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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-2020-252
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-252
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  13 Jul 2020

13 Jul 2020

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This preprint is currently under review for the journal AMT.

Development of a chemical ionization mass spectrometry system for measurement of atmospheric OH radical

Wei Pu1, Zhouxing Zou1, Weihao Wang1, David Tanner2, Zhe Wang3, and Tao Wang1 Wei Pu et al.
  • 1Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China
  • 2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, USA
  • 3Division of Environment and Sustainability, The Hong Kong University of Science and Technology, Hong Kong, China

Abstract. The hydroxyl radical (OH) is the most important oxidant in the atmosphere and plays a central role in tropospheric chemistry. Ambient OH is extremely difficult to measure because of its low concentration and high reactivity. We have developed and optimized a chemical ionization mass spectrometry (CIMS) system to measure OH based on ion-assisted mass spectrometry. A calibration unit was developed based on chemical actinometry to convert detected signals to OH concentration. Different types of ion sources (210Po and corona source) and scavenger gases (propane, C3F6, and NO2) were compared. Radioactive ion source (210Po foils) was chosen for lower detection limits, and propane was selected for high elimination efficiency and the negligible influence on the signal stability. The sensitivity of the CIMS instrument to OH radicals is influenced by the efficiencies of titration reaction, ion conversion, and ion transmission. Through adjusting their efficiencies by changing the flow rates and voltages, optimal sensitivity was determined. The background noise from OH interferences was reduced by adjusting the flow rate of scavenger gas. The CIMS system achieved a detection limit of ~ 0.15×106 molecules cm−3 (signal/noise = 2). The CIMS was then taken out to measure ambient OH radicals at an urban site in Hong Kong in April 2019. An obvious diurnal pattern of OH radicals was observed, with the highest concentration of ~ 6×106 molecules cm−3 at midday and the lowest concentration of ~ 0.25×106 molecules cm−3 at night, with an overall accuracy of about ±51 %. The results demonstrated the capability of our CIMS for OH measurements on clear days. The tests and results from our study provide a useful reference to other researchers who wish to develop and apply the CIMS technique to measure OH and other chemicals.

Wei Pu et al.

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Short summary
The hydroxyl radical (OH) is responsible for the degradation of trace gases and plays key roles in major environmental issues such as photochemical pollution. However, the measurement of atmospheric OH radical is a huge challenge due to its high reactivity. Our study provides systematic optimization of a chemical ionization mass spectrometer (CIMS) for OH measurement as a reference for other CIMS users. The ambient result demonstrates the capability of the CIMS for ambient OH measurement.
The hydroxyl radical (OH) is responsible for the degradation of trace gases and plays key roles...
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