Aerodyne Research

Abstract. We present a state-of-the-art humidity probe for in situ airborne water vapor measurements tailored to the humidity range relevant for the formation of persistent aviation contrail cirrus clouds. Our probe is based on tunable infrared laser direct absorption spectroscopy (TILDAS). We scan a laser in wavelength across an isolated water (H2O) absorption line at 7205.246 cm−1 (1.39 µm) to obtain the H2O volume mixing ratio in sample air at 1 Hz in real time. Our optical design features a combination of single-mode optical fibers and a short-path absorption cell with integrated focusing optics and detector to avoid any absorption path outside of the sample volume. A parallel fiber path and detector capture the laser power profile during the wavelength scan for spectral-baseline normalization. We have built and tested two prototypes and tested them side-by-side in the laboratory against a humidity standard as well as against each other. Without any calibration, we found agreement against the humidity standard of better than 98 % for the relevant H2O range below 200 ppm. The agreement between both instruments when operated in series measuring the same sample air was 99.7 %. The stability of both instruments was quantified to be ±1 % (1σ) during a 5 d long period where both prototypes operated without any temperature control. We show that our probes can measure H2O with linear response over 4 orders of magnitude.

The Interstellar Mapping and Acceleration Probe (IMAP; McComas et al. in Space Sci Rev 214:116, 2018a; McComas et al. in Space Sci Rev 221:100, 2025) is a NASA mission designed to study two of the most important issues in heliophysics: the interaction of the solar wind with the local interstellar medium and the acceleration of energetic particles. These two puzzles are surprisingly intertwined because particle acceleration in the inner heliosphere plays a large role in the interactions at the edge of the heliosphere. The ten instruments on IMAP, situated near the Earth-Sun L1 Lagrange point, include both in situ and remote-sensing observations and are designed to work together to explore these questions. The High-energy Ion Telescope (HIT), described herein, measures in situ ions from hydrogen through nickel at energies ranging from a few MeV/nucleon to some tens of MeV/nucleon, depending upon species. While the primary focus of HIT is Solar Energetic Particles (SEPs), HIT is also sensitive to Anomalous Cosmic Rays (ACRs) and Galactic Cosmic Rays (GCRs) that are also present at these energies. Working with the other IMAP instruments, HIT studies how the variation in SEP intensities, spectra, anisotropies, and composition inform the acceleration processes that generate these particles, and, in turn, how these high-energy particles affect the outer heliosphere. This paper details the design and operation of the HIT instrument.

<strong class="journal-contentHeaderColor">Abstract.</strong> We present a state-of-the-art humidity probe for <em>in situ</em> airborne water vapor measurements tailored to the humidity range relevant for the formation of persistent aviation contrail cirrus clouds. Our probe is based on tunable infrared laser direct absorption spectroscopy (TILDAS). We scan a laser in wavelength across an isolated water (H₂O) absorption line near 7205 cm^-1 (1.39 &micro;m) to obtain the H₂O volume mixing ratio in sample air at 1 Hz in real time. Our novel optical design features a combination of single-mode optical fibers and a short-path absorption cell with integrated focusing optics and detector to avoid any absorption path outside of the sample volume. A parallel fiber path and detector capture the laser power profile during the wavelength scan for spectral-baseline normalization. We have built and tested two prototypes and tested them side-by-side in the laboratory against a humidity standard as well as against each other. Without any calibration, we found agreement against the humidity standard of better than 98 % for the relevant H₂O range below 200 ppm. The agreement between both instruments when operated in series measuring the same sample air was 99.7 %. The stability of both instruments was quantified to be &plusmn;1 % (1 &sigma;) during a 5-day long period where both prototypes operated without any temperature control. We show that our probes can measure H₂O with linear response over 4 orders of magnitude.

Halogen atoms play important but undercharacterized roles in atmospheric oxidation chemistry. Here, we report laboratory measurements of gas- and condensed-phase products formed from the oxidation of α-pinene by hydroxyl radicals (OH), chlorine atoms (Cl), and bromine atoms (Br) in an oxidation flow reactor (OFR). Products were detected using a Vocus proton-transfer time-of-flight reaction mass spectrometer (PTR-ToF-MS) operated with low-pressure ammonium adduct (NH4+) ionization and a Vaporization Inlet for Aerosols (VIA). We applied Positive Matrix Factorization (PMF) to classify precursor and product ions into early and later-generation oxidation products. While some common products were observed across all oxidants, significant compositional differences were also apparent. Vocus:VIA signal ratios were used to estimate volatility trends, revealing that more highly oxygenated compounds and many halogenated products contributed to SOA formation. Cl and Br oxidation led to the formation of oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA), which retained halogen atoms, with Br-derived products exhibiting the lowest carbon oxidation state and the highest halogen retention. Halogenated oxidation products were less volatile than their non-halogenated counterparts. Photochemical modeling suggests that the fates of organic peroxy radicals (RO2) were primarily influenced by RO2 + HO2 reactions for α-pinene/OH, RO2 + Cl, RO2 + HO2, and potentially RO2 isomerization/autooxidation reactions for α-pinene/Cl, and RO2 + Br reactions for α-pinene/Br.

Abstract. Volatile organic compounds (VOCs) are important atmospheric components that contribute to air pollution, but their accurate quantification by proton-transfer-reaction mass spectrometry (PTR-MS) remains challenging. In this work, we coupled a gas chromatograph (GC) prior to PTR-MS and analyzed complex ambient air in urban Shanghai to speciate the PTR signal to identify the VOC species that were responsible for the generation of the ions detected by PTR. We classified 176 individual PTR signals with associated compounds resolved by the GC based on whether they could be used to quantify a VOC species without pre-separation. In this classification, Category I includes 45 decent signal ions that were produced from a single VOC species and thus can be used for reliable quantification, although some of the Category I ions are not the conventionally used protonated quasi-molecular ions (MH+). Category II includes 39 signal ions that were produced from a group of isomers and can be used to quantify the isomeric sum, but with an increased uncertainty if a single calibration factor for one specific isomer is used to represent all structures. Category III includes 92 signal ions that were generated from more than one non-isomeric species (e.g., through protonation, fragmentation, cluster formation) and thus merely gave an upper limit of VOC concentrations. In addition, we propose taking aromatic compounds, for instance, the quantification of selected VOCs, utilizing either non-MH+ or non-Category I ions. Our results help to achieve more comprehensive species identification and reliable VOC quantification in PTR measurements.

Volatile chemical products (VCPs), including organic species emitted from pesticides, coatings, cleaning products, and personal care products, account for more than half of the urban VOC emissions in major North American and European cities. However, VCP emissions, spatial and temporal distributions, and impacts vary widely. Despite being the fourth largest U.S. city, Houston, Texas, lacks measured VCP concentration and emission data. This study presents the first spatial and temporal measurements of selected VCP tracers in Houston, Texas, using a Vocus 2R Chemical Ionization Mass Spectrometer on a mobile platform. Ambient measurements of five major VCP tracers, including D5-siloxane, monoterpenes, para-dichlorobenzene, para-chlorobenzotrifluoride (PCBTF), and 2,2,4-trimethyl-1,3-pentanediol isobutyrate (Texanol), were collected in winter and summer 2023. Several compounds exhibited significantly higher averaged concentrations, with pronounced spatial and seasonal variability, distinguishing Houston from urban areas in the temperate and cooler climate zone. A customized box model was employed to estimate seasonal emissions for the Greater Houston Area, showing that emissions of most VCPs were significantly higher during the summer. This study provides critical insights into the distribution and emission of VCPs in a subtropical metropolitan area, advancing methods for assessing VCP emissions and concentrations across cities and improving understandings of their impacts on air quality, climate, and public health.

<strong class="journal-contentHeaderColor">Abstract.</strong> Ambient air pollution poses a significant threat to public health, particularly in low and middle-income countries, where detailed data on particulate matter (PM) mass and composition are scarce. We conducted a year-long study on PM composition and sources in Eastern Africa (Kigali, Rwanda). The annual mean concentration of PM₁ was 31 &mu;g/m³, with slightly higher concentrations during the dry season. Organic aerosols (OA) contributed 73 % of the observed PM₁ mass, black carbon (BC) 16 %, nitrate 6 %, sulfate and ammonium 2 % each, and chlorine 1 %. BC is approximately 60 % due to fossil fuel and 40 % from biomass burning emissions. Tracer ions detected by the mass spectrometer suggest that photochemistry plays a significant role in the formation of secondary OA during the daytime (6:00 am to 6:00 pm), while primary OA dominates in the morning and evening due to increased anthropogenic activity and shallower boundary layer height. PM₁ in Kigali is primarily composed of Oxygenated Organic Aerosols (OOA, 45 %), Hydrocarbon-like OA (HOA, 32 %), and Biomass Burning OA (BBOA, 23 %). Secondary organic aerosol (SOA) accounted for 47 % and 41 % of PM₁ during the wet and dry seasons, respectively, while primary OA (POA: BBOA + HOA) contributed 53 % and 59 %. This suggests that seasonal changes in PM₁ mass in Kigali are primarily driven by deposition rather than shifts in emissions, chemical processing, or source strengths.

Oxygenated organic molecules (OOMs) originate from both direct emissions and secondary formation via the oxidation of volatile organic compounds (VOCs) emitted from biogenic and anthropogenic sources. OOMs are suggested to play a crucial role in the nucleation and growth of newly formed particles, which in turn influence climate, health and air quality. However, OOMs and the associated nucleation and growth of aerosols in the boundary layer aloft are not considered in atmospheric chemistry models owing to limited observation data. Here, by using unique measurements on the top of a 528 m building tower in Beijing, we show that a variety of OOMs existed, dominated by C6-7 and C5 compounds, which probably arose from multigeneration oxidation of aromatic compounds and isoprene, respectively. Consequently, five possible formation pathways of OOMs were identified using a machine learning approach combined with their diurnal patterns. Further analysis suggested that OOMs, together with sulfuric acid and ammonia, are highly involved in the formation of nanoparticles. Our results complement the current mechanism of ammonia-sulfuric acid nucleation in the boundary layer aloft of anthropogenically influenced regions, highlighting the important role of OOMs in the nucleation and growth of aerosol. We suggest that OOMs should be considered in atmospheric chemistry models for better prediction accuracy.

<strong class="journal-contentHeaderColor">Abstract.</strong> Nitryl chloride (ClNO₂) is a reservoir species of chlorine atoms and nitrogen oxides, both of which play important roles in atmospheric chemistry. To date, all ambient ClNO₂observations have been obtained by chemical ionization mass spectrometry (CIMS). In this work, Thermal Dissociation Tunable Infrared Laser Differential Absorption Spectrometer (TD-TILDAS) is shown to be a viable method for quantifying ClNO₂in laboratory and field settings. This technique relies on the thermal dissociation of ClNO₂ to create chlorine radicals, which undergo fast reactions with hydrocarbons to produce hydrogen chloride (HCl) that is detectable by the TILDAS instrument. Complete quantitative conversion of ClNO₂ to HCl was achieved at temperatures &gt; 400 &deg;C, achieving 1 Hz measurement precision of 11 &plusmn; 1 pptv (3&sigma; limits of detection of 34 &plusmn; 2 pptv) during laboratory comparisons with other ClNO₂ detection methods. After blank- and line loss-corrections, method accuracy is estimated to be within &plusmn; 5 %. Performance metrics of TD-TILDAS during ambient sampling were a 1 Hz precision of 19 &plusmn; 1 pptv and 3&sigma; limits of detection of 57 &plusmn; 3 pptv), which is directly comparable to previously reported ClNO₂ detection by quadrupole CIMS. Thus, TD-TILDAS can provide an alternative analytical approach for a direct measurement of ClNO₂ that can complement existing datasets and future studies. The quantitative nature of TD-TILDAS also makes it a potentially useful tool for the calibration of CIMS instruments. However, interpretation of ambient data may be potentially complicated by interference from unaccounted-for sources of thermolabile chlorine.

Abstract. Mass spectrometry is an established method for studying the chemical composition of gases and particles in the atmosphere. Using this technique, signals corresponding to thousands or even tens of thousands of compounds may be detected from ambient air. The process of identifying all the peaks in the mass spectra is often arduous and time-consuming, in particular when multiple overlapping peaks are present. This manual peak fitting and identification may take even experienced analysts anywhere from weeks to months to complete, depending on the desired accuracy and completeness. In this work, we attempted to automate the fitting and formula assignment workflow and evaluate how far the process can get using a “one-button” algorithm. The algorithm constructed in this work takes in commonly known parameters specific to the instrument type, and by pressing one button it runs and ultimately provides a list of likely peaks for the mass spectrum. The algorithm utilizes weighted-least-squares fitting and a modified version of the Bayesian information criterion along with an iterative formula assignment process. We applied it to synthetic mass spectra and both a gas-phase chemical-ionization mass spectrometer (CIMS) dataset and an aerosol mass spectrometer (AMS) dataset. The results were largely comparable with manual peak fitting and identification done previously but were achieved in a fraction of the time. Erroneous assignments mainly appeared at low-intensity signals, with interference from nearby higher-intensity signals, a case that is challenging also for manual peak fitting. This algorithm provides an excellent starting point for a peak list, which, if needed, can be manually revised. The main result of this study is the algorithm itself. While further improvements and tweaks are possible, the algorithm presented here is currently being implemented into the commonly used Tofware analysis software package to allow easy utilization by the broader community. We hope this can save valuable time of researchers for data interpretation rather than data processing and curation.