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We describe the characterization and field deployment of chemical ionization mass spectrometry (CIMS) using a recently developed focusing ion-molecule reactor (FIMR) and ammonium–water cluster (NH4+⋅H2O) as the reagent ion (denoted as NH4+ CIMS). We show that NH4+⋅H2O is a highly versatile reagent ion for measurements of a wide range of oxygenated organic compounds. The major product ion is the cluster with NH4+ produced via ligand-switching reactions. Other product ions (e.g., protonated ion, cluster ion with NH4+⋅H2O , with H3O+ , and with H3O+⋅H2O) are also produced, but with minor fractions for most of the oxygenated compounds studied here. The instrument sensitivities (ion counts per second per part per billion by volume, cps ppbv -1) and product distributions are strongly dependent on the instrument operating conditions, including the ratio of ammonia (NH3) and H2O flows and the drift voltages, which should be carefully selected to ensure NH4+⋅H2O as the predominant reagent ion and to optimize sensitivities. For monofunctional analytes, the NH4+⋅H2O chemistry exhibits high sensitivity (i.e., >1000 cps ppbv -1) to ketones, moderate sensitivity (i.e., between 100 and 1000 cps ppbv -1) to aldehydes, alcohols, organic acids, and monoterpenes, low sensitivity (i.e., between 10 and 100 cps ppbv -1) to isoprene and C1 and C2 organics, and negligible sensitivity (i.e., <10 cps ppbv -1) to reduced aromatics. The instrumental sensitivities of analytes depend on the binding energy of the analyte– NH4+ cluster, which can be estimated using voltage scanning. This offers the possibility to constrain the sensitivity of analytes for which no calibration standards exist. This instrument was deployed in the RECAP campaign (Re-Evaluating the Chemistry of Air Pollutants in California) in Pasadena, California, during summer 2021. Measurement comparisons against co-located mass spectrometers show that the NH4+ CIMS is capable of detecting compounds from a wide range of chemical classes. The NH4+ CIMS is valuable for quantification of oxygenated volatile organic compounds (VOCs) and is complementary to existing chemical ionization schemes. [ABSTRACT FROM AUTHOR]

Iodide chemical ionization mass spectrometry (CIMS) is a common analytical tool used in both laboratory and field experiments to measure a large suite of atmospherically relevant compounds. Here, we describe a systematic ion molecule reactor (IMR) temperature dependence of iodide CIMS analyte sensitivity for a wide range of analytes in laboratory experiments. Weakly bound iodide clusters, such as HCl, HONO, HCOOH, HCN, phenol, 2-nitrophenol, and acyl peroxynitrate (PAN) detected via the peroxy radical cluster, all exhibit strong IMR temperature dependence of sensitivity ranging from -3.4 % ∘ C -1 to 5.9 % ∘ C -1 (from 37 to 47 ∘ C). Strongly bound iodide clusters, such as Br 2 , N 2 O 5 , ClNO 2 , and PAN detected via the carboxylate anion, all exhibit little to no IMR temperature dependence ranging from 0.2 % ∘ C -1 to -0.9 % ∘ C -1 (from 37 to 47 ∘ C). The IMR temperature relationships of weakly bound clusters provide an estimate of net reaction enthalpy, and comparison with database values indicates that these clusters are in thermal equilibrium. Ground site HCOOH data collected in the summer of 2021 in Pasadena (CA) are corrected and show a reversal in the diel cycle, emphasizing the importance of this correction (35±6 % during the day, -26±2 % at night). Finally, we recommend two approaches to minimize this effect in the field, namely heating or cooling the IMR; the latter technique has the added benefit of improving absolute sensitivity. [ABSTRACT FROM AUTHOR]