Atmospheric Trace Gas Measurements Using Chemical Ionisation Time-of-Flight Mass Spectrometry

Abstract

Atmospheric trace gases whose concentrations range from parts per million by volume (ppt) to parts per quadrillion by volume (ppq) undergo complicated microphysical and chemical processes in the lower atmosphere and play a significant role in climate by indirectly affecting the global radiative feedback through particle formation processes. This work presents the first detailed validation and interpretation of nitric acid (HNO3), hydrogen cyanide (HCN) and some other relevant trace gases measured during the first two campaign deployments of the innovative Time-of-Flight Chemical Ionisation Mass Spectrometer FunMass. The two campaigns span science objectives as versatile as upper tropospheric and lower stratospheric processes above the Asian Monsoon region for the 2017 StratoClim campaign and the nighttime oxidation of isoprene for the 2018 NO3-Isoprene campaign. The Asian Summer Monsoon (ASM) is the dominant circulation system in boreal summer. During high monsoon season, air in the highly polluted Asian boundary is rapidly transported into the Upper Troposphere and Lower Stratosphere (UTLS) by strong convective activities, where it is horizontally retained in the Asian Monsoon Anticyclone (AMA).With the upwelling motion inside the upper part of the AMA, these air pollutants can enter the global stratosphere, potentially affecting the worldwide climate. During the StratoClim aircraft campaign from Kathmandu, Nepal, in July and August 2017, FunMass was deployed onboard the high-altitude research aircraft M55-Geophysica. On August 6 and 8 of the campaign, the first two successful high spatial and temporal resolution insitu measurements of gaseous HNO3 and HCN with high spatial and temporal resolution were carried out inside the AMA. The atmospheric concentrations of HNO3 and HCN were calibrated with reference to gravimetrically controlled permeation devices. HNO3 was further referred to ion-chromatographic analyses. The in-situ measurements show a good agreement with satellite observations, i.e. HNO3 from Aura-MLS and HCN from ACE-FTS. Tracer correlations have been studied with CO and O3 obtained by the airborne instruments COLD and FOZAN, respectively. The HCN observations show significant vertical and horizontal signatures within the AMA which have been analysed by backward trajectory analyses employing the Lagrangian models TRACZILLA and CLaMS. Some of the structures are consistent with the CO measurements indicating quite recent convective events while some segments show CO enhancements without obvious HCN features, which is attributed to different origin regions. Measurements in both flights point to the existence of a layer with enhanced HCN at ~ 365 K potential temperature level which probably is the main convective outflow layer. A filament of high HNO3 and O3 correlated with high HCN signatures between 420 – 437 K was observed uncorrelated with any convective activities. A mean Age of Air (AOA) analysis employing long forward modelling nearly perfectly explains the formation of the filament as isentropic mixing of older HCN rich air from middle or high latitude lower stratosphere with high HCN which was caused by an overall strong enhancement from peatland burning in winter 2015 – 2016. Given its good performance in measuring HNO3 in the UTLS, FunMass was coupled to the SAPHIR chamber to measure HNO3 with an improved concentric-DBD ion source while studying the nighttime oxidation of isoprene (C5H8), one of the most prominent and abundant Biogenic Volatile Organic Compounds (BVOCs), by NO3 radicals in the atmospheric simulation chamber SAPHIR in July and August 2018. This process is deemed to play a major role in Secondary Organic Aerosol (SOA) formation. An online calibration unit employing the established permeation device was connected to the inlet line. HNO3 formation from selected experiment days under very different conditions (with changes in isoprene, NO2, O3, humidity, photolysis, etc.) is discussed. The first detection of organic nitrates is reported herein employing mass peaks corresponding to the respective CO3 – -clusters, especially the Isoprene Nitrate Peroxide (INP) as the first-generation product from the oxidation of isoprene by NO3 radicals. FunMass showed a quite good response and signal-to-noise ratio for the INP measurements and also good to very good correlations with other parallel measurements, mainly by the Br– -CIMS and Vocus-PTR instruments. However, quite a strong variation in absolute responses on different measurement days were identified, which remains up-to-date unexplained. FunMass contributed excellent HNO3 measurements and has shown great potential for the measurement of highly oxidized SOA precursors like INP. Apart from the relevant atmospheric results presented herein, this thesis has shown that FunMass can perform as an extremely sensitive tool for the analysis of various atmospherically relevant processes under extreme conditions in the upper troposphere and lower stratosphere as well as for ground-based and chamber-based measurements. Corrigendum to Figure 3.9 (Page 37) : corrigendum_dc2027.pdf