On the representativeness of coastal aerosol studies to open ocean studies: Mace Head – a case study M. Rinaldi1, M. C. Facchini1, S. Decesari1, C. Carbone1, E. Finessi1, M. Mircea1, S. Fuzzi1, D. Ceburnis2, M. Ehn3, M. Kulmala3, G. de Leeuw3,4,5, and C. D. O'Dowd2 1Istituto di Scienze dell'Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, Bologna, Italy 2School of Physics and Centre for Climate & Air Pollution Studies, Environmental Change Institute, National University of Ireland Galway, Galway, Ireland 3Division of Atmospheric Sciences and Geophysics, Department of Physics, University of Helsinki, Helsinki, Finland 4Research and Development, Climate Change Unit, Atmospheric Modelling and Observations, Finnish Meteorological Institute, Helsinki, Finland 5Unit Environment, Health and Safety, TNO, Utrecht, The Netherlands
Abstract. A unique opportunity arose during the MAP project to compare open ocean
aerosol measurements with those undertaken at the Mace Head Global Atmosphere
Watch Station, a station used for decades for aerosol process research and
long-term monitoring. The objective of the present study is to demonstrate
that the key aerosol features and processes observed at Mace Head are
characteristic of the open ocean, while acknowledging and allowing for
spatial and temporal gradients. Measurements were conducted for a 5-week
period at Mace Head and offshore, on the Research Vessel Celtic Explorer, in
generally similar marine air masses, albeit not in connected-flow scenarios.
The results of the study indicate, in terms of aerosol number size
distribution, higher nucleation mode particle concentrations at Mace Head
than offshore, pointing to a strong coastal source of new particles that is
not representative of the open ocean. The Aitken mode exhibited a large
degree of similarity, with no systematic differences between Mace Head and
the open ocean, while the accumulation mode showed averagely 35% higher
concentrations at Mace Head. The higher accumulation mode concentration can
be attributed equally to cloud processing and to a coastal enhancement in
concentration. Chemical analysis showed similar or even higher offshore
concentrations for dominant species, such as nss-SO4-2, WSOC, WIOC and
MSA. Sea salt concentration differences determined a 40% higher supermicron
mass at Mace Head, although this difference can be attributed to a higher
wind speed at Mace Head during the comparison period. Moreover, the relative
chemical composition as a function of size illustrated remarkable similarity.
While differences to varying degrees were observed between offshore and
coastal measurements, no convincing evidence was found of local coastal
effects, apart from nucleation mode aerosol, thus confirming the integrity of
previously reported marine aerosol characterisation studies at Mace Head.
Citation: Rinaldi, M., Facchini, M. C., Decesari, S., Carbone, C., Finessi, E., Mircea, M., Fuzzi, S., Ceburnis, D., Ehn, M., Kulmala, M., de Leeuw, G., and O'Dowd, C. D.: On the representativeness of coastal aerosol studies to open ocean studies: Mace Head – a case study, Atmos. Chem. Phys., 9, 9635-9646, doi:10.5194/acp-9-9635-2009, 2009.