Abstract
Abstract. 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 RHwater=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.
Highlights
Atmospheric aerosols affect the energy budget of the Earth and climate in different ways: directly through absorption and scattering of heat and light, respectively, and indirectly by affecting cloud formation and lifetime
Use of the renewable fuels (HVO, rapeseed methyl ester (RME)) led to a reduction of soot mode number emissions of 20 % for hydrotreated vegetable oil (HVO) and 56 % for RME; an equivalent black carbon (eBC) reduction of approximately 50 % was observed for both fuels compared to fossil diesel per MJ (Gren et al, 2021)
The soot mode geometric mean diameters (GMDs) were slightly smaller for HVO (GMD = 40 nm, with geometric standard deviation GSD = 2.0) compared to fossil diesel (GMD = 49 nm, GSD = 2.1) and RME (GMD = 49 nm, GSD = 1.9), which means that the soot reduction of HVO originated from both a reduced soot particle size and a reduced number concentration, while the soot reduction of RME was dominated by the reduced number concentration
Summary
Atmospheric aerosols affect the energy budget of the Earth and climate in different ways: directly through absorption and scattering of heat and light, respectively, and indirectly by affecting cloud formation and lifetime. Direct effects, including aerosol optical thickness and light scattering and absorption are being routinely monitored through a global network of ground-based instruments, such as sun photometers (Toledano et al, 2012) and lidars (Althausen et al, 2009). The indirect effects, remain less understood (Boucher et al, 2013; Kreidenweis et al, 2018) due to complexity of the processes within the clouds that contribute to the total effect. One indirect effect of the aerosol particles is related to mixed-phase clouds (MPCs), where certain types of particles may promote formation of ice crystals within them via immersion freezing (Murray et al, 2012). Understanding mixed-phase cloud processes is of the utmost importance as cloud lifetimes and their radiative properties depend strongly on the cloud phase (Korolev et al, 2017).
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