| Author: | Wang, Weihao |
| Title: | Investigations of the atmospheric oxidative capacity with chemical ionization mass spectrometry and chemical box model |
| Advisors: | Wang, Tao (CEE) |
| Degree: | Ph.D. |
| Year: | 2020 |
| Subject: | Chemical ionization mass spectrometry Atmospheric chemistry Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxxii, 225 pages : color illustrations |
| Language: | English |
| Abstract: | The oxidation process, which controls the fate of many atmospheric trace species and the formation of secondary pollutants, is the most important chemical process in the atmosphere. These reactions are dominated by three main groups of gas-phase atmospheric radicals: ROx (OH, HO₂, and RO₂), NO₃, and halogen radicals. It is essential to detect directly or estimate the concentrations of radicals and thus to quantify the atmospheric oxidation capacity. There are two ways to quantify the in-situ radical level: (1) direct detection of target radical and (2) indirect estimation by constraining its sources and sinks using steady-state analysis or a complex chemical box model. For the daytime chemistry, ROx, which includes hydroxyl radicals (OH) and their relevant species peroxy radicals (HO₂ and RO₂), are the dominant oxidants in the atmosphere. In polluted environments, the photolysis of HONO is the dominant primary source of ROx radicals. Field observation studies have suggested that there remain unknown sources for ROx radicals and HONO, indicating the need to improve the techniques for the accurate measurement of ROx radicals and HONO. In the past decade, there has been an emerging recognition that halogen radicals, especially Cl atoms, can play a similar role to OH radicals to modify the oxidation capacity by kick-starting hydrocarbon oxidation and influencing the levels of OH, HO₂, and RO₂ radicals in polluted regions. However, the measurement of reactive halogen species is very challenging, and little is known about the abundance and role of reactive halogen species due to a lack of field studies, apart from ClNO₂, in polluted regions. Thus, there is a need to optimize the techniques and to conduct field observation for these species in the polluted regions. For the nighttime chemistry, the NO3 radical and its relevant species N₂O₅ play important roles in the reactive nitrogen budget and oxidative capacity. It is difficult to accurately simulate the level of NO3 or N2O5 as researchers have found that the heterogeneous kinetic parameters of N2O5 in the ambient environment differ from those in laboratory experiments. Thus, it is important to obtain the accurate heterogeneous kinetic parameter of N₂O₅ under a real ambient environment. In this study, instrumental developments, field observations, and observation-based model simulations were conducted using the Chemical Ionization Mass Spectrometry (CIMS) technique and a newly developed chemical box model. This study has three main objectives: (1) the development of measurement techniques for radicals or their precursors; (2) the investigation of the heterogeneous kinetic parameter of N₂O₅ in the ambient environment; (3) the development of an observation-based chemical box model with an updated chemistry mechanism to simulate the level of free radicals and estimate their impact on atmospheric oxidation capacity. First, a CIMS system was developed and optimized for the indirect measurement of total peroxy radicals (ROx = HO2 + RO2) by converting the ROx radicals into H2SO4 and then detecting H2SO4 via ionization by NO3-. A calibration system was established based on water photolysis. In this study, a new corona discharge ion source was developed, and its stability was tested in the laboratory. Furthermore, different types of OH scavengers (C3F6 and NO2) were compared and tested in the laboratory. C3F6, which was used in previous research, was found to have a larger interference on the chemical background signal than NO2. Moreover, the CIMS was deployed at an urban site at the campus of the Hong Kong Polytechnic University, and it was shown that this system is capable of measuring ambient ROx in a polluted environment. The 2σ detection limit of this instrument in ambient measurement was 1.5×108 molecules/cm3. Additionally, another instrument was developed for the measurement of HONO. Currently, the most widely used technique for the measurement of ambient HONO is the Long Path Absorption Photometer (LOPAP). As a wet chemistry instrument, the LOPAP has the disadvantages of a long temporal resolution and requiring lots of maintenance work. In this study, a CIMS system was developed and optimized for measuring HONO by using I- reagent ions. The developed CIMS technique has the advantages of (1) high sensitivity (2σ LOD <40 ppt), (2) fast response (<1 s), and high-frequency sampling (up to 10 Hz), (3) good selectivity, and (4) convenient maintenance. Laboratory tests were conducted to characterize the system performance, including sensitivity under different humidity and possible interferences. The humidity of the sample air and the pressure of the flow-tube chamber were found to influence the sensitivity to HONO. The 3σ detection limit was 30 pptv for 1-min average data under relative humidity (RH) conditions of 20-80%, with a fast response time of 0.75 s and the measurement uncertainty of 15%. The CIMS was deployed to the North China Plain (NCP) in the winter and compared with a LOPAP. The performance and comparison demonstrate the capability of the CIMS for the fast measurement of HONO in the ambient environment. In order to obtain kinetic data for N₂O₅ on aerosols in the ambient environment, a novel flow-tube experimental device coupled with an iterative chemical box model was developed to determine the direct uptake coefficient of N₂O₅ on ambient aerosols. Laboratory tests and model simulations were performed to characterize the developed system and to investigate the factors affecting γN₂O₅, including mean residence time, wall loss variability with RH, and N2O5 formation and titration with high levels of NO, NOx, and O3. The results demonstrate that this device is capable of accurately measuring the uptake coefficient of N₂O₅ in a polluted environment. The heterogeneous kinetic dataset obtained from the field observation and the novel experimental device can help improve the parameterization of the N₂O₅ heterogeneous process on ambient aerosols. To understand the role of reactive halogen radicals in a polluted region, the iodide-adduct CIMS (I-CIMS) was optimized to measure the precursors of Cl and Br atoms, and reactive halogen species, including Cl2, Br2, BrCl, and HOBr. Additionally, calibration systems for these four species were successfully established. Through sampling-inlet configuration, sampling tubing maintenance, the examination of potential inlet artifacts, and isotopic analysis, this system was found to be reliable for measurement in a polluted environment. Furthermore, during a recent winter campaign at a village site in the NCP, an unexpectedly high level of BrCl (average level >50 ppt) and a very high level of Cl2 (average level >50 ppt) were observed. Moreover, BrCl was found to be the dominant source of both Br and Cl atoms. This measurement result indicates that reactive halogen species are present at a higher level and have a more significant impact in polluted environments than previously thought. Additionally, the different diurnal patterns of BrCl and Cl2 indicate that they may have different activation mechanisms. These unknown halogen activation mechanisms require more study in polluted environments. Subsequently, to better estimate the level of radicals by using the observational data, which were obtained from the developed and optimized CIMS system, and to assess the comprehensive impact of these species on the atmospheric oxidation capacity, an observation-based zero-dimensional chemical box model with the Master Chemical Mechanisms and updated halogen chemistry was developed. The newly developed model was established using the Matlab platform, which has the advantages of developmental efficiency and the availability of extension tools. This software can be conveniently used to simulate in-situ chemical processes with an optimized data input module and an output data structure with concentrations of all species and reaction rates of the whole time series. The coded analysis scripts can be used to perform budget analysis for any species, budget analysis for the oxidation of VOCs, and the calculation of the ozone production/loss rate. This model was used to simulate the impact of halogen chemistry and to analyze the oxidative capacity using the observational data obtained from a previous field study in the NCP. The simulation result suggests a large enhancement in oxidation capacity from the halogen atoms. The total VOCs oxidation rate increased by ~180% for alkanes, ~50% for C2-C6 alkenes, ~40% for aromatics, and ~90% for aldehyde. This study assesses the oxidation capacity in an ambient polluted environment via the development of a measurement technique involving CIMS and an observation-based chemical box model. The CIMS can be applied to detect ROx radicals and other radical precursors, including HONO, and reactive halogen species. Additionally, the N2O5 kinetic parameter in the ambient environment is directly obtained from observational field data, and a novel developed experimental device. This parameter can assist in the simulation of NO₃ and chemistry. A novel observation-based chemical box model was developed in Matlab to simulate the radical levels and analyze the oxidative capacity. The methods used in this study can be applied to assess the oxidative capacity of the atmosphere and thus to assess the process of secondary air pollutant formation. |
| Rights: | All rights reserved |
| Access: | open access |
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