The Atmosphere/Ocean Chemistry Experiment (AEROCE) was a comprehensive multi-disciplinary and multi-institutional research program that focused on a variety of aspects of the atmospheric and marine chemistry over the North Atlantic Ocean (NAO) region.
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Major objectives were to gauge the impact of anthropogenic sources on the chemical and physical properties of the atmosphere, to assess the consequences of these perturbations on natural processes including climate, and, through the use of models, to predict longer term effects. Research focused on two theme areas:
Theme 1: Ozone and oxidants
To understand the role of anthropogenic emissions and natural processes in the ozone budget and the oxidizing capacity of the troposphere over the NAO.
Theme 2: Aerosols and climate
To characterize the chemical and physical properties of aerosols important to the radiative properties of the atmosphere and climate; to study the processes that affect these properties; and to assess the relative importance of natural versus human sources.
Within the context of these two main themes, a substantial effort was made to study atmosphere/ocean chemical fluxes and their effects on biogeochemical processes in the ocean.
AEROCE began in 1988. Sponsored mainly by the U.S. National Science Foundation, the program involved both continuous measurements and intensive field programs, some of which were carried out in conjunction with activities such as NARE and ACE-2. Activities focused to a large extent on field stations: Barbados, West Indies; Bermuda; Izaña, Tenerife, Canary Islands; Mace Head, Ireland; Heimaey, Iceland; Miami, Florida. Several targeted studies were conducted with aircraft. AEROCE facilities have served a broad scientific community beyond AEROCE itself. Although AEROCE ended in 1998, its sites continue to operate independently, although at reduced levels, with support provided from the international community.
Some major findings and accomplishments are listed below.
- Characterized role of US East Coast frontal passages on vertical distribution of pollutants over the western NAO
Springtime aircraft measurements coupled with an ozonesonde network showed substantial NOY, CO, VOC's, and aerosols in layers in the free troposphere between the east coast and Bermuda. Ozone layers were also observed, some of which were attributed to pollution while others originated in the upper troposphere/lower stratosphere (UT/LS). Layering was driven by a mechanism that linked cyclones and associated cold fronts. Convective activity over the east coast in the warm sector (ahead of cold fronts) lifts pollutants from the boundary layer into the free troposphere where ozone can become moderately enhanced through chemical processing. In post cold-front areas subsiding air forms layers that have low specific humidity, high potential vorticity, and elevated ozone from the UT/LS. Subsidence behind a cold front encounters convective clouds ahead of the following cold front. Thus, parcels of polluted continental boundary layer air are found close to layers of air from the UT/LS containing high mixing ratios of O3. These studies show how complex vertical structures evolve during frontal passage and they help clarify the relationship of ozone to pollutants [Prados et al., 1999].
- Characterized the meteorological factors associated with pollution transport from the eastern United States to Bermuda and the western NAO.
Seasonal cycles in the concentrations of pollution-derived elements in aerosols at Bermuda were mainly driven by transport; they were not strongly related to variations in source emissions. Spring maxima in pollutants were caused by rapid transport from North America while fall maxima were the result of air that was slowly transported from North America by large high-pressure systems that stagnated over the lower mid-Atlantic states. Ozone, carbon monoxide and the tracers 210Pb and 7Be exhibited seasonal cycles similar to those of the pollution-derived trace elements. The springtime maximum in ozone over the western NAO is largely driven by transport from the UT/LS over eastern North America [Moody et al., 1995].
- Developed an ozone climatology for the NAO.
A network of ozonesonde stations provided a multi-year picture of ozone distributions over the NAO, which has led to a better understanding of the factors that drive the large-scale variability of ozone over this region and the relative importance of natural and anthropogenic sources [Oltmans et al., 1996].
- Linked atmospheric transport to water column processes.
Records of dust deposition in deep-sea sediment traps in the Sargasso Sea were found to be consistent with measured atmospheric dust loadings measured at Bermuda. The seasonality of the fluxes differed in the atmosphere and the ocean as a result of biological cycles in the surface water. Annual variations in the sediment trap fluxes were linked to changes in the atmospheric transport efficiency from source regions in Africa as opposed to changes in the strengths of the dust sources [Jickells et al., 1998].
- Documented the large-scale temporal (daily, seasonal, interannual) and spatial variability of aerosols, precipitation, and selected gases over the NAO.
The AEROCE data set has been widely used for testing large-scale chemical transport models; it was the only NAO data used in aerosol forcing models in the recent (third) IPCC climate assessment [IPCC, 2001].
- Provided estimates of anthropogenic impacts on the atmospheric cycles of S and N and other substances.
Human activities accounted for a maximum of 85-90% of particulate sulfate (in the westerly winds at Mace Head, Ireland). Even at Barbados, in the easterly Trade Winds, about half of the sulfate was from anthropogenic sources. Similar impacts were observed for particulate nitrogen species (nitrate and ammonium) and selenium [Savoie et al., 1992].
- Showed that mineral dust is the dominant light scattering aerosol over a large region of the tropical and subtropical NAO.
On an annual basis, dust accounts for 56% of the light scattering over the tropical Atlantic, sea-salt 33% and non-sea-salt sulfate 11%. These measurements demonstrate that dust (the most prominent aerosol constituent visible in satellite imagery over the oceans) can have a great impact on radiative processes and (possibly) climate [Maring et al., 2000].
- Documented long-term trends in pollutant transport.
Bermuda data indicate that there has been no substantial change in the concentration of nss-sulfate and nitrate in aerosols and precipitation over the period 1988-1998 although a slight downward trend is suggested. Over this time period the SO2 emissions in the US have decreased about 15%. Such a small change may not be readily discernable because of the confounding effects of transport variability noted above. In contrast, aerosol Pb concentrations decreased by a factor of 4 during AEROCE and a factor of 10 since the 1970s. Similar changes in Pb concentrations were observed in seawater from the NAO [Arimoto et al., 1999].
- Provided aerosol and deposition data that enabled an assessment of the impact of atmospheric inputs on nutrient cycles and primary productivity in the NAO.
AEROCE dust deposition measurements served as a basis of a current working hypothesis – that African dust, by providing the micronutrient Fe, has strongly impacted the nitrogen cycle by enhancing the growth of Trichodesmium sp, a prominent colonial cyanobacterium in the central NAO. The large temporal-spatial variability of African dust could provide a strong modulation to N-nutrient budgets in the NAO. AEROCE aerosol and precipitation data have been widely used by the oceanographic community in their biogeochemical models. The dust-Fe data are central to the current focus on the effects of dust on ocean primary productivity. By modulating ocean productivity, mineral dust could have a substantial impact on the global carbon cycle and atmospheric CO2. Finally, AEROCE measurements show that direct inputs of N-species to the surface ocean via wet deposition can lead to episodic enhancements in primary production [Michaels et al., 1996].
- Showed the impact of long range transport on air quality in the southeastern (and Gulf Coast) United States.
Aerosol studies at Barbados and Miami show that during the summer, on a mass basis, African dust is the major sub-2.5 mm diameter aerosol. Furthermore dust is carried throughout the southeastern United States where it could have implications for air-quality enforcement. Although African dust in itself is unlikely to cause an exceedance of the new EPA PM 2.5 standard, dust in combination with local pollution aerosols could well exceed the new standards. These data suggest that African dust can be a health issue in the southeastern states [Prospero, 1999].
- Showed that long-range dust transport is linked to large-scale climate processes.
AEROCE African dust measurements (and the long term record from Barbados) show that dust concentrations vary greatly from year to year. Concentrations increase sharply (a factor of three to four) during periods of severe drought. The long-term record at Barbados has been shown to be strongly related with the North Atlantic Oscillation (NAO). This relationship can provide the basis for linking dust to paleoclimate forcing processes [Prospero, 1996].
- Provided a unique aerosol data set to the remote sensing community.
AEROCE aerosol data have been widely used by the remote sensing community to develop and test aerosol algorithms. AEROCE data are currently being used as a part of the NASA Global Aerosol Climatology Project. The value of the AEROCE data set lies in the fact that the data were obtained on a daily basis (and thus could be linked to synoptic meteorology) and are available as an integrated data set over a large ocean region [Diaz et al., 2001].
- Provided (in conjunction with satellite data) information on global dust sources and the processes that affect dust mobilization.
The linking of the AEROCE mineral dust record to the TOMS absorbing aerosol product has led to a global assessment of dust sources. The major dust sources are shown to be associated with topographical lows in arid regions. All major dust sources are associated with deep alluvial soils that were deposited during Pleistocene pluvials. This study shows that the paleoclimate record of dust (from ocean sediments and ice cores) must be reinterpreted. Dust does not necessarily imply arid conditions; for significant production, aridity must follow upon a relatively recent (in a geological sense) pluvial period [Ginoux et al., 2001].
- Demonstrated that sea-salt aerosol is a major reaction medium and sink in the chemical evolution of sulfur and nitrogen over the NAO.
Scavenging by sea salt is a potentially important shunt in the sulfur (S) cycle that could limit production and growth of new submicrometer aerosol. Sea salt must be explicitly considered in assessing relationships between S emissions and the associated direct and indirect radiative effects of S-containing particles in marine regions [Turekian et al., 2001]. Studies also led to the first reliable estimates of marine aerosol pH as a function of particle size and implications for S oxidation, nitrogen phase partitioning, and associated radiative effects. HCl and HNO3 phase partitioning strongly buffers the pH of supermicrometer marine aerosol [Keene et al., 1998, 1999].
- Investigated the importance of halogen radical chemistry in the cycling of S, N, O3, and alkanes over the NAO.
Demonstrated that most particulate containing bromine is efficiently volatilized from sea-salt aerosol. This observation is consistent with predictions based on the autocatalytic halogen activation mechanism. Showed that HCl produced from chlorine-atom reactions in the gas phase can be efficiently recycled through acidic aerosol.
- Developed predictive tools to relate chemical and optical properties of precipitation at Bermuda and Barbados with aerosols properties of corresponding scavenged air parcels.
These tools can be used to model the influences of precipitation on the optical properties of marine air over the Western NAO.
AEROCE was uniquely conceived to investigate a broad array of atmosphere-ocean chemistry issues over a large ocean region for an extended time period. The results demonstrate the value of combining long-term measurements with intensive campaigns, highlight the important role that the atmosphere plays in linking continental and ocean chemistry processes, and lay the foundation for future programs in this region.
The program officially started in 1987 with coordinate measurements from four stations, i.e.Barbados, West Indies; Bermuda; Iza˜na, Tenerife, Canary Islands; and Mace Head, Ireland. Five more stations were added in June 1995 to give greater geographical coverage of continuous measurements of bulk aerosol chemical composition and condensation nuclei.
This article was substantially contributed to NOAA by Joseph M. Prospero, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Florida.