Scattering Properties of Aerosols in Clean Marine and Polluted Air

The light scattering properties of aerosols are affected by their chemicalcomposition, size distribution and ambient relative humidity (RH) thus makingthem highly variable in both the spatial and temporal domains, which induceslarge uncertainty in their radiative effects on climate. Long-term aerosolmeasurements are key for developing a better understanding of their radiativeproperties. In the pristine marine environment, sea-spray aerosols are theprecursors for cloud formation, which then act as a reflecting layer for incomingsolar radiation. This increases the overall albedo of the marine environment, thusinducing a cooling effect on the climate which otherwise is towards warmingbecause of the darker ocean surface. The main factors which influence theproduction mechanisms and radiative properties of sea-spray aerosols are windspeed,relative humidity and ocean near-surface layer chemical compositionwhich in turn show seasonal behaviour. Hence it is vital to study the radiativeproperties of sea-spray aerosols with reference to these influencing parameters inorder to reduce uncertainties in the estimation of the Earth¿s radiation budget andto obtain more reliable climate predictions.In the present work, the scattering properties of sea-spray aerosols withreference to above mentioned variables were investigated for clean marine airmasses arriving at the Mace Head Atmospheric Research Station on the westernperiphery of Europe. Ten years (2001-2010) of aerosol light scattering datashowed a clear seasonal trend in the aerosol light scattering coefficient (¿scat) andthe Ångström exponent (Å). ¿scat values were maximum (35.3 Mm-1) in themonth of January and minimum (13.7 Mm-1) in the month of July. A highpositive correlation coefficient of 0.82 was also found for the summer seasonbetween percentage occurrence of lower ¿scat values (5-15 Mm-1) and thepercentage occurrence of relatively large Å values (>1.2). A high positivecorrelation coefficient of 0.88 was found between wind-speed and ¿scat. TheseAbstractvifindings clearly indicate that high ¿scat values in the winter season are due to thecontribution of wind driven sea-spray aerosols in the marine boundary layer.Subsequent studies found that ¿scat and the aerosol light backscatteringcoefficient (¿bscat) are dependent on the square of wind-speed ~ U2. It was alsofound that ¿scat for the low biological activity (LBA) period was approximatelytwice that found for the high biological activity (HBA) period. This differencewas attributed to the combined effect of size distribution and refractive indexwhereas refractive index on its own accounted for 70% of the observeddifferecesA relative humidity (RH) scanning Nephelometer (Humidograph)instrument was built to study the effect of RH on aerosol light scatteringproperties. 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 thanthe f(RH) value 1.77 for polluted air masses. Also, the effect of RH onbackscatter fraction and single scattering albedo was to reduce the former by20% and to increase the latter by 1-5% at 85% RH as compared to dryconditions. By combining the information about aerosol chemical compositionand hygroscopic growth factor (HGF), a new HGF parameterization for seasprayaerosols was established. Using the HGF parameterization along withaerosol density and refractive index as inputs to a Mie radiative code, a dualhygroscopicity state, flipping from high-hygroscopicity and high f(RH) to lowhygroscopicityand low f(RH), of aerosol was found as the organic matter mixingvolume percentage exceeded ~ 50%. The effect of organic enrichment on the topof atmosphere (TOA) direct radiative forcing (¿F) was to reduce the coolingcontribution of sea-spray aerosol by ~4.5 times as compared to pure sea-saltspray.