The third factor, traced by S and NH4
þ, is the result of the
formation of secondary sulfate (SS) in atmosphere from the
photochemical oxidation of sulfur oxides initially emitted as
gases from local emissions (utilities and power plants) and from
long range transport. As shown in Table 1 NH4
þ is higher in PM2.5
than in PM10 probably because of reactions on the filter and
related sampling artifacts. So in PMF2, although the more
‘‘unrealistic’’ data were downweighted by a factor of 2, the
physical constraint that a source contribution decreases with
cutoff diameter could not be obeyed