Thomas Bjerring Kristensen

Cloud condensation nuclei (CCN) modulate the properties and thus the climate effects of clouds. The North African dust plume is an important source of CCN in the North Atlantic trade‐wind zone. Yet, research into the nature and sources of these CCN is sparse, making it difficult to fully assess related cloud effects. Using combined airborne in situ measurements with aerosol modeling we show that ≥60% of CCN in the dust plume were not mineral dust, but co‐transported sulfate particles. We estimate the contribution of anthropogenic sulfur emissions to this particulate sulfate to be 50%–70%. While dust plumes in general are considered natural aerosols, these numbers illustrate that at least one third—but likely more than half—of the CCN were of anthropogenic origin. This points out that even in regions where a natural aerosol absolutely dominates particle mass, a considerable anthropogenic climate impact may be present.

Maritime shipping remains a large source of anthropogenic airborne pollutants, including exhaust particles that can act as cloud condensation nuclei (CCN). The International Maritime Organization (IMO) imposed global fuel sulfur content (FSC) limits on marine fuels in order to target ship exhaust sulfur oxides and particulate matter emissions, but has allowed competing pathways to regulatory compliance; i.e., low FSC fuels versus exhaust after-treatment. Laboratory experiments revealed that these compliance measures have secondary effects on physicochemical properties of exhaust particles, affecting their CCN activity (Santos et al., 2022a; 2022b). We observe that combustion of low FSC fuels results in emissions of highly hydrophobic particles, causing significant reductions in CCN emissions, whereas wet scrubbing leads to an increase in CCN activity. One area of focus is the Arctic region, which has been shown to be particularly susceptible to the effects of climate warming. A steady decrease in observed sea ice cover amplifies the regional warming (Screen and Simmonds, 2010), but also opens the region to increased ship traffic which may result in further climate feedbacks (Stephenson et al., 2018). It is of particular interest to identify how increased ship exhaust particle emissions may affect cloud processes; for example, by facilitating liquid droplet formation and thus, potentially changing the radiative properties of the aerosol and clouds. Here, we investigate how increased shipping activity potentially influences the properties of Arctic mixed-phase clouds. In our study the experimentally observed characteristics of marine particle emissions and their liquid droplet forming potential have been implemented in large eddy simulations. We use the MIMICA model (MISU/MIT Cloud-Aerosol model) (Savre et al., 2014) to simulate a stable stratiform mixed-phase cloud based on the Arctic Summer Cloud Ocean Study (ASCOS) (Tjernström et al., 2014). A range of input parameters for ship aerosol, including size distributions, number concentrations, vertical distributions and hygroscopicities, has been studied to assess the potential impact on cloud properties and regional climate. Santos et al. (2022a). Environ. Sci.: Processes Impacts, 24:1769-1781 Santos et al. (2022b). Environ. Sci.: Atmos., Advance Article Savre et al. (2014). J. Adv. Model. Earth Syst., 6:630-649 Screen and Simmonds (2010). Nature, 464(7293):1334–1337 Stephenson et al. (2018). Geophys. Res. Lett., 45:9898–9908 Tjernström et al. (2014). Atmos. Chem. Phys., 14:2823-2869

Biological aerosol particles affect human health, are essential for microbial and gene dispersal, and have been proposed as important agents for atmospheric processes. However, the abundance and size distributions of atmospheric biological particles are largely unknown. In this study we used a laser-induced fluorescence instrument to measure fluorescent biological aerosol particle (FBAP) concentrations for 18 months (October 2020–April 2022) at a rural, forested site in Sweden. The aim of this study was to investigate FBAP number concentrations (N_FBAP) over time and analyze their relationship with meteorological parameters. N_FBAP was highest in summer and lowest in winter, exhibiting a ∼ 5-fold difference between these seasons. The median N_FBAP was 0.0050, 0.0025, 0.0027, and 0.0126 cm⁻³ in fall, winter, spring, and summer, respectively, and constituted ∼ 0.1–0.5 % of the total supermicron particle number concentration. N_FBAP was dominated by the smallest measured size fraction (1–3 µm), suggesting that the main portions of the biological particles measured were due to single bacterial cells, fungal spores, and bacterial agglomerates. N_FBAP was significantly correlated with increasing air temperature (P<0.01) in all seasons. For most of the campaign N_FBAP was seen to increase with wind speed (P<0.01), while the relationship with relative humidity was for most of the campaign nonsignificant (46 %) but for a large part (30 %) negative (P<0.05). Our results indicate that N_FBAP was highest during warm and dry conditions when wind speeds were high, suggesting that a major part of the FBAP in spring and summer was due to mechanical aerosol generation and release mechanisms. In fall, relative humidity may have been a more important factor in bioaerosol release. This is one of the longest time series of atmospheric FBAPs, which are greatly needed for estimates of bioaerosol background concentrations in comparable regions.

Biological aerosol particles affect human health, are essential for microbial- and gene dispersal, and have been proposed as important agents for atmospheric processes. However, the abundance and size distributions of atmospheric biological particles are largely unknown. In this study we used a laser-induced fluorescence instrument to measure fluorescent biological aerosol particle (FBAP) concentrations for 18 months (October 2020–April 2022) at a rural, forested site in Sweden. The aim of this study was to investigate FBAP number concentrations (N_FBAP) over time and analyze their relationship to meteorological parameters. The N_FBAP was highest in the summer and lowest in winter, exhibiting a ~3-fold difference between these seasons. The median N_FBAP was 0.0050, 0.0025, 0.0027 and 0.0126 cm^-3 in fall, winter, spring, and summer, respectively, and constituted ~0.1–0.5 % of the total supermicron particle number. The N_FBAP were dominated by the smallest measured size fraction (1–3 µm), suggesting that the main portion of the biological particles measured were due to single bacterial cells, fungal spores, and bacterial agglomerates. The N_FBAP were significantly correlated with increasing air temperature (PFBAP was seen to increase with wind speed (PFBAP were highest during warm and dry conditions when wind speeds were high, suggesting that a major part of the FBAP in the spring and summer were due to mechanical aerosol generation and release mechanisms. In the fall, relative humidity may have been a more important factor for bioaerosol release. This is one of the longest time series of atmospheric FBAP, which are highly needed for estimates of bioaerosol background concentrations in comparable regions.

The time dependence of ice-nucleating particle (INP) activity is known to exist, yet for simplicity it is often omitted in atmospheric models as an approximation. Hitherto, only limited experimental work has been done to quantify this time dependency, for which published data are especially scarce regarding ambient aerosol samples and longer timescales. In this study, the time dependence of INP activity is quantified experimentally for six ambient environmental samples. The experimental approach includes a series of hybrid experiments with alternating constant cooling and isothermal experiments using a recently developed cold-stage setup called the Lund University Cold-Stage (LUCS). This approach of observing ambient aerosol samples provides the optimum realism for representing their time dependence in any model. Six ambient aerosol samples were collected at a station in rural Sweden representing aerosol conditions likely influenced by various types of INPs: marine, mineral dust, continental pristine, continental-polluted, combustion-related and rural continental aerosol. Active INP concentrations were seen to be augmented by about 40 % to 100 % (or 70 % to 200 %), depending on the sample, over 2 h (or 10 h). Mineral dust and rural continental samples displayed the most time dependence. This degree of time dependence observed was comparable to, but weaker than, that seen in previous published works. A general tendency was observed for the natural timescale of the freezing to dilate increasingly with time. The fractional freezing rate was observed to decline steadily with the time since the start of isothermal conditions following a power law. A representation of time dependence for incorporation into schemes of heterogeneous ice nucleation that currently omit it is proposed. Our measurements are inconsistent with the simplest purely stochastic model of INP activity, which assumes that the fractional freezing rate of all unfrozen drops is somehow constant and would eventually overpredict active INPs. In reality, the variability of efficiencies among INPs must be treated with any stochastic theory.

We studied ice-nucleating abilities of particulate emissions from a modern heavy-duty diesel engine using three different types of fuel. The polydisperse particle emissions were sampled during engine operation and introduced to a continuous-flow diffusion chamber (CFDC) instrument at a constant relative humidity RH_water=110 %, while the temperature was ramped between −43 and −32 ^∘C (T scan). The tested fuels were EN 590 compliant low-sulfur fossil diesel, hydrotreated vegetable oil (HVO), and rapeseed methyl ester (RME); all were tested without blending. Sampling was carried out at different stages in the engine exhaust aftertreatment system, with and without simulated atmospheric processing using an oxidation flow reactor. In addition to ice nucleation experiments, we used supportive instrumentation to characterize the emitted particles for their physicochemical properties and presented six parameters. We found that the studied emissions contained no significant concentrations of ice-nucleating particles likely to be of atmospheric relevance. The substitution of fossil diesel with renewable fuels, using different emission aftertreatment systems such as a diesel oxidation catalyst, and photochemical aging of total exhaust had only minor effect on their ice-nucleating abilities.

The time dependence of ice-nucleating particle (INP) activity is known to exist, yet for simplicity it is often omitted in atmospheric models as an approximation. Hitherto only limited experimental work has been done to quantify this time dependency, for which published data are especially scarce regarding ambient aerosol samples and longer time scales. In this study, the time dependence of INP activity is quantified experimentally for ambient environmental samples. The experimental approach includes a series of hybrid experiments with alternating constant cooling and isothermal experiments using a recently developed cold-stage setup called the Lund University Cold-Stage (LUCS). This approach of observing ambient aerosol samples provides the optimum realism for representing their time dependence in any model. Six ambient aerosol samples were collected representing aerosol conditions likely influenced by these types of INPs: marine, mineral dust, continental pristine, continental polluted, combustion-related and rural continental aerosol. Active INP concentrations were seen to be augmented by about 40 % to 100 % (or 70 % to 200 %), depending on the sample, over 2 (or 10) hours. This degree of time dependence observed was comparable to that seen in previous published works. Our observations show that the minority of active ice nuclei (IN) with strong time dependency on hourly time scales display only weak time dependence on short time scales of a few minutes. A general tendency was observed for the natural time scale of the freezing to dilate increasingly with time. The fractional freezing rate was observed to steadily declines exponentially with the order of magnitude (logarithm) of the time since the start of isothermal conditions. A representation of time dependence for incorporation into schemes of heterogeneous ice nucleation that currently omit time dependence is proposed.

We studied ice-nucleating abilities of particulate emissions from a modern heavy-duty diesel engine using three different types of fuel. The polydisperse particle emissions were sampled during engine operation and introduced to a continuous-flow diffusion chamber (CFDC) instrument at a constant relative humidity RH_water = 110 %, and temperature was ramped between −43 °C and −32 °C (T-scan). The tested fuels were EN 590 compliant low-sulfur fossil diesel, hydrotreated vegetable oil (HVO) and rapeseed methyl ester (RME), and all were investigated without blending. Sampling was carried out at different stages in the engine exhaust after-treatment system, with and without simulated atmospheric processing using an oxidation flow reactor. In addition to ice-nucleation experiments, we used supportive instrumentation to characterize the emission particles and present six different physical and chemical properties of them. We found that the studied emissions were poor ice-nucleators and substitution of fossil diesel with renewable fuels, using different emission after-treatment systems and photochemical aging of total exhaust had only little effect on their ice-nucleating abilities.

We studied ice-nucleating abilities of particulate emissions from a modern heavy-duty diesel engine using three different types of fuel. The polydisperse particle emissions were sampled during engine operation and introduced to a continuous-flow diffusion chamber (CFDC) instrument at a constant relative humidity RH_water = 110 %, and temperature was ramped between −43 °C and −32 °C (T-scan). The tested fuels were EN 590 compliant low-sulfur fossil diesel, hydrotreated vegetable oil (HVO) and rapeseed methyl ester (RME), and all were investigated without blending. Sampling was carried out at different stages in the engine exhaust after-treatment system, with and without simulated atmospheric processing using an oxidation flow reactor. In addition to ice-nucleation experiments, we used supportive instrumentation to characterize the emission particles and present six different physical and chemical properties of them. We found that the studied emissions were poor ice-nucleators and substitution of fossil diesel with renewable fuels, using different emission after-treatment systems and photochemical aging of total exhaust had only little effect on their ice-nucleating abilities.