Abstract. In this study, we describe the characterization and field deployment of a 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 (counts per second per ppbv, 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) towards ketones, moderate sensitivity (i.e., between 100 and 1000 cps ppbv-1) towards aldehdyes, alcohols, organic acids, and monoterpenes, low sensitivity (i.e., between 10 and 100 cps ppbv-1) towards isoprene and C1 and C2 organics, and negligible sensitivity (i.e., < 10 cps ppbv-1) towards 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 VOCs and is complementary to existing chemical ionization schemes.;
