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Quantifying the atmospheric implications of the solid phase and phase transitions of secondary organic aerosols

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A poorly understood yet large component of atmospheric particles just got less mysterious

Quantifying the flux of secondary organic aerosols (SOAs) is critical to accurately predicting effects on climate and pollution. Pioneering and complementary experimental and modelling studies have filled important knowledge gaps.

Climate Change and Environment

Fog and smoke as well as commercial products like hair spray and spray paint are aerosols (suspensions of particles dispersed in gas, including in the atmosphere). Cloud formation and pollination are also the result of particles in the Earth’s atmosphere. While primary organic aerosols are emitted into the air from sources such as vegetation and combustion of fuels, SOAs are formed in the atmosphere via multiphase chemical processes. Combining innovative experimental and modelling methods, the EU-funded QAPPA project has shed light on critical SOA processes at relevant atmospheric conditions, with a focus on SOAs of biogenic origin in a glassy (amorphous solid or semisolid) state.

Interesting yet largely uncharacterised SOA phase state transitions

Gas-solid and gas-liquid partition processes play critical roles in the transport and residence time of organic pollutants in the atmosphere, important for quantifying effects of SOAs on climate change and air pollution. However, until now, the phase change of SOAs was poorly characterised as was the effect of the glassy phase on various atmospheric processes. One of the reasons for the dearth of data is precisely the difficulty in obtaining and integrating it. According to project coordinator Annele Virtanen, “Field measurements at varying environments and conditions challenged our methodology. In addition to technical improvements, we also needed to develop new data analysis approaches to resolve the multidimensional dependency of different factors.”

Revealing previously unknown dependencies

Building on previous pioneering experimental and modelling work published in the prestigious peer-reviewed journal Nature, the team overcame obstacles and stretched the boundaries of what could be extracted from measured data. As Virtanen explains, “QAPPA successfully resolved and quantified the effects of the amorphous solid or semisolid phase SOA at atmospherically relevant conditions. We observed that the glassy phase is the prevalent state of atmospheric SOA at lower humidity. When the humidity reaches atmospheric values, which in many environments can be relatively high, the particles liquefy and the role of particle phase diffusion limitations in central processes diminishes.” Results show that, at temperatures above 0 oC, partitioning of organic vapours is dominated by the vapour pressure of partitioning vapours, a key source of uncertainty in models. In addition, at these temperatures, water uptake by organic aerosols is not significantly affected by the glassy phase. In contrast, the team’s ongoing experiments demonstrate that, at subzero temperatures, the water uptake and probably also ice nucleation of organic aerosols can be affected by the glassy state and particle phase diffusion limitations. Particulate matter acting as nucleation sites for cloud droplet and ice particle formation affect precipitation and the reflective properties of clouds. Virtanen plans to focus future efforts on low temperatures and the role of glassy SOA in ice nucleation. In the meantime, Virtanen says, “Although the physical phase of SOAs does not play a major role in many processes in the natural environment under typical conditions, it may have a significant role in laboratory measurements. This should be kept in mind when the lab results are interpreted and especially when they are used to develop and improve parametrisations for models.”

Keywords

QAPPA, secondary organic aerosols (SOA), phase, glassy, atmospheric, particle, aerosol, temperature, state, atmosphere, nucleation, ice, partitioning, gas, amorphous solid, modelling

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