Comment on egusphere-2022-1108

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Atmospheric cluster ions are important constituents in the atmosphere. Concentrations and compositions of cluster ions govern their effects on atmospheric chemistry, air quality, and human health. However, quantitative research on ion composition is rare, especially in an urban atmosphere where pollution levels and human populations are intense. In this study, we measure negative cluster ion compositions using an atmospheric pressure interface high-resolution time-of-flight mass spectrometer in urban Beijing. We demonstrate the feasibility of quantifying cluster ion compositions with simultaneous <em>in-situ</em> measurements by a neutral cluster and air ion spectrometer. The median concentrations of negative cluster ions smaller than 1.6 nm were 85 (61&ndash;112 for 25&ndash;75 %) cm^-3, decreasing significantly with an increasing condensation sink (CS). These concentrations are far lower than those observed at comparatively clean sites due to the higher CS in polluted environments. The ions NO₃^- and HSO₄^-, together with organic ions with the adducts of NO₃^- and HSO₄^-, were the most abundant in urban Beijing, and the organic ions in the atmosphere were similar in composition to those oxygenated organic molecules charged in a chemical ionization mass spectrometer with NO₃^- as the reagent ions. It was shown that the ambient atmosphere is a natural ion-molecular reaction chamber with NO₃^- and HSO₄^- as the main reagent ions. Compared to the clean sites, negative cluster ions in Beijing are composed of more NO₃^- and CHON organic ions due to higher NO_x concentrations and higher fractions of CHON molecules in overall oxygenated organic species. Using dynamic equilibrium equations to examine the fate of HSO₄^- and C₃H₃O₄^- in the atmosphere, we found that their main sources to be the ionization of H₂SO₄ and C₃H₄O₄ by NO₃^- and their main loss being the condensational loss onto aerosol particles (73&ndash;75 %), followed by ion-molecule reaction losses (19 %), and ion-ion recombination losses (6&ndash;8 %).