| Aerosol light attenuation on quartz fibre filters has been measured since
February 1989 at the Mace Head Atmospheric Research station near Carna, Co.
Galway, Ireland, using an Aethalometer.
The frequency of occurrence of the hourly averaged aerosol absorption data
is found to be bimodally distributed. The two modes result from clean marine
air and anthropogenically polluted continental air both being advected to
the station dependent on the prevailing wind direction. The hourly averages
of the marine portion of the aerosol light absorption are found to follow
closely a lognormal distribution with a geometric mean of
0.310 Mm-1. The hourly averages of continental sector aerosol
absorption are neither normally nor lognormally distributed and have an
arithmetic mean of 6.36 Mm-1, indicating the presence of
anthropogenic sources for BC east of the Mace Head station.
The time series of the monthly averaged attenuation coefficient σatt of
both marine and continental sector aerosol shows an increase from 1989 to
1997 and a levelling off thereafter.
The monthly maximum of marine sector σatt is found in May. Trend and
seasonal characteristics of the clean marine aerosol attenuation
coefficients observed at Mace Head appear to be driven by meteorological
factors, as indicated by rainfall data and by trends in the North Atlantic
Oscillation (NAO) indices. The observed increasing trends of the continental
sector σatt from 1989 up to 1997 are possibly related to changes in BC
emissions over Ireland, calculated from UNSTAT (2002) fuel consumption data. |