Articles | Volume 9, issue 7
Research article
08 Jul 2016
Research article |  | 08 Jul 2016

Evaluation of NO+ reagent ion chemistry for online measurements of atmospheric volatile organic compounds

Abigail R. Koss, Carsten Warneke, Bin Yuan, Matthew M. Coggon, Patrick R. Veres, and Joost A. de Gouw

Abstract. NO+ chemical ionization mass spectrometry (NO+ CIMS) can achieve fast (1 Hz and faster) online measurement of trace atmospheric volatile organic compounds (VOCs) that cannot be ionized with H3O+ ions (e.g., in a PTR-MS or H3O+ CIMS instrument). Here we describe the adaptation of a high-resolution time-of-flight H3O+ CIMS instrument to use NO+ primary ion chemistry. We evaluate the NO+ technique with respect to compound specificity, sensitivity, and VOC species measured compared to H3O+. The evaluation is established by a series of experiments including laboratory investigation using a gas-chromatography (GC) interface, in situ measurement of urban air using a GC interface, and direct in situ measurement of urban air. The main findings are that (1) NO+ is useful for isomerically resolved measurements of carbonyl species; (2) NO+ can achieve sensitive detection of small (C4–C8) branched alkanes but is not unambiguous for most; and (3) compound-specific measurement of some alkanes, especially isopentane, methylpentane, and high-mass (C12–C15) n-alkanes, is possible with NO+. We also demonstrate fast in situ chemically specific measurements of C12 to C15 alkanes in ambient air.

Short summary
Using laboratory and field experiments, we have explored how the technique of NO+ chemical ionization mass spectrometry can be used to measure volatile organic compounds (VOCs) in the troposphere. Results include the design and operation of the instrument, an evaluation of the technique’s utility for atmospheric measurement, and a guide for data interpretation. Use of this technique will improve our understanding of VOC chemistry.