Frank McGovern

The 2nd Aerosol Characterization Experiment (ACE-2): general overview and main results

This overview summarizes the objectives of the Aerosol Characterization Experiments (ACEs) of the International Global Atmospheric Chemistry (IGAC) project and the research strategy implemented in the second of this series of experiments (ACE-2). ACE-2 took place from 16 June to 24 July 1997, over the sub-tropical North-East Atlantic. It provided an opportunity to study the properties, processes and effects of contrasting aerosol types in this region, including background marine and anthropogenic pollution aerosol in the marine boundary layer (MBL), and background aerosol and mineral dust in the overlaying free troposphere (FT). The major achievements of ACE-2 include:(a) identification of entrainment, in-cloud scavenging and coagulation as the major processes transforming a pollution aerosol transported within the MBL; (b) the first documentation of the indirect radiative effect of aerosols at the scale of a cloud ensemble in continental pollution outflow; (c) observation of a wide range in the contribution of organic material to the sub-micron aerosol mass, with possibly the highest contribution in the free tropospheric; (d) improved understanding of the role of condensing HCl, HNO3 and NH3 as a growth mechanism of sub-micron aerosols in polluted air masses advecting over the ocean. A close connection was observed between meteorological factors (such as horizontal and vertical wind speed, boundary layer development, entrainment, humidity fields) and aerosol and cloud characteristics. In the ACE-2 region, these meteorological factors, rather than aerosol microphysics and chemistry, often dominated the shaping of the aerosol size distribution and/or their effect on radiation and clouds. The ACE-2 data presently analyzed provide a qualitative, and in many cases a quantitative understanding of the complex gas/aerosol/cloud system in the sub-tropical marine environment. This will guide future model development. Some major data sets are still to be analyzed.

Anthropogenic influences on the chemical and physical properties of aerosols in the Atlantic subtropical region during July 1994 and July 1995

Elevated pollution levels have been observed at marine boundary layer (MBL) and free troposphere (FT) sites on the island of Tenerife. These have been linked to air mass transport from continental areas, with over 40% of the measurements being directly influenced by continental sources. The influence of different meteorological patterns on this transport has been examined. Saharan dust was an important component of the FT aerosol. In the MBL, modification of aerosol light scattering and cloud nucleating properties correlated strongly with enhanced non-sea-salt (nss)SO42 levels, linked to transport from Europe. Regression analysis of cloud condensation nuclei (CCN) concentrations estimated at supersaturation levels of 0.15% and 0.75% gave CCN = 95 nssSO−24 + 139(0.15%) and CCN = 142 nssSO−24 + 256(0.75%), respectively. Regression analysis of submicron aerosol scattering at 530 nm gave a sulfate mass scattering efficiency value of 6.3 m2 g−1. Sea salt was the dominant supermicron scattering species in the MBL. A mass scattering efficiency of 2.9 m2 g−1 is estimated for Na+.

An analysis of the ionic composition of Irish precipitation and background air quality since 1980 based on samples collected at Valentia Observatory, Co. Kerry, Ireland

This work has investigated the ionic composition of precipitation samples based on measuring Na, K, Mg, Ca, Cl, SO4-S, NH4-N, NO3-N (mg/L) and the trends of SO2-S, SO4-S, and NO2-N (μg/m) concentrations in air samples collected on a daily basis at Valentia Observatory, Cahirciveen, Co. Kerry, Ireland and analysed in the Met Eireann laboratory over the period 1980 2004. The time series of annual average SO2-S and SO4-S concentrations (μg/m) for the air data shows a distinct downward trend. The NO2-N concentration (μg/m) levels have remained relatively stable since measurements commenced in 1989. No such obvious trends are visible from the original precipitation data; however, by investigating the contribution of sea-salt sulphate to the total sulphate levels it is possible to conclude that the non-sea salt sulphate levels (i.e. anthropogenic contribution) have decreased since the measurements have commenced. The location of the site on the south west coast results in exposure to the Atlantic Ocean and this results in considerable contribution of ions from the sea that is clearly shown from the wind sector analysis of the precipitation samples for Na, K, Mg, Ca and Cl. The air data shows that the highest concentrations for the SO2-S and SO4-S and NO2-N are correlated with winds coming from an easterly and northeasterly direction (which is also confirmed from the non-sea salt sulphate levels for the precipitation results). The lowest concentrations were found for westerly directions. This has also been proven correct with the use of back trajectory analysis, which has allowed tracing the movement of air parcels using ECMWF analysis fields.

Results of the pre-ace 2 campaigns in the subtropical North Atlantic

Publisher Summary This chapter helps in measuring aerosol properties in the free troposphere and marine boundary layer of the Northeast Atlantic, in order to survey and classify the properties of the various air masses advecting to the region, which will serve as the base for the 1997 ACE-2 experiment. It provides an overview of chemical composition, size distributions, and radiative properties for both sites. European pollution is found to affect the aerosol properties in the marine boundary layer and the free troposphere of the studied area, leading to higher nonsea-salt sulfate concentration, higher scattering coefficient, higher fight absorption, and more numerous condensation cloud nuclei (CCN). Aerosol chemistry and size distribution show that clean marine air is advected to the Tenerife MBL, with interruptions due to pollution plumes from Europe. During the 1994 and 1995 campaigns, well-defined clean and polluted episodes lasted 4 to 6 days, and no more than two major pollution outbreaks reached the site in a time span of 5 weeks. The free troposphere is characterized in July by strong dust episodes (sometimes mixed with anthropogenic inputs from Europe) alternating with cleaner periods during which significant concentrations of NH4+ and SO42- are still observed.