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Title: Measurement of nitryl chloride with a chemical ionization mass spectrometer in polluted China environment : investigation of its abundance, origin, and impact on oxidative chemistry
Authors: Tham, Yee Jun
Advisors: Wang, Tao (CEE)
Keywords: Nitro compounds.
Air -- Pollution -- China.Air -- Pollution -- Health aspects -- China.
Issue Date: 2017
Publisher: The Hong Kong Polytechnic University
Abstract: Nitryl chloride (ClNO₂) is an important source of highly reactive chlorine radical that may affect the atmospheric oxidative chemistry in the daytime. However, the abundance of ClNO₂ and its roles are not fully understood under different environmental conditions. This thesis presents the first detailed investigation of ClNO₂ and its impact in polluted China environments based upon field measurements of ClNO₂ and its precursor, N₂O₅, using chemical ionization mass spectrometry (CIMS). Preliminary studies of ClNO₂ were conducted in Tung Chung and Hok Tsui (both in Hong Kong) with a thermal decomposition (TD) CIMS between 2011 and 2012. ClNO₂ shows a clear diurnal pattern with significant concentrations being observed during the night-time. Formation of ClNO₂ was persistent throughout the year with the highest level of ClNO₂ observed during the autumn season followed by spring, winter, and summer, which is consistent with the seasonal variation of NOx, O₃, and particle chloride. The variations of the ClNO₂ were found to be influenced by the air masses origin with elevated ClNO₂ mixing ratios observed in plumes from Hong Kong and the Pearl River Delta region and lower concentrations in marine air. Calculations suggest that photolysis of ClNO₂ contributed up to 0.02 pptv/s of Cl atom production rate in the early morning and exceeded the OH production rate via O3 photolysis by a factor of 3. These results provide an initial understanding of ClNO₂ and its potential importance in Asian coastal regions. To improve the quality of measurement, we then reconfigured and optimized the TD-CIMS to an unheated version of CIMS. A series of testing were performed, including calibrations, background zeroing and inlet transmission efficiency. The optimized CIMS demonstrates high sensitivity (~1 Hz/pptv) with a low detection limit of 5 pptv for both N₂O₅ and ClNO₂ measurement. The sampling line was also optimized and was suggested to be replaced and washed daily to reduce the artifacts from aerosol deposition on the inlet surface. Field verification of the in-situ CIMS N₂O₅, measurements with observations by a cavity ring-down spectroscopy (CRDS) showed excellent agreement (slope = 0.99; R2 = 0.93). These testing results indicate that the optimized CIMS is capable of accurately detecting both N₂O₅ and ClNO₂ under polluted China environment.
In keeping the works above, the optimized CIMS was deployed to the peak of Tai Mo Shan (957 m above sea level) in late autumn of 2013. The measurement found significant levels of ClNO₂ (>400 pptv) and its precursor, N₂O₅ (>1000 pptv) on 6 out of the 12 consecutive nights with full CIMS data. The highest ever reported levels of ClNO₂ (4.7 ppbv, 1 min average) and N₂O₅ (7.7 ppbv, 1 min average) were observed in one night. High-resolution meteorological simulations revealed that this ClNO2 and N₂O₅-laden air originated from the surface of urban/industrial areas north of the site with enriched anthropogenic chloride as well as NOx and aerosol. We demonstrated that the fast production of NO3 and moderate N₂O₅ uptake and ClNO₂ yield initiated the very high concentration of ClNO₂, which led to a large production rate of Cl atom (up to 0.59 pptv/s) in the next morning. Daytime ClNO₂ and N₂O₅ were also observed in this measurement. The heterogeneous uptake of daytime N₂O₅ on fog/cloud droplets was shown to be a potentially important production mechanism for soluble nitrate and additional source of VOC oxidants during the daytime. Elevated ClNO₂ mixing ratios (>350 pptv) were measured in an inland site of northern China during the summer CAREBEIJING 2014 campaign. The ClNO₂ mixing ratio of up to 2070 pptv (1-min average) was observed on 20-21 June 2014 and was characterized with a larger N₂O₅ heterogeneous loss rate and ClNO₂ production rate compared to the campaign average condition. A salient feature of this observation is that the abundant ClNO₂ concentration kept increasing even after sunrise and reached a peak 4 hours later. Such highly sustained ClNO₂ peaks in the morning are discrepant from the typical diurnal pattern previously observed in other places. As evidenced by the meteorological and chemical analysis, the sustained ClNO₂ morning peaks were caused by the significant ClNO₂ production in the residual layer at night followed by downward mixing after the break-up of the nocturnal boundary layer at sunrise. Calculations showed that ~1.7-4.0 ppbv of ClNO₂ would exist in the residual layer in order to sustain the observed morning ClNO₂ peaks at the surface site. Photolysis of the ClNO₂ was shown to be a dominant radical source and contributed up to 46%, 37%, 10%, 7% and 13% of the integrated oxidation of alkane, alkyne, alkene, aromatic and OVOC, respectively, throughout the morning. Overall, the findings of this thesis reveal that ClNO₂ is ubiquitous over the polluted regions of China and is a crucial pollutant to be considered when assessing photochemical and haze pollutions in China.
Description: PolyU Library Call No.: [THS] LG51 .H577P CEE 2017 Tham
xx, 186 pages :color illustrations
Rights: All rights reserved.
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