Investigating atmospheric trace gases from sources to impacts with time-of-flight chemical ionization mass spectrometry

Abstract

Time-of-flight chemical ionization mass spectrometry (ToF-CIMS) has become one of the most powerful and widely utilized tools for real-time measurement of atmospheric trace gases. This thesis focuses on the investigation of formic acid (HCOOH), nitrous acid (HONO), and molecular bromine (Br2) using an iodide-adduct ToF-CIMS. By integrating field measurements across diverse geographic regions with laboratory experiments and box model simulations, this research advances our understanding of the budgets and environmental impacts of these three compounds. The findings demonstrate that even trace gases at parts-per-trillion (ppt) levels can exert significant influence on atmospheric chemistry. These insights provide a robust foundation for improving air quality management strategies and designing effective pollution mitigation measures. HCOOH is among the most prevalent organic acids in the atmosphere, where it plays a significant role in influencing atmospheric acidity and aqueous-phase chemistry. Despite its ubiquity, sources of HCOOH remain poorly understood. To address this gap, atmospheric concentrations of HCOOH were measured at a coastal site in southern China from August 13 to October 31, 2021. The findings revealed average HCOOH concentrations of 191 ± 167 ppt in marine air masses and 996 ± 433 ppt in coastal air masses, indicating a marked increase in coastal environments. A notable finding from this study was the strong linear correlation observed between HCOOH concentrations and the surface area densities of submicron particulate matter specifically in coastal air masses. This relationship suggests a significant link between particulate matter and HCOOH generation. Further, post-campaign laboratory experiments demonstrated that the photochemical aging of ambient aerosols, enhanced by heterogeneous reactions with ozone, significantly contributes to HCOOH formation. These reactions yielded high concentrations of HCOOH at a rate of 0.185 ppb h⁻¹ under typical noon conditions. Additionally, the role of nitrate photolysis in HCOOH production was highlighted. The photolysis of nitrate is an efficient source of OH radicals, which subsequently oxidize organic compounds to produce HCOOH. To quantitatively assess the impact of these findings, the identified particle-phase source of HCOOH was incorporated into a photochemical model. This integration led to a tripling of the net HCOOH production rate compared to that predicted by the default Master Chemical Mechanism (MCM). These results underscore the importance of considering the photochemical aging of aerosols as a significant source of HCOOH in atmospheric chemistry-transport models. HONO serves as a crucial precursor to hydroxyl radicals (OH), which are vital for daytime atmospheric photochemistry and play a significant role in determining the oxidative capacity of the air. One notable source of HONO in the troposphere is the photolysis of particulate nitrate ( pNO3− ). However, accurately determining the photolysis rate constant of pNO3− (jpNO3−) has been challenging due to considerable uncertainties. One of the primary complications in previous laboratory measurements of jpNO3− using aerosol filters is the “shadow effect”, a phenomenon where light extinction within layers of aerosol can skew results. To address this issue, we developed a novel method that corrects for the shadow effect on the photolysis rate constant specifically for HONO production (jpNO3−→HONO ) under tropical noontime sunlight conditions (solar zenith angle θ = 0°). This method utilizes aerosol filters with identical chemical compositions but varies in aerosol loadings. We applied this refined method to measure jpNO3−→HONO during the winter haze period over the North China Plain (NCP). After making the necessary corrections for the shadow effect, the average normalized jpNO3−→HONO at 5 °C increased significantly from 5.89 × 10−6 s⁻¹ to 1.72 × 10−5 s⁻¹. The jpNO3−→HONO decreased with increasing pH and nitrate proportions in PM2.5, showing no correlation with the absolute nitrate concentrations. Based on these observations, we developed a parameterization for jpNO_3→HONO that can be used in model simulations to accurately predict HONO production in the NCP and similar atmospheric environments. Reactive bromine compounds significantly contribute to ozone depletion and affect ground-level air quality by participating in photochemical reactions. However, their abundance and sources outside the polar regions remain poorly understood. Observations of anthropogenic bromine emissions are particularly sparse. In this study, we report substantial levels of Br2, with a maximum concentration of 23.4 ppt, detected at a High-Tech Industrial Park in the Chinese Blue Silicon Valley, Qingdao. The average Br2 concentration following the reopening of the area was significantly higher than during the Coronavirus disease 2019 (COVID-19) lockdown, indicating that the source is strongly linked to human activities. The alignment of wind direction of high Br2 concentrations with the location of the pharmaceutical facilities, along with strong correlations with brominated organic compounds and other chemical markers of methylating agents, implicates pharmaceutical processes as the probable source of these emissions. To our knowledge, this is the first reported observation of reactive bromine emissions from pharmaceutical activities. These episodic emissions occasionally persisted after sunset, contributing to approximately 20% of isoprene oxidation in the early morning. This source presents a significant concern for air quality in regions with extensive pharmaceutical manufacturing and is expected to become increasingly important as the pharmaceutical industry continues to grow. The collective insights from these studies underscore the roles that specific trace gases play in atmospheric chemistry and their broader implications for environmental health and regulatory measures. Each study not only deepens our understanding of individual trace gas dynamics but also helps in refining atmospheric models that predict the fate of these pollutants. The use of ToF-CIMS has proven invaluable in tracing these gases from their sources to their ultimate impact on air quality. This work advances our understanding of atmospheric chemistry and demonstrates the critical need for innovative analytical techniques in resolving the complexities of air pollution and its management. By highlighting the diverse sources and transformation pathways of trace gases, this thesis also paves the way for targeted environmental interventions aimed at reducing human exposure to harmful pollutants and mitigating the broader impacts of air pollution on climate change.