Articles | Volume 8, issue 1
Research article
05 Jan 2015
Research article |  | 05 Jan 2015

Application of high-resolution time-of-flight chemical ionization mass spectrometry measurements to estimate volatility distributions of α-pinene and naphthalene oxidation products

P. S. Chhabra, A. T. Lambe, M. R. Canagaratna, H. Stark, J. T. Jayne, T. B. Onasch, P. Davidovits, J. R. Kimmel, and D. R. Worsnop

Abstract. Recent developments in high-resolution time-of-flight chemical ionization mass spectrometry (HR-ToF-CIMS) have made it possible to directly detect atmospheric organic compounds in real time with high sensitivity and with little or no fragmentation, including low-volatility, highly oxygenated organic vapors that are precursors to secondary organic aerosol formation. Here, using ions identified by high-resolution spectra from an HR-ToF-CIMS with acetate reagent ion chemistry, we develop an algorithm to estimate the vapor pressures of measured organic acids. The algorithm uses identified ion formulas and calculated double bond equivalencies, information unavailable in quadrupole CIMS technology, as constraints for the number of possible oxygen-containing functional groups. The algorithm is tested with acetate chemical ionization mass spectrometry (acetate-CIMS) spectra of O3 and OH oxidation products of α-pinene and naphthalene formed in a flow reactor with integrated OH exposures ranged from 1.2 × 1011 to 9.7 × 1011 molec s cm−3, corresponding to approximately 1.0 to 7.5 days of equivalent atmospheric oxidation. Measured gas-phase organic acids are similar to those previously observed in environmental chamber studies. For both precursors, we find that acetate-CIMS spectra capture both functionalization (oxygen addition) and fragmentation (carbon loss) as a function of OH exposure. The level of fragmentation is observed to increase with increased oxidation. The predicted condensed-phase secondary organic aerosol (SOA) average acid yields and O/C and H/C ratios agree within uncertainties with previous chamber and flow reactor measurements and ambient CIMS results. While acetate reagent ion chemistry is used to selectively measure organic acids, in principle this method can be applied to additional reagent ion chemistries depending on the application.

Short summary
Authors of this publication used acetate chemical ionization mass spectrometry (acetate-CIMS) to measure experimental products of alpha-pinene and naphthalene oxidation formed in a flow reactor. Acetate-CIMS instrumentation is selective toward carboxylic acid compounds which are readily formed in atmospheric photooxidation processes. Spectral information was used to identify previously measured products of both systems and to estimate their volatilities.