Abstract
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.
Highlights
Volatile organic compounds (VOCs) are central to the formation of ozone and secondary organic aerosol and can have direct human health effects
In H3O+ CIMS, air is mixed with hydronium (H3O+) ions in a drift tube region
We report the sensitivity and spectral simplicity of NO+ CIMS, relative to H3O+ CIMS, for nearly 100 atmospherically relevant VOCs, including a wide range of functional groups, and provide product ion distributions for several representative compounds
Summary
Volatile organic compounds (VOCs) are central to the formation of ozone and secondary organic aerosol and can have direct human health effects. Many environmentally important species require measurement precision of better than 100 parts per trillion (ppt). H3O+ chemical ionization mass spectrometry (H3O+ CIMS), more commonly known as proton-transfer-reaction mass spectrometry (PTR-MS), is a well-established approach to measuring VOCs (de Gouw and Warneke, 2007; Jordan et al, 2009b). VOCs are ionized by transfer of the proton from H3O+ to the VOC These instruments are capable of VOC measurements that are fast, sensitive, and chemically detailed (Jordan et al, 2009b; Graus et al, 2010; Sulzer et al, 2014; Yuan et al, 2016).
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