Infrared Spectroscopic Evidence for a Heterogeneous Reaction between Ozone and Sodium Oleate at the Gas–Aerosol Interface: Effect of Relative Humidity

Abstract

The heterogeneous ozonolysis of sodium oleate aerosols in an aerosol flow tube is reported under different relative humidity (RH%) conditions. Submicron sodium oleate particles were exposed to a known ozone concentration and the consumption of sodium oleate was monitored by infrared spectroscopy. When the experimental results are treated as a surface‐mediated reaction (i.e., following a Langmuir–Hinshelwood type mechanism), the following parameters are obtained: at low RH%, KnormalO3 = (3 ± 1) × 10−16 cm3 molecule−1 and k max I = (0.046 ± 0.006) s−1; at high RH%, KnormalO3 = (6 ± 2) × 10−16 cm3 molecule−1 and k max I = (0.21 ± 0.05) s−1. From these pseudo–first‐order coefficients, the reactive uptake coefficients for dry and aqueous sodium oleate aerosols are calculated as (1.5 − 0.5) × 10−7 and (1.7 − 0.7) × 10−6, respectively. Hydrated oleate aerosols display both an increase in the ozone trapping ability and an increase in the effective rate reaction at the droplet surface compared to dry aerosol surfaces. These observations may provide an explanation for some of the variability observed between lab studies of dry ozonolysis and real‐world, atmospheric lifetimes of oleic acid–related species.

An organic crystalline state in ageing atmospheric aerosol proxies: spatially resolved structural changes in levitated fatty acid particles

“…Water is known to affect the reaction mechanism of ozonolysis and affect the product distribution in oxidised oleic acid particles (Al-Kindi et al, 2016;Vesna et al, 2009). In our experiments, ozonolysis was carried out under dry conditions (< 5 % RH) in order to negate any effect the presence of water might have on the ozone uptake and reaction rate (Berkemeier et al, 2016;He et al, 2017;Nájera et al, 2015).…”

An organic crystalline state in ageing atmospheric aerosol proxies: spatially resolved structural changes in levitated fatty acid particles

“…Water is known to affect the reaction mechanism of ozonolysis and affect the product distribution in oxidised oleic acid particles (Al-Kindi et al, 2016;Vesna et al, 2009). In our experiments, ozonolysis was carried out under dry conditions (< 5 % RH) in order to negate any effect the presence of water might have on the ozone uptake and reaction rate (Berkemeier et al, 2016;He et al, 2017;Nájera et al, 2015).…”

“…As a surface-active molecule, oleic acid is able to form, in contact with water, complex self-assembled structures such as organogels (in organic solvents) (Nikiforidis et al, 2015), vesicles (Blöchliger et al, 1998) and even helices (Ishimaru et al, 2005). Mixed with its sodium salt (sodium oleate) and water, oleic acid can also form lyotropic liquid crystalline (LLC) phases (Tiddy, 1980).…”

Chemical functionality at the liquid surface of pure unsaturated fatty acids

“…It is therefore of particular relevance to atmospheric chemists to gain an insight into the preference of alkene groups for the surface, as compared to other groups within fatty acids, such as the carboxylic acid group and alkane groups. Studies of model systems for aerosols, however, sometimes make assumptions about fatty acids on the surfaces of these based on work on bulk liquid samples 45,46 or solid salts of these acids, 47 without explicit knowledge about the distribution and orientation of fatty acid functional groups at the gas-liquid interface. Interfaces are, however, known to be energetically, and as result oen chemically, distinct from the bulk of a system 7,8,48 and an understanding of the surface preferences of different molecules and functional groups is vital in order to improve the modelling of surfaces, including those of atmospheric relevance.…”