Ammonium adduct chemical ionization to investigate anthropogenic oxygenated gas-phase organic compounds in urban air

Abstract

Abstract. Volatile chemical products (VCPs) and other non-combustion-related sources have become important for urban air quality, and bottom-up calculations report emissions of a variety of functionalized compounds that remain understudied and uncertain in emissions estimates. Using a new instrumental configuration, we present online measurements of oxygenated organic compounds in a US megacity over a 10 d wintertime sampling period, when biogenic sources and photochemistry were less active. Measurements were conducted at a rooftop observatory in upper Manhattan, New York City, USA using a Vocus chemical ionization time-of-flight mass spectrometer, with ammonium (NH4+) as the reagent ion operating at 1 Hz. The range of observations spanned volatile, intermediate-volatility, and semi-volatile organic compounds, with targeted analyses of ∼150 ions, whose likely assignments included a range of functionalized compound classes such as glycols, glycol ethers, acetates, acids, alcohols, acrylates, esters, ethanolamines, and ketones that are found in various consumer, commercial, and industrial products. Their concentrations varied as a function of wind direction, with enhancements over the highly populated areas of the Bronx, Manhattan, and parts of New Jersey, and included abundant concentrations of acetates, acrylates, ethylene glycol, and other commonly used oxygenated compounds. The results provide top-down constraints on wintertime emissions of these oxygenated and functionalized compounds, with ratios to common anthropogenic marker compounds and comparisons of their relative abundances to two regionally resolved emissions inventories used in urban air quality models.

Chemical ionization mass spectrometry utilizing ammonium ions (NH4+ CIMS) for measurements of organic compounds in the atmosphere

“…We find minimal dependence of sensitivity on sample relative humidity (RH) as shown in Fig. S1 in the Supplement, consistent with Khare et al (2022) and observations made when running the Vocus in H 3 O + mode (Krechmer et al, 2018). This is mainly because a large amount of water vapor (20 sccm) is deliberately added to the FIMR.…”

“…Among the seven compounds tested here, the sensitivities of six oxygenated compounds exhibit a negative dependence on the temperature between 310 and 370 K. The reduced VOC, isoprene, exhibits a positive dependence. Similar to isoprene, α-pinene sensitivity also increases with temperature in the 303-350 K window as recently reported in Khare et al (2022). Here we examine the opposite trends of temperature-dependent sensitivity between acetone and α-pinene because their NH + 4 affinities are available in the literature (Sect.…”

“…One ionization scheme under active development utilizes the ammonium ion (NH + 4 ) chemistry. Several recent studies have demonstrated its capability to detect a range of oxygenated organic compounds, including alcohols, aldehydes, ketones, and even the short-lived peroxy radicals (RO 2 ) (Blake et al, 2006;Lindinger et al, 1998;Canaval et al, 2019;Hansel et al, 2018;Müller et al, 2020;Zaytsev et al, 2019;Berndt et al, 2018;Khare et al, 2022). One reason NH + 4 chemistry is attractive is that it detects oxygenated organic compounds in the positive mass spectrometer mode, in contrast to existing reagent ions (i.e., I − , CF 3 O − , and NO − 3 ) which are operated in negative mode.…”

“…Zaytsev et al (2019) and Müller et al (2020) demonstrated the feasibility of such rapid switching in laboratory conditions. The application of NH + 4 CIMS in recent studies has largely focused on laboratory studies (Berndt et al, 2018;Zaytsev et al, 2019), but its deployment in field measurements and intercomparison with other analytical instruments are scarce (Khare et al, 2022).…”

Ammonium-adduct chemical ionization to investigate anthropogenic oxygenated gas-phase organic compounds in urban air

“…Data availability. The data that support the results presented in this paper are publicly accessible and could be found online at https://doi.org/10.6084/m9.figshare.21507798 (Khare, 2022).…”