The surface properties of SOA generated from limonene and toluene using specific molecular probes: exploration of a new experimental technique

Abstract. A new experimental technique of characterizing the aerosol-atmosphere surface has been explored using three examples: the secondary organic aerosols (SOA) resulting from the reaction of limonene with O₃ and from the photooxidation of toluene in comparison with the combustion aerosol (soot) from a toluene diffusion flame. Rather than investigating the bulk composition of the aerosol by complete chemical analysis and identification of the many dozens if not more of constituent compounds we have interrogated the type and number of functional groups located at the aerosol surface by interacting them with specific molecular probes such as O₃, NO₂, N(CH₃)₃, and NH₂OH to probe for the presence of oxidizable sites, acidic sites and carbonyl functionalities, respectively, that are present on the surface of the aerosol particle. In practice, typical amounts of one to a few mg of laboratory-generated SOA of limonene, toluene and soot have been deposited on a PTFE membrane filter that subsequently has been transferred to a molecular flow reactor used for the titration reaction of the surface functional groups by the molecular probes. Absolute amounts N_i with i=O₃, NO₂, N(CH₃)₃, NH₂OH of probe molecules taken up by the filter sample measured using molecular beam sampling mass spectrometry have been converted into the number of surface group functionalities per unit surface area S using the aerosol particle distribution function (PDF) of limonene and toluene SOA and the BET total surface area of toluene flame soot to result in N_i/S. Arguments are presented that support the transfer of the PDF of the suspended to the aerosol collected on the Teflon filter.