark room size of 66.58m3
density of air at 20C is 1.2041 kg/m3
mass of air = (66.58)(1.2041) = 80.17kg
converting %v/v to %w/w
%v/v of CO in air = 1E-6 %
mw of CO in air = (%v/v CO)(28.01) = (1E-6)(28.01)/100 = 2.80E-7
%w/w of CO = (mw of CO in air)(100) / (mw of air)
%w/w of CO = (2.80E-7)(100) / (28.972)
%w/w of CO = 9.668E-7 %
mass of CO = (9.668E-9)(80.17) = 7.751E-7 kg or 7.75E-4 grams
my answer is astronomically wrong, sorry
if using your figure 48E-6 g/m3
mass of CO = (66.58)(48E-6) = 0.003 grams