Verified answer

the density of air at STP is 1.29 g/L

http://www.google.com/#sclient=psy&hl=en&rlz=1R2GF...

at 25 Celsius, the volume will exand, & it will become less dense by a ratio of T1 / T2 (Charles' law):

1.29 g/L @ 273 Celsius / 298 Celsius = 1.18 grams /Litre

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the cubic meter of air in your 80ug/m^3 has a mass of approximately:

1000 litres @ 1.18 g/L = 1180 grams

find grams of SO3 / gram of air:

(80 e-6 grams SO3) / (1180 g air) = 6.78 e-8 g SO3 per gram of air

find the ppm by mass:

(6.78 e-8 g SO3 per gram of air) (1 e6) = 6.78 e-2

your answer, rounded to 2 sig figs is

0.068 ppm SO3 in air , by mass

the " 80ug/m^3" has only 1 sig fig showing,

you might be expected to round your answer off to

0.07 ppm

0.0678 gra