The Azores Islands: A unique location for ground-based measurements in the MBL and FT of the central North Atlantic

Contributed by
R.E. Honrath, Michigan Technological University, Houghton, USA, and Paulo Fialho, Universidade dos Açores.

The Azores Islandsthe only islands in the central North Atlantic that are distant from all surrounding continentshave historically been important for studies of the North Atlantic atmosphere. Prior to the advent of satellite observations, they provided weather data critical to the accuracy of forecasts of European weather. Today, they provide a unique base for observations of the impacts on atmospheric composition of emissions from the surrounding continents. As part of the NARE program, ground-based measurements of CO and O₃ [Parrish et al., 1998] and NO_Y [Peterson et al., 1998], as well as ozone sondes [Oltmans et al., 1996] were made in 1993 on the island of Terceira. The Azores have also served as a base for airborne studies (e.g., those described by Law et al., this issue) and shipboard studies [e.g., Huebert et al., 1996]. Here, we briefly discuss Azores ground-based measurements, with an emphasis on a new mountaintop site designed to probe the free troposphere.

The 1993 measurements were made at 1000 m on the island of Terceira. This was low enough that the air sampled originated in the surrounding marine boundary layer (MBL), with the exception of short periods of active vertical mixing due to frontal passages. Periods of correlated variations in CO and O₃ were observed during spring, indicating significant impacts from pollutant transport [Parrish et al., 1998]. However, this signature of long-range transport was much weaker during summer. NO_Y measurements were made during August only, and were similarly unaffected by long-range transport [Peterson et al., 1998].

Interpretation of measurements in the MBL to identify impacts of long-range transport is not straightforward for three reasons. First, the structure of the near-shore boundary layer over oceans and other large water bodies is largely determined by land-water temperature differences, and this can result in the lofting of continental emissions above the MBL and a disconnection between surface-air composition and that of the air above [Angevine et al., 1996; Honrath et al., 1997]. This process restricts North American pollutant export into the Atlantic MBL during summer. Second, pollutant lifetimes in the MBL are shorter than in the free troposphere (FT), due to the presence of the ocean-water surface, high concentrations of water vapor and therefore of OH radical, and elevated aerosol levels. This reduces the spatial scale of impacts from long-range transport in the MBL, relative to that of transport in the overlying FT. Finally, in remote midlatitude marine regions like the central North Atlantic, MBL composition and structure are largely determined by subsidence from the FT. For this reason, concentrations in the MBL of compounds like O₃, nitrogen oxides, and CO are determined by a balance between the source from

the FT (which depends on concentrations in the FT) and the sink in the MBL. Events of changing concentrations in the overlying FT, which may be caused by long-range transport, are therefore reflected by changing concentrations in the MBL, but the magnitude and timing of these changes is different for each compound because of differences in their lifetimes in the MBL. The result is that interspecies correlations in the MBL must be interpreted with caution [Peterson et al., 1998]. Nevertheless, ground-based measurements can be an extremely valuable complement to aircraft studies. In particular, the opportunity for continuous observations for an extended period of time allows determination of the frequency and duration of pollutant transport events and calculation of the distribution of concentrations in background air.

The Pico International Atmospheric Chemistry Observatory (PICO-NARE) was developed with these issues in mind to provide information on the frequency and duration of events carrying O₃ and O₃ precursors to the lower FT of the Azores region, and to determine regional background levels of these compounds for comparison with global model predictions. PICO-NARE is the result of a collaboration between Michigan Tech and the University of the Azores, and was set up in July 2001 on the summit of Pico mountain on the Azores island of Pico. At an altitude of 2225m, this is the only location in the central North Atlantic high enough for ground-based observations frequently in the FT. (Water vapor soundings deployed from the adjacent island of Terceira indicate that the MBL height during August typically ranged from 1to 2km [Oltmans et al., 1996], and the summit of Pico is often observed above the MBL-capping cloud layer.)

Measurements on Pico mountain are logistically difficult, as the nearest road ends 1000 m below the summit, and the mountain is a restricted area for safety and environmental reasons. Equipment is limited to a 2 by 2.5 by 2 m instrument enclosure and instrument inlets. The system is powered by a small diesel generator located 1000 m lower in elevation, via a 2.5 km power cable. All instruments are fully automated and are controlled and accessed via a GSM (cellular) internet connection. The regional government of the Azores has granted permission for 2 years of operation.

Measurements during the first year will focus on the relatively simple and reliable observations of O₃ and the combustion tracers CO and aerosol black carbon, plus standard meteorological parameters and automated sampling of whole air for determination of non-methane hydrocarbons. However, the site was designed with capacity for a limited number of additional measurements, and it is our hope that the potential value of PICO-NARE as a platform for observations of the impacts of continental emissions on the lower FT and MBL of the central North Atlantic region will be fully realized. Scientists wishing to conduct collaborative research during Year 2 (June 2002–2003) and those desiring additional information on PICO-NARE may contact the project's web site for additional information.

References

Angevine, W. M., M. Trainer, S. A. McKeen, and C. M. Berkowitz, Mesoscale meteorology of the New England coast, Gulf of Maine, and Nova Scotia: Overview, J. Geophys. Res., 101, 28893–28901, 1996.
Honrath, R. E., C. I. Sweet, and C. J. Plouff, Surface exchange and transport processes governing atmospheric PCB levels over Lake Superior, Environ. Sci. Tech., 31, 842-852, 1997.
Huebert, B. J., A. Pszenny, and B. Blomquist, The ASTEX/MAGE Experiment, J. Geophys. Res., 101, 4319–4329, 1996.
Oltmans, S. J., H. Levy II, J. M. Harris, J. T. Merrill, J. L. Moody, J. A. Lathrop, E. Cuevas, M. Trainer, M. S. O'Neill, J. M. Prospero, H. Vomel, and B. J. Johnson, Summer and spring ozone profiles over the North Atlantic from ozonesonde measurements, J. Geophys. Res., 101, 29,179-29,200, 1996.
Parrish, D. D., M. Trainer, J. S. Holloway, J. E. Yee, M. S. Warshawsky, F. C. Fehsenfeld, G. Forbes, and J. L. Moody, Relationships between ozone and carbon monoxide at surface sites in the North Atlantic region, J. Geophys. Res., 103, 13357-13376, 1998.
Peterson, M. C., R. E. Honrath, D. D. Parrish, and S. J. Oltmans, Measurements of nitrogen oxides and a simple model of NO_y fate in the remote North Atlantic marine atmosphere, J. Geophys. Res., 103, 13,489-13,503, 1998.


IGACtivities No. 24 August 2001	The Azores Islands: A unique location for ground-based measurements in the MBL and FT of the central North Atlantic Contributed by R.E. Honrath, Michigan Technological University, Houghton, USA, and Paulo Fialho, Universidade dos Açores. The Azores Islandsthe only islands in the central North Atlantic that are distant from all surrounding continentshave historically been important for studies of the North Atlantic atmosphere. Prior to the advent of satellite observations, they provided weather data critical to the accuracy of forecasts of European weather. Today, they provide a unique base for observations of the impacts on atmospheric composition of emissions from the surrounding continents. As part of the NARE program, ground-based measurements of CO and O₃ [Parrish et al., 1998] and NO_Y [Peterson et al., 1998], as well as ozone sondes [Oltmans et al., 1996] were made in 1993 on the island of Terceira. The Azores have also served as a base for airborne studies (e.g., those described by Law et al., this issue) and shipboard studies [e.g., Huebert et al., 1996]. Here, we briefly discuss Azores ground-based measurements, with an emphasis on a new mountaintop site designed to probe the free troposphere. The 1993 measurements were made at 1000 m on the island of Terceira. This was low enough that the air sampled originated in the surrounding marine boundary layer (MBL), with the exception of short periods of active vertical mixing due to frontal passages. Periods of correlated variations in CO and O₃ were observed during spring, indicating significant impacts from pollutant transport [Parrish et al., 1998]. However, this signature of long-range transport was much weaker during summer. NO_Y measurements were made during August only, and were similarly unaffected by long-range transport [Peterson et al., 1998]. Interpretation of measurements in the MBL to identify impacts of long-range transport is not straightforward for three reasons. First, the structure of the near-shore boundary layer over oceans and other large water bodies is largely determined by land-water temperature differences, and this can result in the lofting of continental emissions above the MBL and a disconnection between surface-air composition and that of the air above [Angevine et al., 1996; Honrath et al., 1997]. This process restricts North American pollutant export into the Atlantic MBL during summer. Second, pollutant lifetimes in the MBL are shorter than in the free troposphere (FT), due to the presence of the ocean-water surface, high concentrations of water vapor and therefore of OH radical, and elevated aerosol levels. This reduces the spatial scale of impacts from long-range transport in the MBL, relative to that of transport in the overlying FT. Finally, in remote midlatitude marine regions like the central North Atlantic, MBL composition and structure are largely determined by subsidence from the FT. For this reason, concentrations in the MBL of compounds like O₃, nitrogen oxides, and CO are determined by a balance between the source from the FT (which depends on concentrations in the FT) and the sink in the MBL. Events of changing concentrations in the overlying FT, which may be caused by long-range transport, are therefore reflected by changing concentrations in the MBL, but the magnitude and timing of these changes is different for each compound because of differences in their lifetimes in the MBL. The result is that interspecies correlations in the MBL must be interpreted with caution [Peterson et al., 1998]. Nevertheless, ground-based measurements can be an extremely valuable complement to aircraft studies. In particular, the opportunity for continuous observations for an extended period of time allows determination of the frequency and duration of pollutant transport events and calculation of the distribution of concentrations in background air. The Pico International Atmospheric Chemistry Observatory (PICO-NARE) was developed with these issues in mind to provide information on the frequency and duration of events carrying O₃ and O₃ precursors to the lower FT of the Azores region, and to determine regional background levels of these compounds for comparison with global model predictions. PICO-NARE is the result of a collaboration between Michigan Tech and the University of the Azores, and was set up in July 2001 on the summit of Pico mountain on the Azores island of Pico. At an altitude of 2225m, this is the only location in the central North Atlantic high enough for ground-based observations frequently in the FT. (Water vapor soundings deployed from the adjacent island of Terceira indicate that the MBL height during August typically ranged from 1to 2km [Oltmans et al., 1996], and the summit of Pico is often observed above the MBL-capping cloud layer.) Measurements on Pico mountain are logistically difficult, as the nearest road ends 1000 m below the summit, and the mountain is a restricted area for safety and environmental reasons. Equipment is limited to a 2 by 2.5 by 2 m instrument enclosure and instrument inlets. The system is powered by a small diesel generator located 1000 m lower in elevation, via a 2.5 km power cable. All instruments are fully automated and are controlled and accessed via a GSM (cellular) internet connection. The regional government of the Azores has granted permission for 2 years of operation. Measurements during the first year will focus on the relatively simple and reliable observations of O₃ and the combustion tracers CO and aerosol black carbon, plus standard meteorological parameters and automated sampling of whole air for determination of non-methane hydrocarbons. However, the site was designed with capacity for a limited number of additional measurements, and it is our hope that the potential value of PICO-NARE as a platform for observations of the impacts of continental emissions on the lower FT and MBL of the central North Atlantic region will be fully realized. Scientists wishing to conduct collaborative research during Year 2 (June 2002–2003) and those desiring additional information on PICO-NARE may contact the project's web site for additional information. References Angevine, W. M., M. Trainer, S. A. McKeen, and C. M. Berkowitz, Mesoscale meteorology of the New England coast, Gulf of Maine, and Nova Scotia: Overview, J. Geophys. Res., 101, 28893–28901, 1996. Honrath, R. E., C. I. Sweet, and C. J. Plouff, Surface exchange and transport processes governing atmospheric PCB levels over Lake Superior, Environ. Sci. Tech., 31, 842-852, 1997. Huebert, B. J., A. Pszenny, and B. Blomquist, The ASTEX/MAGE Experiment, J. Geophys. Res., 101, 4319–4329, 1996. Oltmans, S. J., H. Levy II, J. M. Harris, J. T. Merrill, J. L. Moody, J. A. Lathrop, E. Cuevas, M. Trainer, M. S. O'Neill, J. M. Prospero, H. Vomel, and B. J. Johnson, Summer and spring ozone profiles over the North Atlantic from ozonesonde measurements, J. Geophys. Res., 101, 29,179-29,200, 1996. Parrish, D. D., M. Trainer, J. S. Holloway, J. E. Yee, M. S. Warshawsky, F. C. Fehsenfeld, G. Forbes, and J. L. Moody, Relationships between ozone and carbon monoxide at surface sites in the North Atlantic region, J. Geophys. Res., 103, 13357-13376, 1998. Peterson, M. C., R. E. Honrath, D. D. Parrish, and S. J. Oltmans, Measurements of nitrogen oxides and a simple model of NO_y fate in the remote North Atlantic marine atmosphere, J. Geophys. Res., 103, 13,489-13,503, 1998.
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