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 VCPs in a U.S. megacity over a 10-day 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/functionalized compounds with ratios to common anthropogenic marker compounds and compares their relative abundances to two regionally-resolved emissions inventories used in urban air quality models.

A Chemical Ionization Mass Spectrometry Utilizing Ammonium Ions (NH4+ CIMS) for Measurements of Organic Compounds in the Atmosphere

“…To account for instrument variability, the ion signals are typically normalized to the changing reagent ion signals. However, previous studies using Vocus in H 3 O + and NH + 4 • H 2 O chemistry did not normalize the signals to reagent ions (Krechmer et al, 2018;Khare et al, 2022), because the BSQ serves as a high-pass band filter and substantially reduces the signal intensity of reagent ions. In this study, we find that without normalization, the comparisons between NH + 4 CIMS and GC-MS exhibit significant difference between day and night (Figure S8a and b), which is consistent with the diurnal trend of reagent ion NH + 4 • H 2 O (Figure S8c).…”

“…In this section, we compare the sensitivities of our NH + 4 CIMS (denoted as NOAA NH + 4 CIMS) to two other NH + 4 CIMS and a PTR using H 3 O + chemistry. The other two NH + 4 CIMS include a PTR3 instrument with a different IMR design from our Vocus (Zaytsev et al, 2019) (denoted as PTR3 NH + 4 CIMS) and a Vocus instrument with the same IMR design as ours but operated 410 under different conditions (Khare et al, 2022) (denoted as Khare NH + 4 CIMS). The PTR instrument is from our lab (denoted NOAA H 3 O + CIMS), which uses the same FIMR as our NH + 4 CIMS and was calibrated along with our NH + 4 CIMS using the same calibration methods.…”

“…In particular, the ethylene glycol sensitivity is lower by a factor of 100. The lower sensitivity in Khare et al (2022) is likely because they used a higher NH 3 /H 2 O ratio than The sensitivity ratio of NOAA H 3 O + CIMS to NOAA NH + 4 CIMS does spans a wide range from 1 to 10 4 . In general, the sensitivity ratio anti-correlates with the sensitivity of NOAA NH + 4 CIMS within each chemical class.…”

“…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 ) (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 inter-comparison with other analytical instruments are scarce (Khare et al, 2022).…”