Imad El Haddad’s research while affiliated with Paul Scherrer Institute and other places

A schematic of the “Himalayan aerosol factory”. Courtesy of Sole Lätti (https://kuvittajat.fi/).

Exposure to anthropogenic atmospheric aerosol is a major health issue, causing several million deaths per year worldwide. The oxidation of aromatic hydrocarbons from traffic and wood combustion is an important anthropogenic source of low-volatility species in secondary organic aerosol, especially in heavily polluted environments. It is not yet established whether the formation of anthropogenic secondary organic aerosol involves mainly rapid autoxidation, slower sequential oxidation steps or a combination of the two. Here we reproduced a typical urban haze in the ‘Cosmics Leaving Outdoor Droplets’ chamber at the European Organization for Nuclear Research and observed the dynamics of aromatic oxidation products during secondary organic aerosol growth on a molecular level to determine mechanisms underlying their production and removal. We demonstrate that sequential oxidation is required for substantial secondary organic aerosol formation. Second-generation oxidation decreases the products’ saturation vapour pressure by several orders of magnitude and increases the aromatic secondary organic aerosol yields from a few percent to a few tens of percent at typical atmospheric concentrations. Through regional modelling, we show that more than 70% of the exposure to anthropogenic organic aerosol in Europe arises from second-generation oxidation.

Solid fuel (SF) combustions, including coal and biomass, are important sources of pollutants in the particle and gas phase and therefore have significant implications for air quality, climate, and human health. In this study, we systematically examined gas-phase emissions, using the Vocus proton-transfer-reaction time-of-flight (PTR-TOF) mass spectrometer, from a variety of solid fuels, including beech logs, spruce/pine logs, spruce/pine branches and needles, straw, cow dung, and coal. The average emission factors (EFs) for organic vapors ranged from 4.8 to 74.2 gkg-1, depending on the combustion phases and solid fuel types. Despite slight differences in modified combustion efficiency (MCE) for some experiments, increasing EFs for organic vapors were observed with lower MCE. The relative contribution of different classes showed large similarities between the combustion phases in beech logs stove burning, relative to the large change in EFs observed. The CxHyOz family is the most abundant group of the organic vapor emitted from all SF combustion. However, among these SF combustions, a greater contribution of nitrogen-containing species and CxHy families (related to polycyclic aromatic hydrocarbons) is observed in the organic vapors from cow dung burning and coal burning, respectively. Intermediate-volatility organic compounds (IVOCs) constituted a significant fraction of emissions in solid fuel combustion, ranging from 12.6 % to 39.3 %. This was particularly notable in the combustion of spruce/pine branches and needles (39.3 %) and coal (31.1 %). Using the Mann–Whitney U test on the studied fuels, we identified specific potential new markers for these fuels based on the Vocus measurements. The product from pyrolysis of coniferyl-type lignin and the extract of cedar pine needle were identified as markers in the open burning of spruce/pine branches and needles (e.g., C10H14O2, C11H14O2, C10H10O2). The product (C9H12O) from the pyrolysis of beech lignin was identified as the potential new marker for beech log stove burning. Many series of nitrogen-containing homologues (e.g., C10H11–21NO, C12H11–21N, C11H11–23NO, and C15H15–31N) and nitrogen-containing species (e.g., acetonitrile, acrylonitrile, propanenitrile, methylpentanenitrile) were specifically identified in cow dung burning emissions. Polycyclic aromatic hydrocarbons (PAHs) with 9–12 carbons were identified with significantly higher abundance from coal burning compared to emissions from other studied fuels. The composition of these organic vapors reflects the burned solid fuel types and can help constrain emissions of solid fuel burning in regional models.

Aerosols play a critical role in the Arctic's radiative balance, influencing solar radiation and cloud formation. Limited observations in the central Arctic leave gaps in understanding aerosol dynamics year-round, affecting model predictions of climate-relevant aerosol properties. Here, we present the first annual high-time-resolution observations of submicron aerosol chemical composition in the central Arctic during the Arctic Ocean 2018 (AO2018) and the 2019–2020 Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expeditions. Seasonal variations in the aerosol mass concentrations and chemical composition in the central Arctic were found to be driven by typical Arctic seasonal regimes and resemble those of pan-Arctic land-based stations. Organic aerosols dominated the pristine summer, while anthropogenic sulfate prevailed in autumn and spring under haze conditions. Ammonium, which impacts aerosol acidity, was consistently less abundant, relative to sulfate, in the central Arctic compared to lower latitudes of the Arctic. Cyclonic (storm) activity was found to have a significant influence on aerosol variability by enhancing emissions from local sources and the transport of remote aerosol. Local wind-generated particles contributed up to 80 % (20 %) of the cloud condensation nuclei population in autumn (spring). While the analysis presented herein provides the current central Arctic aerosol baseline, which will serve to improve climate model predictions in the region, it also underscores the importance of integrating short-timescale processes, such as seasonal wind-driven aerosol sources from blowing snow and open leads/ocean in model simulations. This is particularly important, given the decline in mid-latitude anthropogenic emissions and the increase in local ones.

Biomass burning organic aerosol (BBOA) particles are a major contributor to atmospheric particulate matter with various effects on climate and public health. Quantifying these effects is limited by our understanding of the BBOA particles’ evolving chemical composition during atmospheric aging, driven by their exposure to atmospheric oxidants. This study explores the role of ozone (O3) as atmospheric oxidant in processing primary BBOA particles. We exposed particulate emissions from beech, spruce and pine wood fires to O3 in an oxidative flow reactor, monitoring their chemical evolution using high-resolution time-of-flight aerosol mass spectrometry (HR-AMS) and extractive electrospray ionization time-of-flight mass spectrometry (EESI-ToF). We found that the oxidative state of the particles increased with O3 exposure, as shown by the consistent, albeit minor, rise in O/C ratios. Analysis of the EESI-ToF data revealed specific molecular groups containing 18 and 20 carbon atoms, likely mainly abietic, linoleic, and oleic acid, as highly reactive toward O3 and driving the increase in oxidative state. At higher relative humidity, increased oxidation and loss of reactive species indicate that enhanced O3 diffusion into particles allows the ozonolysis to progress further, highlighting humidity's role in overcoming diffusion barriers that limit the ozonolysis in dry conditions. This study provides qualitative insights into the oxidative processing of primary BBOA particles in different phase states, presenting O3 as selective oxidant. Further research could focus on quantifying the progression of the ozonolysis, in particular the change in diffusion rates depending on relative humidity conditions or particle sizes.

While photochemical aging is known to alter secondary organic aerosol (SOA) properties, this process remains poorly constrained for anthropogenic SOA. This study investigates the photodegradation of SOA produced from the hydroxyl radical-initiated oxidation of naphthalene under low- and high-NOx conditions. We used state-of-the-art mass spectrometry (MS) techniques, including extractive electrospray ionization and chemical ionization MS, for the in-depth molecular characterization of gas and particulate phases. SOA were exposed to simulated irradiation at different stages, i.e., during formation and growth. We found a rapid (i.e. >30 min) photodegradation of high-molecular-weight compounds in the particle-phase. Notably, species with 20 carbon atoms (C20) decreased by 2/3 in the low-NOx experiment which was associated with particle mass loss (∼12%). Concurrently, the formation of oligomers with shorter carbon skeletons in the particle-phase was identified along with the release of volatile products such as formic acid and formaldehyde in the gas-phase. These reactions are linked to photolabile functional groups within the naphthalene-derived SOA products, which increases their likelihood of being degraded under UV light. Overall, photodegradation caused a notable change in the molecular composition altering the physical properties (e.g., volatility) of naphthalene-derived SOA.