Scattering Properties of Aerosols in Clean Marine and Polluted Air

Abstract

The light scattering properties of aerosols are affected by their chemical composition, size distribution and ambient relative humidity (RH) thus making them highly variable in both the spatial and temporal domains, which induces large uncertainty in their radiative effects on climate. Long-term aerosol measurements are key for developing a better understanding of their radiative properties. In the pristine marine environment, sea-spray aerosols are the precursors for cloud formation, which then act as a reflecting layer for incoming solar radiation. This increases the overall albedo of the marine environment, thus inducing a cooling effect on the climate which otherwise is towards warming because of the darker ocean surface. The main factors which influence the production mechanisms and radiative properties of sea-spray aerosols are windspeed, relative humidity and ocean near-surface layer chemical composition which in turn show seasonal behaviour. Hence it is vital to study the radiative properties of sea-spray aerosols with reference to these influencing parameters in order to reduce uncertainties in the estimation of the Earth¿s radiation budget and to obtain more reliable climate predictions. In the present work, the scattering properties of sea-spray aerosols with reference to above mentioned variables were investigated for clean marine air masses arriving at the Mace Head Atmospheric Research Station on the western periphery of Europe. Ten years (2001-2010) of aerosol light scattering data showed a clear seasonal trend in the aerosol light scattering coefficient (¿scat) and the Ångström exponent (Å). ¿scat values were maximum (35.3 Mm-1) in the month of January and minimum (13.7 Mm-1) in the month of July. A high positive correlation coefficient of 0.82 was also found for the summer season between percentage occurrence of lower ¿scat values (5-15 Mm-1) and the percentage occurrence of relatively large Å values (>1.2). A high positive correlation coefficient of 0.88 was found between wind-speed and ¿scat. These Abstract vi findings clearly indicate that high ¿scat values in the winter season are due to the contribution of wind driven sea-spray aerosols in the marine boundary layer. Subsequent studies found that ¿scat and the aerosol light backscattering coefficient (¿bscat) are dependent on the square of wind-speed ~ U2. It was also found that ¿scat for the low biological activity (LBA) period was approximately twice that found for the high biological activity (HBA) period. This difference was attributed to the combined effect of size distribution and refractive index whereas refractive index on its own accounted for 70% of the observed differeces

A relative humidity (RH) scanning Nephelometer (Humidograph) instrument was built to study the effect of RH on aerosol light scattering properties. It was found that the aerosol light scattering enhancement (f(RH)) values for the clean marine air masses was 2.22 at 85% RH which is higher than the f(RH) value 1.77 for polluted air masses. Also, the effect of RH on backscatter fraction and single scattering albedo was to reduce the former by 20% and to increase the latter by 1-5% at 85% RH as compared to dry conditions. By combining the information about aerosol chemical composition and hygroscopic growth factor (HGF), a new HGF parameterization for seaspray aerosols was established. Using the HGF parameterization along with aerosol density and refractive index as inputs to a Mie radiative code, a dual hygroscopicity state, flipping from high-hygroscopicity and high f(RH) to lowhygroscopicity and low f(RH), of aerosol was found as the organic matter mixing volume percentage exceeded ~ 50%. The effect of organic enrichment on the top of atmosphere (TOA) direct radiative forcing (¿F) was to reduce the cooling contribution of sea-spray aerosol by ~4.5 times as compared to pure sea-salt spray.