Hygroscopic growth and cloud droplet activation of marine hydrogels

Over the last decade, a large number of studies have been devoted to reduction of the uncertainty in anthropogenic aerosol radiative forcing of climate. However, a lesser-known uncertainty in climate predictions as a result of natural aerosols has only recently been explored. Current regional and climate models attempt to characterize aerosol and trace gas emissions from the Earth's various ecospheres and quantify the uncertainties associated with these emissions. Some of the largest effects of natural aerosols on climate radiative forcing occur over the oceans where boundary layer clouds cover the vast expanse of the Earth's surface and have some of the highest sensitivity of cloud albedo to cloud condensation nuclei (CCN) concentration. The existence of physical relationships between marine biota, gas emissions, aerosols, clouds, and radiative forcing has been hypothesized for over several decades, yet remains highly uncertain. The chemical composition of 50 to 200 nm sized sea spray particles, most critical to climate models, is often described as purely organic, purely sea-salt or mixture of both. The CCN activation potential of these particles has also been described as considerably lower than sea-salt or considerably higher than sea-salt by different investigators. Characterization of sub-micron sea spray aerosol chemical properties, their water uptake potential, and CCN activation efficiency is crucial for improved air quality and climate change assessments. Here we report measurements of hygroscopic growth and cloud droplet activation properties of pure and internally mixed xanthan gum (XG) aerosol as a proxy for aerosolized marine hydrogel. Xanthan gum is a bacterial biopolymer that is used as a reference standard to quantify transparent exopolymer particles in ocean water. We use the hygroscopicity tandem differential mobility analyzer (HTDMA) method to characterize water uptake (for up to 98% relative humidity (RH) values) and the continuous-flow streamwise thermal-gradient CCN method to characterize the CCN activity of pure XG and XG internally mixed with calcium carbonate, calcium nitrate, and sodium chloride. Results of our study show that pure XG aerosol has a growth factor of ~1.17 at 90% RH, corresponding to hygroscopicity parameter kappa ~0.1. When small amounts of sodium chloride (representing 5% of the total XG mass) are added, the values for hygroscopicity and growth factor increased to ~0.15 and ~1.3, respectively. Kappa values from hygroscopic growth are consistent with those derived from CCN measurements. Water uptake and CCN activity of the organic polymer XG behaves as expected from Flory-Huggins theory. Overall kappa values for XG aerosol are much lower than that of sea-salt and well within the expected range of other known atmospheric organic aerosol. Kappa values of internal mixtures of XG and inorganic compounds are well-explained by the volume-weighted mixing rule for kappa.

To assess the atmospheric implications of our findings, Monte-Carlo simulations were carried out for cloud droplet activated fraction of a bimodal lognormal size distribution. Inputs parameters for the lognormal distribution, i.e., total number, geometric standard deviation, and the geometric mean diameter for the nuclei and accumulation modes, were prescribed to be representative of tropospheric sub-micron aerosols. The resulting size distributions were integrated to find the total number of CCN at a given supersaturation. Results of our study show that the presence of minute amount of sodium chloride cores (at less than 24 nm in diameter, a size that is currently untraceable by the most sophisticated chemical detection instruments) can considerably change cloud droplet activation potential of aerosolized marine hydrogels.