New Insights of Nighttime Chemistry and the Formation Mechanisms of Nitrate Aerosols

다국어 초록 (Multilingual Abstract)

In Northeast Asia, the chemical composition of wintertime PM2.5 has undergone significant changes, with nitrate emerging as a predominant component during high-pollution episodes. Understanding the precise formation mechanisms of nitrate is crucial for developing effective air quality strategies in South Korea. Atmospheric ammonium nitrate （NH4NO3） is produced through the reaction between ammonia （NH3） and nitric acid （HNO3）, where HNO3 is generated via daytime photochemical and nighttime heterogeneous reactions. Particularly at night, the heterogeneous hydrolysis of dinitrogen pentoxide （N2O5） serves as a major pathway, playing a crucial role in wintertime nitrate formation in urban atmospheres. However, the lack of observational data on key nocturnal reactive nitrogen species, such as N2O5, nitryl chloride （ClNO2）, and nitrous acid （HONO）, has limited a comprehensive understanding of these mechanisms in South Korea. Furthermore, nitrate formation is influenced by thermodynamic conditions such as aerosol liquid water content （ALWC） and aerosol acidity （pH）, exhibiting non-equilibrium behaviors in the actual atmosphere.
Therefore, this study aims to elucidate the nocturnal nitrate formation mechanisms in urban atmospheres and evaluate the key physicochemical factors controlling these processes. The main contributions of this study are summarized in three parts.
First, an in situ calibration and quantification method was established for N2O5, ClNO2, and HONO-for which commercial standard gases are unavailable-using an iodide chemical ionization mass spectrometer （I-CIMS）. Securing calibration curves with high linearity provided a reliable foundation for measuring trace ambient compounds at ppt-ppb levels.
Second, utilizing the established I-CIMS analytical technique, observational data collected in Ansan, an urban site within the Seoul Metropolitan Area （SMA）, South Korea, were analyzed to elucidate nocturnal chemical characteristics and nitrate formation processes. During the winter and early spring （January–March 2024） observations, the average nocturnal concentrations of N2O5 and ClNO2 were 154 ppt and 833 ppt, respectively. Pronounced N2O5 accumulation was observed under stagnant conditions with elevated residual ozone. The NO3 radical production rate was higher in March than in January due to the combined effects of rising ozone and temperature. Case analyses confirmed that nitrate formation is heavily dictated by both the heterogeneous uptake efficiency of N2O5 and the NO3 production rate.
Third, based on an intensive wintertime field campaign （December 2025–January 2026）, the impacts of ALWC and pH were evaluated. Under high-ALWC conditions, N2O5 was rapidly consumed on aerosol surfaces, promoting nitrate formation, whereas low-ALWC conditions led to atmospheric N2O5 accumulation. Notably, unlike the thermodynamic equilibrium model （ISORROPIA）, field observations revealed that the gas-particle partitioning of nitrate exhibited continuous transitions across the pH–ALWC space. The model generally overestimated ALWC and excessively partitioned nitrate into the particle phase under high-pH conditions, thereby underestimating gas-phase HNO3. This indicates that the model inadequately represents the kinetic limitations and non-equilibrium characteristics of the ambient atmosphere.
In conclusion, this study established a comprehensive field observational dataset for N2O5, ClNO2, and HONO, the key species driving wintertime nitrate formation in South Korean urban environments. By integrating quantification techniques, chemical kinetics, and thermodynamic analyses, the nocturnal nitrate formation mechanisms were comprehensively elucidated. The empirical verification that nitrate formation is a non-equilibrium process strongly regulated by ALWC and pH highlights that current air quality models inadequately capture nocturnal heterogeneous chemistry. Refining these mechanisms—specifically by improving the parameterization of ALWC and pH—will provide a crucial scientific basis for predicting wintertime PM2.5 episodes and establishing effective air quality management policies.

목차 (Table of Contents)

• Chapter Ⅰ. Introduction 1
• 1. Introduction 2
• 2. Background 6
• 3. Analytical Method and Measurement Principles of Chemical Ionization Mass Spectrometer (CIMS) 9
• 3.1 CIMS Measurement Principles 9
• 3.2 Data analysis and normalization of I-CIMS 13
• Chapter Ⅱ. Calibration and Quantification of I-CIMS 14
• Abstract 15
• 1. N2O5 Calibration 16
• 1.1 N2O5 generation 16
• 1.2 Calculation of N2O5 18
• 1.3 Calibration Result of N2O5 21
• 2. ClNO2 Calibration 24
• 2.1 ClNO2 generation 24
• 2.2 Calculation of ClNO2 26
• 2.3 Calibration Result of ClNO2 27
• 3. HONO Calibration 30
• 3.1 HONO generation 30
• 3.2 Calculation of HONO 32
• 3.3 Calibration Result of HONO 33
• 4. The quality control and quality assurance (QC/QA) of I-CIMS 35
• 5. Conclusions 44
• Supplementary information 45
• Chapter Ⅲ. Insight of Nighttime Chemistry and Formation Mechanism of Nitrates Aerosol 46
• Abstract 47
• 1. Introduction 48
• 2. Materials and Methods 52
• 2.1 Site description 52
• 2.2 Instruments 54
• 2.2.1 Iodide chemical ionization mass spectrometer 54
• 2.2.2 High-resolution time-of-flight aerosol mass spectrometer 56
• 2.2.3 HNO3 System 59
• 2.2.4 Other measurements 62
• 2.3 Calculation of NO3∙ production rate 65
• 2.4 Calculation of N2O5 uptake coefficient 66
• 2.5 ClNO2 yield and nitrate formation potential 69
• 2.6 Model simulation and Estimation of γ(N₂O₅) 70
• 3. Results 74
• 3.1 Overview of observations 74
• 3.2 Diurnal variation of species related to nitrate nighttime chemistry 82
• 3.3 NO3· production and its effect on nitrate nighttime chemistry 86
• 3.4 N2O5 uptake coefficient: Observational Characteristics and Model Comparison 90
• 3.5 Case study 96
• 4. Conclusions 108
• Chapter Ⅳ. Influence of Aerosol Liquid Water Content (ALWC) and Aerosol Acidity (pH) on Nitrate Formation 110
• Abstract 111
• 1. Introduction 112
• 2. Materials and Methods 115
• 2.1 Measurement site and period 115
• 2.2 Instruments 116
• 2.2.1 Iodide chemical ionization mass spectrometer 116
• 2.2.2 HR-ToF-AMS 117
• 2.2.3 Other measurements 118
• 2.2.4 Meteorological parameters 119
• 2.3 Proxy-Based Estimation of Aerosol Liquid Water Content (ALWC) 120
• 2.4 Aerosol Acidity (pH) 123
• 2.5 ISORROPIA 125
• 2.6 Calculation of Nitrate formation Metrics .127
• 3. Results 128
• 3.1 Overview of observations 128
• 3.2 Influence of ALWC and pH on Nitrate Partitioning 133
• 3.3 Impact of Thermodynamic Conditions on Nighttime Nitrate Production 137
• 3.4 Observation-Model Comparison of Nitrate Partitioning 140
• 4. Conclusions and Implications 146
• Chapter Ⅴ. Conclusions 148
• References 151
• LIST OF PUBLICATIONS 178