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) 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 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.