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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/amt-2017-286
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
20 Sep 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).
An ion-neutral model to investigate chemical ionization mass spectrometry analysis of atmospheric molecules – application to a mixed reagent ion system for hydroperoxides and organic acids
Brian G. Heikes1, Victoria Treadaway1, Ashley S. McNeill1,2, Indira K. C. Silwal3,4, and Daniel W. O'Sullivan3 1Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, 02882, USA
2Department of Chemistry, The University of Alabama, Tuscaloosa, AL, 35401, USA
3Chemistry Department, United States Naval Academy, Annapolis, MD, 21402, USA
4Department of Chemistry, University of Maine, Orono, 04469, USA
Abstract. An ion-neutral chemical kinetic model is described and used to simulate the negative-ion chemistry occurring within a mixed-reagent ion chemical ionization mass spectrometer (CIMS). The model objective was the establishment of a theoretical basis to understand ambient pressure (variable sample flow and reagent ion carrier gas flow rates), water vapor, ozone and oxides of nitrogen effects on ion-cluster sensitivities for hydrogen peroxide (H2O2), methyl peroxide (CH3OOH), formic acid (HFo) and acetic acid (HAc). The model development started with established atmospheric ion chemistry mechanisms, thermodynamic data and reaction rate coefficients. The chemical mechanism was augmented with additional reactions and their reaction rate coefficients specific to the analytes. Some existing reaction rate coefficients were modified to enable the model to match laboratory and field campaign determinations of ion-cluster sensitivities as functions of CIMS sample flow rate and ambient humidity. Relative trends in predicted and observed sensitivities are compared as instrument specific factors preclude a direct calculation of instrument sensitivity as a function of sample pressure and humidity. Predicted sensitivity trends and experimental sensitivity trends suggested the model captured the reagent ion and cluster chemistry and reproduced trends in ion-cluster sensitivity with sample flow and humidity observed with a CIMS instrument developed for atmospheric peroxide measurements (PCIMS). The model was further used to investigate the potential for isobaric compounds as interferences in the measurement of the above species. For ambient ozone (O3) mixing ratios more than 50 times those of hydrogen peroxide, H2O2, O3 (H2O) was predicted to be a significant isobaric interference to the measurement of O2 (H2O2) at m/z 66. O3 and NO give rise to species and cluster ions, CO3 (H2O) and NO3 (H2O), respectively, which interfere in the measurement of CH3OOH) using O2 (CH3OOH) at m/z 80. The CO3 (H2O) interference assumed one of its O atoms was 18O and present in the cluster in proportion to its natural abundance. The model results indicated monitoring water vapor mixing ratio, m/z 78 for CO3 (H2O) and m/z 98 for isotopic CO3 (H2O)2 can be used to determine when CO3 (H2O) interference is significant. Similarly, monitoring water vapor mixing ratio, m/z 62 for NO3 (H2O) and m/z 98 for NO3 (H2O)2 can be used to determine when NO3 (H2O) interference is significant.

Citation: Heikes, B. G., Treadaway, V., McNeill, A. S., Silwal, I. K. C., and O'Sullivan, D. W.: An ion-neutral model to investigate chemical ionization mass spectrometry analysis of atmospheric molecules – application to a mixed reagent ion system for hydroperoxides and organic acids, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2017-286, in review, 2017.
Brian G. Heikes et al.
Brian G. Heikes et al.
Brian G. Heikes et al.

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Short summary
A unique CIMS utilizing a multi-species neg. ion chemistry was designed to measure peroxides and organic acids throughout the troposphere from an aircraft. Prior CIMS have used a single reagent ion and the multi-ion approach has the potential to be a significant step forward, although one with analytical risk. Here, a chemical kinetics numerical model is presented which simulates its past water vapor and pressure dependence, providing a test-bed to characterize its use in future environments.
A unique CIMS utilizing a multi-species neg. ion chemistry was designed to measure peroxides and...
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