Sea salt aerosol production via sublimating wind-blown saline snow particles over sea ice: parameterizations and relevant microphysical mechanisms

Abstract

Blowing snow over sea ice has been proposed as a significant source of sea salt aerosol (SSA) (Yang et al., 2008). In this study, using snow salinity data and blowing snow and aerosol particle measurements collected in the Weddell Sea sea ice zone (SIZ) during a winter cruise, we perform a comprehensive model-data comparison with the aim of validating proposed parameterizations. Additionally, we investigate possible physical mechanisms involved in SSA production from blowing snow. A global chemical transport model, p-TOMCAT, is used to examine the model sensitivity to key parameters involved, namely blowing-snow size distribution, snow salinity, sublimation function, surface wind speed, relative humidity, air temperature and ratio of SSA formed per snow particle. As proposed in the parameterizations of Yang et al. (2008), the SSA mass flux is proportional to the bulk sublimation flux of blowing snow and snow salinity. To convert the bulk sublimation flux to SSA size distribution requires (1) sublimation function for snow particles, (2) blowing-snow size distribution, (3) snow salinity and (4) ratio of SSA formed per snow particle.The optimum model-cruise aerosol data agreement (in diameter range of 0.4-12 µm) indicates two possible microphysical processes that could be associated with SSA production from blowing snow. The first one assumes that one SSA is formed per snow particle after sublimation, and snow particle sublimation is controlled by the curvature effect or the so-called "air ventilation" effect. The second mechanism allows multiple SSAs to form per snow particle and assumes snow particle sublimation is controlled by the moisture gradient between the surface of the particle and the ambient air (moisture diffusion effect). With this latter mechanism the model reproduces the observations assuming that one snow particle produces ∼ 10 SSA during the sublimation process. Although both mechanisms generate very consistent results with respect to observed aerosol number densities, they correspond to completely different microphysical processes and show quite different SSA size spectra, mainly in ultra-fine and coarse size modes. However, due to the lack of relevant data, we could not, so far, conclude confidently which one is more realistic, highlighting the necessity of further investigation.

Pan-Arctic surface ozone: modelling vs. measurements

“…The ozone photochemistry scheme applied to the model has been detailed in previous studies (Law et al, 1998(Law et al, , 2000Savage et al, 2004), with updates including an isoprene chemistry scheme, same as the one implemented in the UKCA model by Young et al (2009) according to the method of Pöschl et al (2000); a hydrolysis reaction of N 2 O 5 on aerosols and cloud droplets (Yang et al, 2005); a tropospheric-bromine scheme involving both gaseous-phase reactions (Yang et al, 2005) and heterogeneous reactions (Yang et al, 2010); and a Fast-J photolysis scheme developed by Voulgarakis et al (2009b), which is not used in this study. They found that N 2 O 5 hydrolysis can cause net NO x loss at high latitudes by up to 60 % in the Northern Hemisphere and ∼ 80 % in the Southern Hemisphere (Yang et al, 2005).…”

Pan-Arctic surface ozone: modelling vs measurements

“…A process-based SSA transport, dry and wet deposition scheme has been implemented in the model by Levine et al (2014) based on the work of Reader and McFarlane (2003). The open ocean sea spray emission scheme follows Jaeglé et al, (2011), and the sea-ice-sourced SSA scheme follows the latest work of Yang et al (2019). Both open-ocean-sourced and sea-icesourced SSA (denoted as OO and SI, respectively) are tagged in 21 size bins covering dry NaCl diameter of 0.02-20 μm in 35 order to track their history for online calculation of their surface density for heterogeneous reaction rates.…”

“…We assume that the evaporation rate of blowing snow particles is controlled by the moisture gradient between the surface of the particle and the ambient air, i.e., the classic mechanism in Yang et al (2019). Other physical parameters include blowing snow size distribution (a shape parameter α=3 and a scale parameter β=37.5 µm), SSA production ratio N =20 (i.e., 20 SSA particles formed from one saline wind-blown snow particle during sublimation).…”

“…On a global scale, the uncertainty of the sea spray (from open ocean) source can be a factor of four (Lewis and Schwartz, 2004). On sea ice, the blowing snow related SSA production is sensitive to both snow salinity and bulk 35 sublimation flux calculated (as a complex function of near surface wind speed, temperature and relative humidity, etc) (Yang et al, 2019). We lack snow data on Arctic sea ice to constrain the dataset used in this study.…”

“…The blowing snow parameters used in this study (including blowing snow particle size distribution, number of SSA formed per saline snow particle, evaporation function involved, etc) mainly affect SSA particle size spectrum rather than mass (Yang et al, 2019), therefore they may only slightly affect both SSA and bromine mass loading when a cut-off size for SSA is applied (a dry NaCl diameter of 10 µm is applied here). Fig.…”