Characteristics of Negative Cluster Ions in an Urban Environment
Abstract. 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 in-situ measurements by a neutral cluster and air ion spectrometer. The median concentrations of negative cluster ions smaller than 1.6 nm were 85 (61–112 for 25–75 %) 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 NOx 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 (73–75 %), followed by ion-molecule reaction losses (19 %), and ion-ion recombination losses (6–8 %).
Rujing Yin et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2022-1108', Anonymous Referee #1, 31 Dec 2022
- RC2: 'Comment on egusphere-2022-1108', Anonymous Referee #2, 19 Jan 2023
Rujing Yin et al.
Rujing Yin et al.
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The manuscript by Yin et al. describes measurements of atmospheric ions in Beijing. Notably, they quantify the concentrations of ions of specific composition through a rigorous calibration process. For select ions, a steady-state analysis is applied to show how the time series of these specific ions are well represented by known chemical and physical processes and that condensational loss represents the largest ion loss term. Ion composition is compared to the composition of neutral gases measured by a nitrate chemical ionization mass spectrometer (CIMS) and similar to past findings, it is found that there is generally good agreement in the types of organic molecules identified as ambient ions and as neutral gases albeit with different intensities. These measurements from an urban environment are also compared to measurements from the boreal forest with similarities and differences discussed.
In my opinion, the most important contribution of this manuscript is the push towards making quantitative measurements of the chemically resolved composition of ambient ions and in using these quantitative measurements to evaluate our understanding of ion sources and sinks. Quantifying specific ions is a challenging undertaking that few have pursued in recent years (specifically when using time-of-flight mass spectrometers). Although some would argue that the science to effort pay off of such measurements may be minor, I think that there are some important questions that could be addressed if such measurements became more routine. The other aspects of the manuscript (discussion of composition, comparison to other environments, etc.) are only superficially explored and require a more comprehensive analysis before they would provide general insight into atmospheric composition and chemistry. In short, I think this manuscript contributes a technical advancement to the field and thus may be more appropriate for a different journal or as a technical note. In my opinion, to be suitable for publication in a more general journal, more detailed analysis regarding composition is necessary and the scientific motivation and insights of the analysis need to be clearly established in the manuscript.
Other minor comments
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