ClearfLo - London(2012)

Urban pollution such as that currently endured in London has an adverse effect on human health. However, an understanding of the chemical processes responsible for such poor air quality is incomplete. The Clean Air for London (ClearfLo) campaign in Jan/Feb 2012 and July/August 2012 aims to deliver a comprehensive range of atmospheric measurements across London at both ground based and elevated sites. These observations complemented by modelling will be used in an effort to improve predictive capability for air quality in London. The main aims and objectives of ClearfLo with respect to radical species are to measure such species concurrent with their sources and sinks and to quantify the oxidative budget of London’s urban atmosphere both in winter and in summer.

The University of Leeds is responsible for measurements of gas phase radicals (i.e. OH, HO₂ and RO₂). The comprehensive suite of field measurements planned during the ClearfLo campaigns will be followed by a detailed synthesis of the winter and summer datasets to enable the oxidative budget of the winter and summer atmospheres to be determined. These field observations will be used to constrain a detailed zero-dimensional model based on the Master Chemical Mechanism and the model will be used to calculate the oxidative budget of London’s urban atmosphere.

Further information can be found at http://www.clearflo.ac.uk/

Hill Cap Cloud Thuringia (HCCT) - Germany (2010)

The ground based FAGE instrument took part in the HCCT-2010 campaign, which ran from the 13^th September until 24^th October 2010. The project is funded by the Leibniz-Institut fur Tropospharenforschung (IFT) and involves collaboration with a number of international partners.

Clouds play a crucial role in the chemistry of the atmosphere, occupying, on average ~ 15 % of the volume of the lower atmosphere. Modelling studies have shown that aqueous phase chemistry in clouds can influence gas phase radical chemistry and in turn can cause significant reductions in the oxidative capacity. The HCCT project aims to quantify the uptake of OH and HO₂ radicals in cloud droplets and to study the dependency of this uptake efficiency on the different microphysical and chemical conditions encountered during different cloud events. The instrument was located at Mt. Schmucke, Thuringia in Germany; a site where hill cap clouds frequently occur. The photograph shows the FAGE instrument measuring during a cloud event; the detection cell is co-located at the top of the 22 m tower alongside cloud water samplers and other instrumentation.

Analysis of the measurements made during the campaign are underway.

Seasonal Oxidant Study (SOS) - Cape Verde (2009)

In 2009 the AirFAGE instrument was used in its ground configuration to make measurements of OH and HO₂ at the Cape Verde Observatory.

Seasonal influences on OH and HO₂ chemistry were investigated during three intensive measurement campaigns made in three distinct seasonal periods; SOS1 during February-March (generally dry with clear sky), in June during SOS2 (clear and dry), and SOS3 in September (some sustained periods of heavy rainfall). Preliminary data analysis suggests mean midday concentrations of OH were 3.6, 3.5 and 4.6 × 10⁶ cm^-3 for SOS1, 2 and 3; the corresponding mean noontime concentrations of HO₂ were 1.8, 2.1 and 1.8 × 10⁸ cm^-3. Compared to measurements made in May-June 2007, during the RHAMBLE project the peak concentration of HO₂ for SOS2 seem similar, whereas the concentration of OH in SOS2 appears about 30 % lower.

OP3 - Borneo (2008)

The Oxidant and Particle Photochemical Processes project over a South-East Asian tropical rain forest took place in Borneo in 2008, conisting of two intensive ground based campaigns and an aircraft campaign to coincide with the second ground-based measurement period.

The FAGE group took part in all three measurement campaigns. Despite low ozone concentrations, limiting primary production of OH from ozone photolysis, ground based observations found significant OH levels - up to 8.7 x 10⁶ molecule cm^-3. In addition, the OH lifetime instrument measured high OH reactivity, corresponding to a lifetime for OH approaching 10 ms, indicating that significant additional OH sources must be present.

OH concentrations up to 6 x 10⁵ molecule cm^-3 were also recorded on several nights after j(O¹D) had dropped to zero, suggesting a non-photolytic source of OH.

COBRA - Canada (2008)

Direct measurements of OH and HO₂ radical concentrations were made, using the University of Leeds FAGE instrument, as part of the COBRA (Combined Impact of Iodine and Bromine Release on the Arctic Atmosphere) campaign, on the coast of Hudson Bay during spring 2008.

Mean noontime OH and HO₂ concentrations of (0.765 ± 1.05) x 10⁶ molecule cm^-3 and (1.34 ± 0.62) x 10⁸ molecule cm^-3 respectively, were observed. These observations were used in conjunction with a box model, constrained to supporting observations, to asses the radical budget in this cold, northerly environment. HCHO photolysis was found to be the dominant daytime radical source, providing 74 % of the observed HO_x, with observed HCHO concentrations requiring the presence of a considerable (>80%) surface source. Model simulations also suggest significant roles for halogen chemistry and the heterogeneous loss of HO₂. The formation of HO₂NO₂ has been identified as playing an important role as a radical reservoir, reducing HO_x concentrations during the day, and thus impacting both local oxidising capacity and ozone production. The sensitivity of the local chemistry to uncertainties in these processes is explored, the majority of which are not represented in global chemistry models.

RHaMBLe - Cape Verde Campaign (2007)

The Reactive Halogen Marine Boundary Layer project aimed to quantify the impact of reactive halogen species at both coastal and open ocean locations. The second leg of the project, focussing on open ocean impacts, took place at the Cape Verde Observatory in 2007.

Located ~ 500 km off the West coast of Africa, the site predominantly samples background, tropical marine boundary layer air.

During the project the ground-based FAGE instrument made measurements of OH and HO₂. Owing to high humidity and low solar zenith angle experienced at the site midday OH concentrations reached around 1 x 10⁷ molecule cm^-3 and HO₂ levels were as high as 7 x 10⁸ molecule cm^-3 on some days.

Although the halogen oxide concentration in this region is low, steady state analysis of the HO_x radicals highlights that reaction of HO₂ with IO and BrO radicals acts as a major HO₂ sink in this region, whilst photolysis of the reaction products (HOI and HOBr) is an important OH source.

This work demonstrates the importance of halogen chemistry in the remote MBL and that to accurately predict global OH levels this chemistry should be included in model simulations.

References

Whalley, L.K.; Furneaux, K.L.; Goddard, A.; Lee, J.D.; Mahajan, A.; Oetjen, H.; Read, K.A.; Kaaden, N.; Carpenter, L.J.; Lewis, A.C.; Plane, J.M.C.; Saltzman, E.S.; Wiedensohler, A.; Heard, D.E. The chemistry of OH and HO2 radicals in the boundary layer over the tropical Atlantic Ocean, Atmospheric Chemistry and Physics, 2010, 10, 1555-1576

Read, K.A.; Mahajan, A.S.; Carpenter, L.J.; Evans, M.J.; Faria, B.V.E.; Heard, D.E.; Hopkins, J.R.; Lee, J.D.; Moller, S.J.; Lewis, A.C.; Mendes, L.; McQuaid, J.B.; Oetjen, H.; Saiz-Lopez, A.; Pilling, M.J.; Plane, J.M.C. Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean, Nature, 2008, 453, 7159, 2332-1235

CHABLIS Campaign (2005)

The CHABLIS project took place at the Halley Research Station, Antarctica from Jan 2004 - Feb 2005. The ground-based FAGE instrument was deployed at the site during the austral summer of 2005 and made round-the-clock measurements of OH and HO₂ for 6 weeks. Peak OH and HO₂ concentrations were ~ 8 x 10⁵ molecule cm^-3 and 3.8 x 10⁷ molecule cm ^-3 respectively.

Photolysis of O₃ and formaldehyde were found to be the dominant source of the radicals, whilst, NO_x, originating from snowpack photochemistry, and the halogen oxides present, were the dominant species leading to radical inter-conversion and enhancement of OH.

References

Bloss, W.J.; Lee, J.D.; Heard, D.E.; Salmon, R.A.; Bauguitte, S.J.-B.; Roscoe, H.K.; Jones, A.E. , Observations of OH and HO₂ radicals in coastal Antarctica, Atmospheric Chemistry and Physics,, pp.4171-4185, 2007, 7

NAMBLEX Campaign (2002)

The North Atlantic Marine Boundary Layer Experiment took place at Mace Head, Ireland in the summer of 2002. One of the main aims of the NAMBLEX campaign involved the comparison of concentration measurements of a range of free radical species (OH, HO₂, RO₂ and NO₃) with calculations of a zero-dimensional box model based on the MCM, allowing a quantitative test of the basic understanding of oxidation processes in clean and moderately polluted MBL air.

Through this work the importance of halogen chemistry and aerosols upon HO_x levels was demonstrated.

References

PUMA Campaign (1999/2000)

The Pollution in the Urban West Midlands project was sponsered by NERC under the Urban Regeneration in the Environment thermatic program. The aim of the project was to comprehensively monitor urban air quality over two four week field campaigns in the summer of 1999 and winter of 2000 at a central site within the West Midlands. Measurements of OH and HO₂ in urban areas are scarce, and this campaign provided the first example of such measurements in the UK.

References

Emmerson K.M.; Carslaw N.; Carpenter L.J.; Heard, D.E.; Lee J.D.L.; Pilling, M.J. , Urban Atmospheric Chemistry During the PUMA Campaign 1: Comparison of modelled OH and HO2 concentrations with measurements, Journal of Atmospheric Chemistry, , pp.142-164 , 2005 , Kluwer Academic Publishers , 52

Heard D.E.; Carpenter L.J.; Creasey D.J.; Hopkins J.R.; Lee J.D.L.; Lewis A.C.; Pilling M.J.; Seakins P.W.; Carslaw N.; Emmerson, K.M. , High levels of the hydroxyl radical in the winter urban troposphere, Geophysical Research Letters, , pp.L18112 , 2004 , American Geophysical Union , 31