A combined gas- and particle-phase analysis of highly oxygenated organic molecules (HOMs) from α-pinene ozonolysis

Highly oxygenated organic molecules (HOMs) are important for the formation of secondary organic aerosol (SOA), which poses serious health risks and exerts great influence on Earth's climate. However, the speciation of particle-phase HOMs and its relationship with gas-phase HOM formation has been limited by the lack of suitable analytical techniques. Here, combining a novel particle evaporation inlet, the VIA (Vaporization Inlet for Aerosols), with a nitrate chemical ionization mass spectrometer (NO3-CIMS), gas- and particle-phase HOM products of a-pinene ozonolysis were studied under different conditions. Within the 50 min residence time of our Teflon chamber, we observed enhancement of C-16-C-19 HOM dimers in particles compared to the HOMs that were condensing. In particular, gas-phase dimer formation was considerably suppressed in experiments with the addition of CO or NO, but dimers still made up a considerable fraction of the observed SOA. In addition to the generally shorter carbon skeletons of the particle-phase dimers (i.e., C-16-C-19) compared to the gas phase (C-19-C-20), average O/C ratios of the HOMs (especially in the dimer range) also decreased slightly in the particle phase. C17H26Oz compounds, which have often been reported by previous offline measurements, dominate the particle-phase HOM mass spectra in a-pinene ozonolysis experiments. Our results indicate that these C-17 compounds might be related to particle-phase processes within 1 h after HOM condensation. However, the new VIA-NO3-CIMS system used in this work will require more detailed characterization to better understand how the thermal desorption and wall effects may modify the measured particle-phase HOM distributions. Nevertheless, organic nitrate, for example, measured by this novel VIA-NO3-CIMS system was consistent with the measurements of an Aerodyne aerosol mass spectrometer (AMS), showing the capability of this system as a promising technique for particle-phase HOM measurements. Taken together, we believe that this system is a promising technique for combined online gas- and particle-phase HOM measurements.

Automated summary

Highly oxygenated organic molecules (HOMs) are crucial for the formation of secondary organic aerosol (SOA), but the lack of suitable analytical techniques has limited our understanding of particle-phase HOM speciation and its relationship with gas-phase HOM formation. This study used a novel VIA-NO3-CIMS system to investigate the gas- and particle-phase HOM products of a-pinene ozonolysis. The results showed that gas-phase dimer formation was suppressed with the addition of CO or NO, but particle-phase dimers still constituted a considerable fraction of the observed SOA. Overall, the VIA-NO3-CIMS system showed promise for combined online gas- and particle-phase HOM measurements.