NERC ACSOE: Experiments at Mace Head, Ireland

Experiments at Mace Head, Ireland

(The following are excerpts from the 1996 ACSOE Implementation Plan)

The University College Galway Atmospheric Research Station at Mace Head was the site for ground based components of both OXICOA and MAGE experiments. The following are excerpts from the ACSOE Implementation Plan.

OXICOA: OXIDISING CAPACITY OF THE OCEANIC ATMOSPHERE

Introduction

The OXICOA component of ACSOE studied various aspects of the oxidising capacity of the atmosphere concerned with tropospheric ozone; HOx and ROx chemistry; nitrate radical chemistry; and halogen chemistry.

EASE: Eastern Atlantic Spring/Summer Experiments

Objectives of EASE

Investigate the chemical production and loss mechanisms of atmospheric oxidants in the marine boundary layer.
Determine the role of photochemically produced radical species in the ozone cycle in the marine boundary layer.
Investigate the speciation of oxidised nitrogen and its consequences for long range transport and photochemical processing.
Characterise the air masses arriving off the West coast of Ireland and determine whether trace gas composition is indicative of the source of the air mass.
Quantify the extent of NOy chemistry in the marine atmosphere over the Atlantic Ocean.
Investigate the extent of halogenation chemistry occurring over the Atlantic Ocean.
Study the extent of NO3 chemistry, particularly its impact on DMS and hydrocarbons

The ozone record at Mace Head on the west coast of Ireland shows episodes of ozone production and loss superimposed on a seasonal cycle characterised by a spring maximum and a summer minimum. Two comprehensive experimental campaigns were conducted to study tropospheric photochemistry of ozone and HOx/ROx free radicals at this field site: EASE 96 took place during the summer (July-August 1996) when background ozone is normally at a minimum. EASE 97 was carried out when ozone levels should be at a maximum (March-June 1997) due to wide-scale ozone production in the background atmosphere from precursors emitted on the continents. Studies of the extent of NO3 chemistry, particularly its impact on DMS concentrations, was undertaken in 1996 and 1997, as well as studies of halogen chemistry. During the EASE 97 campaign there was the opportunity for combined ground, ship and air-borne measurements during a joint OXICOA/MAGE campaign which mirrored some of the activities planned for ACE later that year. In EASE 96 the Cranfield Jetstream aircraft was deployed in experiments to characterise the vertical and horizontal homogeneity of the air mass.

The dominance of the ozone production or loss processes is dependent on the composition of the air mass advected to the site. Ozone loss occurs in clean polar or sub-tropical air due to deposition and chemical sinks. In polluted air high NOx concentrations and photochemical processing result in ozone production. This is typical of air advected from Europe or of long range transport of well aged polluted air from North America especially in springtime. We took advantage of the meteorology at Mace Head to study oxidant production and loss processes at a single well equipped experimental site in a variety of air mass types. Trajectory analysis allowed us to characterise the air masses reaching the site by source region.

The complex atmospheric chemistry which determines OH concentrations was also studied. We used the chemical schemes developed in TIGER and LOIS which involved the observation of the physical and chemical parameters affecting OH radical concentration and the inclusion of these in detailed chemical modelling. A central part of the EASE 96 experiment was the first experimental deployment of the University of Leeds FAGE system for measurement of the OH and HO2 radicals. The direct measurement of these species provided an excellent opportunity for testing the chemical models used to determine OH concentrations.

Of great importance for both ozone and OH are the oxidised nitrogen species which play a pivotal role in oxidant chemistry: NOx can lead to either production or loss of ozone, is important in the cycling of HO2 to OH and acts as a sink for OH. In polluted air NOx produces ozone, but at the very low concentrations typical of the remote marine environment, ozone is destroyed. The transport of oxidised nitrogen species to the remote marine environment is often via stable, photochemically produced organic reservoir species. These are less reactive than NOx and can be transported over long distances in the free troposphere but will thermally decompose to yield NO2. During EASE 96 we studied the production of these species from photochemical air pollution and their transport in the marine atmosphere. By conducting the EASE 96 experiments in spring and summer seasons we wexperienced different levels of photochemical activity, during spring the predominant species was PAN while in summer a wider range of organic nitrates were present. Chemical modelling of polluted air masses were an essential feature of the balancing of the NOy budget as itidentified those species which could not be observed experimentally.

Nitrate radicals may be an important reactant for removing various gases particularly DMS and olefins from the atmosphere in addition to OH. Measurements of NO3 was made at Mace Head both before and during the expected maximum which occured in DMS concentration in June. Measurements of halogen oxide radicals were also attempted at Mace Head in conjunction with a survey of potential organic halogen source gases.

ACSOE Home