Comparison of advanced offline and in situ techniques of organic aerosol composition measurement during the CalNex campaign

Our understanding of formation processes, physical
properties, and climate/health effects of organic aerosols
is still limited in part due to limited knowledge of organic
aerosol composition. We present speciated measurements of
organic aerosol composition by two methods: in situ thermaldesorption
proton-transfer-reaction mass spectrometry (TDPTR-MS)
and offline two-dimensional gas chromatography
with a time-of-flight mass spectrometer (GC × GC/TOFMS).
Using the GC × GC/TOF-MS 153 compounds were
identified, 123 of which were matched with 64 ions observed
by the TD-PTR-MS. A reasonable overall correlation
of 0.67 (r
2
) was found between the total matched TDPTR-MS
signal (sum of 64 ions) and the total matched
GC × GC/TOF-MS signal (sum of 123 compounds) for the
Los Angeles area. A reasonable quantitative agreement between
the two methods was observed for most individual
compounds with concentrations which were detected at levels
above 2 ng m−3 using the GC × GC/TOF-MS. The analysis
of monocarboxylic acids standards with TD-PTR-MS
showed that alkanoic acids with molecular masses below
290 amu are detected well (recovery fractions above 60 %).
However, the concentrations of these acids were consistently
higher on quartz filters (quantified offline by GC × GC/TOFMS)
than those suggested by in situ TD-PTR-MS measurements,
which is consistent with the semivolatile nature of the
acids and corresponding positive filter sampling artifacts.