Chemical meteorology – linking weather and atmospheric composition from urban to global scales
1
248 Meeting report Chemical meteorology – linking weather and atmospheric composition from urban to global scales Weather – September 2012, Vol. 67, No. 9 The atmosphere has been described by chemists as ‘the worst possible reaction chamber’ and it is easy to understand why. It is never the same for long, and the vari- ation in meteorological processes not only influences the chemistry of atmospheric trace gases, both natural and man-made, but also their emissions, transport and fate. This atmospheric reaction chamber is mixed, often inefficiently, with sources and sinks appearing and disappearing periodi- cally. And the net result affects us all, from the known health impacts of poor urban air quality to the build-up of greenhouse gases and climate change. Making sense of this maelstrom, for the purposes of understanding or predicting atmospheric composition from the scale of city streets to the planet as a whole, requires knowledge of how all these factors inter- relate. This topic of chemical meteorology was the subject of a National Meeting hosted by the RMetS in conjunction with the National Centre for Atmospheric Science and co-sponsored by the Royal Society of Chemistry, held at Imperial College, London, on 18 April 2012. The meeting was con- vened to encourage discussion between meteorologists and atmospheric composi- tion specialists at the interface of their dis- ciplines and to highlight ongoing scientific challenges. The convener, Oliver Wild (Lancaster University), introduced the concept of chemical meteorology and its importance in a changing world, underlining the key challenges which bridge spatial and tempo- ral scales. He explained how the meeting could contextualise these linkages by bring- ing together speakers on topics ranging from the urban environment to global-scale transport. At small scales within the city environ- ment, the dynamics of street canyons and the effect of the urban heat island act to locally mix pollutants from a wide range of emission sources such as cars, chimneys, trees, plants, and even cooking. Stephen Belcher (University of Reading) described how important these processes are in accu- rately predicting urban air quality in order to enable us to mitigate the health impacts that extreme pollution events can trigger. Studies show that life expectancy and quality of life may be significantly affected by such exposure, perhaps reducing life by several months. The Clear air for London (ClearfLo) project, a major field campaign currently underway, brings together scien- tists from several universities and agencies with the aim of building a long-term meas- urement dataset from which to analyse and interpret urban processes. Initial results pre- sented by Professor Belcher for January/ February 2012 suggest that the London boundary layer responds to forcings (in terms of turbulence characteristics) in a similar way to rural environments. This is an important finding that gives confidence in effectively ‘parameterising’ the urban boundary layer in chemical-dynamical models, greatly simplifying the modelling framework. Natural emission and removal of key pol- lutants such as ammonia, sulphur dioxide and ozone are processes that are highly sensitive to changes in local temperature and the presence of clouds and rainfall, and these were described by Mhairi Coyle, (Centre for Ecology & Hydrology, Edinburgh.) Important exchange processes in plant leaf stomata are also controlled by chemical meteorology. Stomata reduce their uptake of tropospheric ozone when deprived of water, instead emitting greater amounts of volatile organic compounds such as iso- prene, a gas that leads to increased forma- tion of ozone. This biospheric response is thought to have been a significant contribu- tory factor to the dangerous levels of ozone and other pollutants observed during the infamous August 2003 heatwave. The role of the tropical oceans in cleans- ing the atmosphere was described by Lucy Carpenter (University of York), who pre- sented findings from five years of data col- lected at Cape Verde, a cross-roads between pristine Atlantic air (dominant in summer), outflow of Saharan dust (more common in winter), and air from as far away as North America and Europe. The observations hint at hitherto unknown processes such as the removal of ozone by reactions occurring on the surface of Saharan aerosol, reinforcing the idea that the tropical Atlantic marine biosphere is not deficient in iron, unlike many other marine environments. This is due to the large deposition of iron in Saharan dust over the area – a process con- trolled by the regional meteorology and wind patterns, which may be expected to change as the climate warms. Local influences on atmospheric composi- tion due to urban dynamics and emissions are often superimposed on a background from distant sources. Nick Savage (UK Met Office) exemplified this when he discussed air-quality forecasts from the Met Office Air Quality Model and compared them with observations. The model performs very well over the UK most of the time, but occasional influxes of highly polluted air from North America and the Continent prove more challenging to reproduce. The importance of long-range, intercon- tinental transport of pollutants was dis- cussed by Ruth Doherty (University of Edinburgh) who showed how these path- ways and relationships evolve, both sea- sonally and climatologically, noting that the position and intensity of sources (such as ‘megacities’) with respect to these path- ways ultimately determine the magnitude of intercontinental transport. Dr Doherty described how regions such as the west coasts of North America and China act as confluences of pollution in warm con- veyor-belt frontal uplift, with subsequent global redistribution by the jet streams. Future changes in climate might be expected to alter these transport path- ways, although the effect of climate-driven changes in the formation and destruction of oxidants is thought likely to have a larger impact on their future atmos- pheric concentrations than changes in transport. These talks were complemented by a number of interesting posters from students and young scientists on a range of topics relating meteorology and air quality, cover- ing both observations and modelling approaches. Simultaneous monitoring of atmospheric composition and meteorological variables across a wide range of scales is urgently needed to test, validate and improve com- puter models which are relied upon to deliver targeted policy advice on both air quality and climate change. Providing this remains a significant challenge. Correspondence to: Grant Allen University of Manchester [email protected] © Royal Meteorological Society, 2012 DOI: 10.1002/wea.1961
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Transcript of Chemical meteorology – linking weather and atmospheric composition from urban to global scales
248
Meeting report Chemical meteorology – linking weather and atmospheric composition from urban to global scales
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The atmosphere has been d escribed by chemists as ‘the worst possible reaction chamber’ and it is easy to understand why. It is never the same for long, and the vari-ation in meteorological processes not only influences the chemistry of atmospheric trace gases, both natural and man-made, but also their emissions, transport and fate. This atmospheric reaction chamber is mixed, often inefficiently, with sources and sinks appearing and disappearing periodi-cally. And the net result affects us all, from the known health impacts of poor urban air quality to the build-up of greenhouse gases and climate change.
Making sense of this maelstrom, for the purposes of understanding or predicting atmospheric composition from the scale of city streets to the planet as a whole, requires knowledge of how all these factors inter-relate. This topic of chemical meteorology was the subject of a National Meeting hosted by the RMetS in conjunction with the National Centre for Atmospheric Science and co-sponsored by the Royal Society of Chemistry, held at Imperial College, London, on 18 April 2012. The meeting was con-vened to encourage discussion between meteorologists and atmospheric composi-tion specialists at the interface of their dis-ciplines and to highlight ongoing scientific challenges.
The convener, Oliver Wild (Lancaster University), introduced the concept of chemical meteorology and its importance in a changing world, underlining the key challenges which bridge spatial and tempo-ral scales. He explained how the meeting could contextualise these linkages by bring-ing together speakers on topics ranging from the urban environment to global-scale transport.
At small scales within the city environ-ment, the dynamics of street canyons and the effect of the urban heat island act to locally mix pollutants from a wide range of emission sources such as cars, chimneys, trees, plants, and even cooking. Stephen Belcher (University of Reading) described how important these processes are in accu-rately predicting urban air quality in order to enable us to mitigate the health impacts that extreme pollution events can trigger. Studies show that life expectancy and quality of life may be significantly affected by such exposure, perhaps reducing life by
several months. The Clear air for London (ClearfLo) project, a major field campaign currently underway, brings together scien-tists from several universities and agencies with the aim of building a long-term meas-urement dataset from which to analyse and interpret urban processes. Initial results pre-sented by Professor Belcher for January/February 2012 suggest that the London boundary layer responds to forcings (in terms of turbulence characteristics) in a similar way to rural environments. This is an important finding that gives confidence in effectively ‘parameterising’ the urban boundary layer in chemical-dynamical models, greatly simplifying the modelling framework.
Natural emission and removal of key pol-lutants such as ammonia, sulphur dioxide and ozone are processes that are highly sensitive to changes in local temperature and the presence of clouds and rainfall, and these were described by Mhairi Coyle, (Centre for Ecology & Hydrology, Edinburgh.) Important exchange processes in plant leaf stomata are also controlled by chemical meteorology. Stomata reduce their uptake of tropospheric ozone when deprived of water, instead emitting greater amounts of volatile organic compounds such as iso-prene, a gas that leads to increased forma-tion of ozone. This biospheric response is thought to have been a significant contribu-tory factor to the dangerous levels of ozone and other pollutants observed during the infamous August 2003 heatwave.
The role of the tropical oceans in cleans-ing the atmosphere was described by Lucy Carpenter (University of York), who pre-sented findings from five years of data col-lected at Cape Verde, a cross-roads between pristine Atlantic air (dominant in summer), outflow of Saharan dust (more common in winter), and air from as far away as North America and Europe. The observations hint at hitherto unknown processes such as the removal of ozone by reactions occurring on the surface of Saharan aerosol, reinforcing the idea that the tropical Atlantic marine biosphere is not deficient in iron, unlike many other marine environments. This is due to the large deposition of iron in Saharan dust over the area – a process con-trolled by the regional meteorology and wind patterns, which may be expected to change as the climate warms.
Local influences on atmospheric composi-tion due to urban dynamics and emissions are often superimposed on a background from distant sources. Nick Savage (UK Met Office) exemplified this when he discussed air-quality forecasts from the Met Office Air Quality Model and compared them with observations. The model performs very well over the UK most of the time, but occasional influxes of highly polluted air from North America and the Continent prove more challenging to reproduce.
The importance of long-range, intercon-tinental transport of pollutants was dis-cussed by Ruth Doherty (University of Edinburgh) who showed how these path-ways and relationships evolve, both sea-sonally and climatologically, noting that the position and intensity of sources (such as ‘megacities’) with respect to these path-ways ultimately determine the magnitude of intercontinental transport. Dr Doherty described how regions such as the west coasts of North America and China act as confluences of pollution in warm con-veyor-belt frontal uplift, with subsequent global redistribution by the jet streams. Future changes in climate might be expected to alter these transport path-ways, although the effect of climate-driven changes in the formation and destruction of oxidants is thought likely to have a larger impact on their future atmos-pheric concentrations than changes in transport.
These talks were complemented by a number of interesting posters from students and young scientists on a range of topics relating meteorology and air quality, cover-ing both observations and modelling approaches.
Simultaneous monitoring of atmospheric composition and meteorological variables across a wide range of scales is urgently needed to test, validate and improve com-puter models which are relied upon to deliver targeted policy advice on both air quality and climate change. Providing this remains a significant challenge.
Correspondence to: Grant Allen
University of Manchester
© Royal Meteorological Society, 2012
DOI: 10.1002/wea.1961
