Primary fine aerosol emissions from a variety of urban sources have be
en quantitatively characterized via high-resolution gas chromatography
to obtain organic mass distribution fingerprints. To assess the degre
e of secondary organic aerosol formation in urban areas, a transport m
odel is used to predict the distribution of ambient organic aerosol ch
aracteristics that would exist at various sites in the Los Angeles Bas
in if the primary organic emissions were transported without chemical
reaction. Comparisons between the model predictions and ambient measur
ements show substantial agreement for the nonpolar organics, suggestin
g that ambient concentrations of this organic fraction result directly
from primary emissions. In contrast, ambient concentrations of fine a
cidic organic aerosols are significantly underpredicted by the model,
indicating that secondary formation is important for acidic organics.
On the basis of the observed differences between model predictions and
measured properties of acidic organics, it is estimated, using monthl
y averages, that up to 18-27 % of the elutable organic aerosol present
in the Los Angeles atmosphere may be secondary in origin.