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Author(s): Theodore W. Wilson [sup.1] , Luis A. Ladino [sup.2] , Peter A. Alpert [sup.3] , Mark N. Breckels [sup.4] , Ian M. Brooks [sup.1] , Jo Browse [sup.1] , [...], The presence of ice in clouds can influence cloud lifetime, precipitation and radiative properties; here, organic material at the sea-air interface, possibly associated with phytoplankton cell exudates, is shown to nucleate ice under conditions relevant for ice cloud formation in the atmospheric environment. Ice-forming 'biological' particles in the marine atmosphere The presence of ice in clouds can have an influence on cloud lifetime and radiative properties. It remains uncertain to what extent sea spray may act as ice nucleating particles that facilitate the formation of ice in clouds. This study finds that organic material at the sea-air interface, probably derived from phytoplankton cell exudates, nucleates ice under conditions relevant for ice cloud formation in the atmospheric environment. Model simulations suggest that marine organic material may be an important source of ice nucleating particles in remote marine environments such as the Southern Ocean. The amount of ice present in clouds can affect cloud lifetime, precipitation and radiative properties.sup.1,2. The formation of ice in clouds is facilitated by the presence of airborne ice-nucleating particles.sup.1,2. Sea spray is one of the major global sources of atmospheric particles, but it is unclear to what extent these particles are capable of nucleating ice.sup.3,4,5,6,7,8,9,10,11. Sea-spray aerosol contains large amounts of organic material that is ejected into the atmosphere during bubble bursting at the organically enriched sea-air interface or sea surface microlayer.sup.12,13,14,15,16,17,18,19. Here we show that organic material in the sea surface microlayer nucleates ice under conditions relevant for mixed-phase cloud and high-altitude ice cloud formation. The ice-nucleating material is probably biogenic and less than approximately 0.2 micrometres in size. We find that exudates separated from cells of the marine diatom Thalassiosira pseudonana nucleate ice, and propose that organic material associated with phytoplankton cell exudates is a likely candidate for the observed ice-nucleating ability of the microlayer samples. Global model simulations of marine organic aerosol, in combination with our measurements, suggest that marine organic material may be an important source of ice-nucleating particles in remote marine environments such as the Southern Ocean, North Pacific Ocean and North Atlantic Ocean.

A global model perturbed parameter ensemble of 60 simulations was used to explore how combinations of six parameters related to secondary organic aerosol (SOA) formation affect particle number concentrations and organic aerosol mass. The parameters represent the formation of organic compounds with different volatilities from biogenic and anthropogenic sources. The most plausible parameter combinations were determined by comparing the simulations against observations of the number concentration of particles larger than 3 nm diameter (N3), the number concentration of particles larger than 50 nm diameter (N50), and the organic aerosol (OA) mass concentration. The simulations expose a high degree of model equifinality in which the skill of widely different parameter combinations cannot be distinguished against observations. We therefore conclude that, based on the observations we have used, a 6-parameter SOA scheme is under-determined. Nevertheless, the model skill in simulating N3 and N50 is clearly determined by the low and extremely low volatility compounds that affect new particle formation and growth, and the skill in simulating OA mass is determined by the low volatility and semi-volatile compounds. The biogenic low volatility class of compounds that grow nucleated clusters and condense on all particles is found to have the strongest effect on the model skill in simulating N3, N50 and OA. The simulations also expose potential structural deficiencies in the model: we find that parameter combinations that are best for N3 and N50 are worst for OA mass, and the ensemble exaggerates the observed seasonal cycle of particle concentrations - a deficiency that we conclude requires an additional anthropogenic source of either primary or secondary particles. [ABSTRACT FROM AUTHOR]

The effect of observational constraint on the ranges of uncertain physical and chemical process parameters was explored in a global aerosol-climate model. The study uses 1 million variants of the HadGEM3-UKCA climate model that sample 26 sources of uncertainty, together with over 9000 monthly aggregated grid-box measurements of aerosol optical depth, PM2.5, particle number concentrations, sulphate and organic mass concentrations. Despite many compensating effects in the model, the procedure constrains the probability distributions of parameters related to secondary organic aerosol, anthropogenic SO2 emissions, residential emissions, sea spray emissions, dry deposition rates of SO2 and aerosols, new particle formation, cloud droplet pH and the diameter of primary combustion particles. Observational constraint rules out nearly 98 % of the model variants. On constraint, the ± 1σ (standard deviation) range of global annual mean direct radiative forcing, RFari, is reduced by 33 % to -0.14 to -0.26 W m-2, and the 95 % credible interval (CI) is reduced by 34 % to -0.1 to -0.32 W m-2. For the global annual mean aerosol-cloud radiative forcing, RFaci, the ± 1σ range is reduced by 7 % to -1.66 to -2.48 W m-2, and the 95 % CI by 6 % to -1.28 to -2.88 W m-2. The tightness of the constraint is limited by parameter cancellation effects (model equifinality) as well as the large and poorly defined representativeness error associated with comparing point measurements with a global model. The constraint could also be narrowed if model structural errors that prevent simultaneous agreement with different measurement types in multiple locations and seasons could be improved. For example, constraints using either sulphate or PM2.5 measurements individually result in RFari ± 1σ ranges that only just overlap, which shows that emergent constraints based on one measurement type may be over-confident. [ABSTRACT FROM AUTHOR]

Ice nucleating particles (INP) are known to affect the amount of ice in mixed-phase clouds, thereby influencing many of their properties. The atmospheric INP concentration changes by orders of magnitude from terrestrial to marine environments, which typically contain much lower concentrations. Many modelling studies use parameterizations for heterogeneous ice nucleation and cloud ice processes that do not account for this difference because they were developed based on measurements predominantly from terrestrial environments. Errors in the assumed INP concentration will influence the simulated amount of ice in mixed-phase clouds, leading to errors in top-of-atmosphere radiative flux and ultimately the climate sensitivity of climate models. Here we develop a global model of INP concentrations relevant for mixed-phase clouds based on laboratory and field measurements of ice nucleation by K-feldspar (an ice-active component of desert dust) and marine organic aerosols (from sea spray). The simulated global distribution of INP concentrations based on these two-species agrees much better with currently available ambient measurements than when INP concentrations are assumed to depend only on temperature or particle size. Underestimation of INP concentrations in some terrestrial locations may be due to neglect of INP from other terrestrial sources. Our model indicates that, on a monthly or yearly average basis, desert dusts dominate the contribution to the INP population over much of the world, but marine organics become increasingly important in the world's remote oceans and can dominate in the Southern Ocean at some time of the year. Furthermore, we show that day-to-day variability is important and since desert dust aerosol tends to be sporadic, marine organics dominate the INP population on many days per month in much of the mid and high latitude northern hemisphere. This study advances our understanding of which aerosol species need to be included in order to adequately describe the global and regional distribution of INP in models, which will guide ice nucleation researchers on where to focus future laboratory and field work. [ABSTRACT FROM AUTHOR]