Molecular characterization of gaseous and particulate oxygenated compounds at a remote site in Cape Corsica in the western Mediterranean Basin
Authors: Michoud Vincent , Hallemans Elise , Chiappini Laura , Leoz-Garziandia Eva , Colomb Aurélie , Dusanter Sébastien , Fronval Isabelle , Gheusi François , Jaffrezo Jean-Luc , Léonardis Thierry , Locoge Nadine , Marchand Nicolas , Sauvage Stéphane , Sciare Jean , Doussin Jean-François 
Published: May 2021

Abstract. The characterization of the molecular composition of organic carbon in both
gaseous and aerosol is key to understanding the processes involved in the
formation and aging of secondary organic aerosol. Therefore a technique
using active sampling on cartridges and filters and derivatization followed
by analysis using a thermal desorption–gas chromatography–mass spectrometer (TD–GC–MS) has been used. It is aimed at studying the molecular composition of
organic carbon in both gaseous and aerosol phases (PM2.5) during an
intensive field campaign which took place in Corsica (France) during the
summer of 2013: the ChArMEx (Chemistry and Aerosol Mediterranean Experiment)
SOP1b (Special Observation Period 1B) campaign. These measurements led to the identification of 51 oxygenated (carbonyl and
or hydroxyl) compounds in the gaseous phase with concentrations between 21 and 3900 ng m−3 and of 85 compounds in the
particulate phase with concentrations between 0.3 and 277 ng m−3. Comparisons of these measurements with collocated data using other
techniques have been conducted, showing fair agreement in general for most
species except for glyoxal in the gas phase and malonic, tartaric, malic and
succinic acids in the particle phase, with disagreements that can reach up to
a factor of 8 and 20 on average, respectively, for the latter two acids. Comparison between the sum of all compounds identified by TD–GC–MS in the
particle phase and the total organic matter (OM) mass reveals that on
average 18 % of the total OM mass can be explained by the compounds
measured by TD–GC–MS. This number increases to 24 % of the total water-soluble OM (WSOM) measured by coupling the Particle Into Liquid Sampler (PILS)-TOC (total organic carbon) if we consider only the sum of the
soluble compounds measured by TD–GC–MS. This highlights the important
fraction of the OM mass identified by these measurements but also the
relative important fraction of OM mass remaining unidentified during the
campaign and therefore the complexity of characterizing exhaustively the
organic aerosol (OA) molecular chemical composition. The fraction of OM measured by TD–GC–MS is largely dominated by
di-carboxylic acids, which represent 49 % of the PM2.5 content
detected and quantified by this technique. Other contributions to PM2.5
composition measured by TD–GC–MS are then represented by tri-carboxylic
acids (15 %), alcohols (13 %), aldehydes (10 %), di-hydroxy-carboxylic
acids (5 %), monocarboxylic acids and ketones (3 % each), and
hydroxyl-carboxylic acids (2 %). These results highlight the importance of
polyfunctionalized carboxylic acids for OM, while the chemical processes
responsible for their formation in both phases remain uncertain. While not
measured by the TD–GC–MS technique, humic-like substances (HULISs) represent the
most abundant identified species in the aerosol, contributing for 59 % of
the total OM mass on average during the campaign. A total of 14 compounds were detected and quantified in both phases, allowing the
calculation of experimental partitioning coefficients for these species. The
comparison of these experimental partitioning coefficients with theoretical
ones, estimated by three different models, reveals large discrepancies
varying from 2 to 7 orders of magnitude. These results suggest that the
supposed instantaneous equilibrium being established between gaseous and
particulate phases assuming a homogeneous non-viscous particle phase is
questionable.