|Title:||Development of chemical ionization mass spectrometry for in-situ measurement of hydroxyl (OH) and hydroperoxyl (HO2) radicals and their impact on atmospheric chemistry|
|Advisors:||Wang, Tao (CEE)|
|Subject:||Hydroxyl group -- Measurement|
Chemical ionization mass spectrometry
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Civil and Environmental Engineering|
|Pages:||168 pages : color illustrations, map|
|Abstract:||The hydroxyl radical (OH) and hydroperoxyl radical (HO2) play a central role in tropospheric chemistry. OH and HO2, or HOx, interconversion removes many reduced trace gases and forms secondary air pollutants. Measuring atmospheric concentrations of HOx is a significant challenge due to its chemical properties. Additionally, the OH sources and sinks are poorly understood under low nitrogen oxides (NOx) conditions. This study developed and optimized a chemical ionization mass spectrometer system (CIMS) with a calibration system to measure HOx and investigate their impact on the atmosphere. Various components of the CIMS were compared to improve performance. The sensitivity of CIMS was improved by optimizing the flow rates of various gases and voltages. The sensitivity tests of the CIMS are explicitly described.|
The first field campaign was conducted in urban Hong Kong in April 2019 to test the optimized CIMS for OH measurement. An obvious diurnal pattern of OH radicals was observed, with the highest concentration of 6 × 106 cm-3 at midday and the daytime detection limits at around 8 × 105 cm-3. The overall uncertainty for this day is about ± 51%. The results demonstrated the capability of our CIMS for OH measurements.
The second field campaign was performed in autumn 2020 at a coastal site in Hong Kong. The measured average noontime OH concentration for the study period was 4.9 ± 2.1 × 106 cm-3. The 0D-Box model with comprehensive observational constraints reproduced daytime observed OH when the air parcels originated from the continental regions but overpredicted daytime observed OH for coastal air parcels by 142% on average. Missing reactivity for OH was proposed to be the cause of the model overprediction in low levels of NOx and volatile organic compounds (VOCs). This finding implies the existence of unmeasured chemical species in the aged coastal air parcels. However, the lack of HO2 measurement hindered further analysis.
Thus, in the third campaign in the winter of 2021-22 at the same coastal site, the CIMS was further developed for simultaneous measurement of ambient OH, HO2, and H2SO4 gases. A new data analysis method was developed to monitor the residual problem of the injection gases after switching measurement targets. Eleven days of results were obtained with the daily maximum concentration of HO2, OH, and H2SO4 varying from 0.6 to 4.63 × 108 cm-3, 2.6 to 15.42 × 106 cm-3 and 4.11 to 10.79 × 106 cm-3, respectively. This campaign shows the successful development of CIMS for HO2 and H2SO4 measurement and the potential to simultaneously measure other species without isotopic 34SO2 addition. This made our CIMS unique around the world.
Overall, this work developed the CIMS for HOx and H2SO4 measurement, provided explicit details for the development and optimization processes of CIMS, performed the first comprehensive measurement of HOx by CIMS in Asia, investigated the OH chemistry in the coastal environment of China, emphasized the knowledge gap of the aged air mass, and highlighted the importance of HOx radical measurement to scrutinize our understanding of the atmosphere.
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