Abstract of PhD Thesis

Remote Sensing of Aerosol Optical Depth in North Atlantic Marine Air

Jane Mulcahy, National University of Ireland, Galway (2007)

Aerosol optical depth (AOD) measurements were made at the Mace Head atmospheric research station and over the North Atlantic region, using ground-based and satellite remote sensing techniques, for the period 2002-2004. Two aerosol satellite retrieval algorithms, the Bremen Aerosol Retrieval (BAER) and the Moderate Resolution Imaging Spectroradiometer (MODIS) algorithms, were evaluated over the northeast Atlantic Ocean. Ground-based measurements of AOD were provided by a Precision Filter Radiometer (PFR) at Mace Head. BAER AOD exhibited an underestimating trend over ocean surfaces and cloud contamination was highlighted as a significant source of error, increasing the AOD by approximately 0.12±0.16. The MODIS AOD showed an overall better agreement with PFR measurements. Over ocean surfaces, 78.6 % and 91.6 % of the retrieved AOD (λ=870 nm) lay within the expected uncertainty of ±0.03±0.05·AOD, for the Terra and Aqua MODIS sensors, respectively.Time series analysis of three years of AOD measurements, made in clean marine air mass environments at Mace Head, determined a mean AOD (λ=500 nm) of 0.14±0.06 and a mean °Angstrom exponent was 0.40±0.29. For continentally-influenced air masses, higher values for the mean AOD (λ=500 nm) and °Angstrom exponent were found, at 0.19±0.12 and 1.07±0.32, respectively. The influence of wind speed (U) on the clean marine AOD was investigated. A strong correlation with U2 (r2=0.97) was determined over a wide wind speed range of 4-18 m s−1. An equally strong anti-correlation (r2=0.93) was observed between the °Angstrom exponent and wind speed, indicating that supermicron particles such as sea salt are the dominant AOD contributor at high wind speeds. These correlations were determined during low biological activity periods. During high biological activity periods, the AOD-U relationship broke down due to the increasing contribution of the wind speed independent submicron secondary aerosol to the AOD at λ <500 nm. The AOD-U parameterizations were then used to derive sea spray AOD across the North Atlantic ocean using satellite-derived wind speeds from QuikSCAT. The sea spray AOD reached values of up to 0.3 under high wind speeds, clearly demonstrating how sea spray AOD can rival anthropogenic AOD.