Real-time measurement of phase partitioning of organic compounds using a proton-transfer-reaction time-of-flight mass spectrometer coupled to aCHARON inlet

Understanding the gas-particle partitioning of semivolatileorganic compounds (SVOCs) is of crucial importance in the accuraterepresentation of the global budget of atmospheric organic aerosols. In thisstudy, we quantified the gas- vs. particle-phase fractions of a large numberof SVOCs in real time in an urban area of East China with the use of aCHemical Analysis of aeRosols ONline (CHARON) inlet coupled to a high-resolution proton-transfer-reaction time-of-flight mass spectrometer(PTR-ToF-MS). We demonstrated the use of the CHARON inlet for highlyefficient collection of particulate SVOCs while maintaining the intactmolecular structures of these compounds. The collected month-long datasetwith hourly resolution allows us to examine the gas-particle partitioning ofa variety of SVOCs under ambient conditions. By comparing the measurementswith model predictions using instantaneous equilibrium partitioningtheory, we found that the dissociation of large parent molecules during thePTR ionization process likely introduces large uncertainties to the measuredgas- vs. particle-phase fractions of less oxidized SVOCs, and therefore,caution should be taken when linking the molecular composition to theparticle volatility when interpreting the PTR-ToF-MS data. Our analysissuggests that understanding the fragmentation mechanism of SVOCs andaccounting for the neutral losses of small moieties during the molecularfeature extraction from the raw PTR mass spectra could reduce, to a largeextent, the uncertainties associated with the gas-particle partitioningmeasurement of SVOCs in the ambient atmosphere.

Automated summary

Understanding the gas-particle partitioning of SVOCs is crucial for accurately representing atmospheric organic aerosols. This study used a CHARON inlet coupled with a PTR-ToF-MS to quantify the gas vs. particle fractions of SVOCs in real time. The results showed that the dissociation of large parent molecules during PTR ionization process introduces uncertainties to the measured gas vs. particle fractions of less oxidized SVOCs. It is important to consider the fragmentation mechanism and neutral losses of SVOCs to reduce the uncertainties associated with gas-particle partitioning measurements.