What are reactive gases?
The reactive gases as a group are very diverse and include surface ozone (O3), carbon monoxide (CO), volatile organic compounds (VOCs), oxidised nitrogen compounds (NOx, NOy), and sulphur dioxide (SO2). All of these compounds play a major role in the chemistry of the atmosphere and as such are heavily involved in inter-relations between atmospheric chemistry and climate, either through control of ozone and the oxidising capacity of the atmosphere, or through the formation of aerosols. The global measurement base for most of them is entirely unsatisfactory, the only exceptions being surface ozone and carbon monoxide.
Scientific Advisory Group on Reactive Gases met on 9th October 2009 at Hohenpeissenberg, Germany to discuss the activities of the group and stes of its re-establishemnt.
Data on reactive gases can be found at the WMO-GAW World Data Centre for Greenhouse Gases (WDCGG).
In the GAW programme one focuses on these reactive gases:
Surface ozone (O3)
Spectrometer AIRS measures the increased contents of carbon monoxide in the upper troposhere (above 500 hPa) above the fires in Russia in the operational regime
Forest fires started to the West from Moscow around July 24. Only an insignificant exceeding of CO content above the European part of Russia was observed by that time. Then the content grew and the area of fires increased. In figure 1 the distribution of CO concentration in the middle atmosphere (spectrometer is sensitive to the heights from 2 to 10 km with the maximum of sensitivity to approximately 5 km) for August 3, 2010, is displayed. CO abundance is growing at the moment of writing this communication and comparable in magnitude with Siberian fires of 2003. Figure 2 represents the picture of changes in the total amount of carbon monoxide above Western Russia (40ºN - 70ºN, 30ºE-90ºE) in megatons up to the present moment (red curve). 2009 is chosen as a year with normal seasonal behavior of CO content. An example of year with the fires is 2002, for which there are similar measurements of MOPITT/Terra. On August 1, 2010 the excess content almost reached the maximum values of 2002. The rate of growth (~0.7 Mt/day) characterizes the rate of emission; in this year it is approximately 3 times higher than in 2002, when peat fires predominated.
Not considering water vapour, tropospheric ozone is currently the third most important greenhouse gas after CO2 and CH4 [Houghton et al., 2001] and is central to the physics, chemistry, and radiative processes in the troposphere. Tropospheric ozone profile information is available from ozone sonde measurements. Surface (ground-level) ozone significantly influences the formation of photochemical smog, and it is an irritant with effects both on the biota and human health.
Our knowledge of trends in the global distribution of surface ozone is still incomplete and observed trends have varied both temporally and spatially [Oltmans et al., 2006]. The Global GAW stations are distributed relatively evenly, but overall, most surface ozone monitoring stations are still located in northern mid-latitudes. There is a need for more remote stations measuring ozone in the middle of continents (e.g., continental Asia), in the tropics and in the southern hemisphere.
Regular performance audits at many of the Global GAW stations show that these stations are providing measurements of the required quality. The existing Data Quality Objectives are met by almost all stations.
The World Data Center for Greenhouse Gases (WDCGG) continues to archive surface ozone and the body of available data is steadily increasing.
The tropospheric burden of carbon monoxide, like that of many other trace gases, has been increasing due to man’s activities, although its upward trend ceased around 1995. Average CO abundances for the NH and SH are approximately 110 and 60 nmole/mole (ppb). The lifetime of CO is on the order of a few months only, and its significance in atmospheric chemistry lies mainly in its competition with many other gaseous pollutants—importantly the greenhouse gas CH4—for the hydroxyl radical (OH, CO + OH → CO2 + H). Increased CO emissions cause higher CO burdens and more reaction with OH, leaving less OH for cleansing the troposphere of other reduced gases. In the background troposphere, about one third of all OH is removed by CO that reacts rapidly with OH (contributing to the latter’s very short lifetime of 1 second only).
Until recently, we were mostly informed about tropospheric CO concentrations via surface measurements. Now, results from remote sensing and an increasing number of aircraft flights give improved global coverage and some vertical resolution. Since the launch of the MOPITT satellite instrument, followed by SCIAMACHY, AIRS and others, we have a much better picture of large scale continental pollution plumes. The vertical resolution of satellite based remote sensing is limited to several km at best, and vertical profiles coordinated with satellite overpasses are needed to better define vertical variability. Before the satellite, surface and aircraft measurements are combined, their relative calibration must be accurately determined.
Measurement of VOCs is complex due to the many different molecules present in the atmosphere. The measurement of many of these species is important for air quality purposes, however, only a few molecules can be measured routinely in the background atmosphere.
The WMO/GAW Experts Workshop
on Volatile Organic Compounds (VOCs) was held in Geneva, Switzerland, from 30 January to 1 February 2006, to discuss the needs of a manageable VOC measurement programme for GAW (GAW Report 171). A core set of molecules was identified, taking into account their ease of measurement in a flask network, and their usefulness in providing information on many processes such as emissions from defined sources, long-range transport, and chemical loss processes (see table below). In addition, a basic flask network was identified making use of existing networks used to provide greenhouse gas measurements for GAW via NOAA (NOAA and INSTAAR Global VOCs network), and for regional VOC measurements in Europe via EMEP. The core species measured in this network, with a frequency of the order of one per week would be supplemented by more frequent measurements of a wider range of species at a small number of well-maintained sites in Europe and North America, and on mid Atlantic islands. The GAW data base on VOCs will include measurements made from aircraft, both research aircraft and in-service aircraft operating in the CARIBIC project. Also measurements of formaldehyde (CH2O) and glyoxal (1,2-ethanedione, HCOOCH) will be made at specific sites for the ground truthing of data produced by satellites. The GAW Workshop proposed the following molecules, as shown in the Table below:
The sum of nitric oxide (NO) and nitrogen dioxide (NO2) has traditionally been called NOx. Likewise the sum of many oxidised nitrogen species, both organic and inorganic but excluding nitrous oxide (N2O) and ammonia (NH3), acetonitrile (CH3CN) and hydrocyanic acid (HCN) have traditionally been referred to as NOy. Their measurement in the global atmosphere is very important since NO has a large influence on both ozone and on the hydroxyl radical (OH). NO2 is now being measured globally from satellites and these measurements suggest that substantial concentrations of this gas are present over most of the continents. A large reservoir of fixed nitrogen is present in the atmosphere as NOy. The influence of the deposition of this reservoir on the biosphere is not known at present but could be substantial. There are efficient in-situ measurement techniques for NO and NOy, and to a lesser extent NO2. A large amount of data on these species has been collected in the past in association with the control of regional pollution. The global data base is more limited and consists mostly of aircraft measurements collected over the world oceans.
Sulphur dioxide (SO2) is the main precursor to the sulphate aerosol which exerts a large influence on world climate. It is a requlatory pollutant controlled in many countries for human health effects by monitoring networks that may or may not be operated by NHMSs. Many measurements have been made in association with its role as a regional pollutant, particularly its role as a precursor to acid rain. Many measurements are available from integrating techniques such as filter observations used by regional networks such as EMEP, CAPMoN and NADP. However, there are very few measurements in the background atmosphere. This is a very unsatisfactory situation that has a number of causes, in particular the lack of a suitable instrument for regular measurement at the low concentrations found there. It is important to remedy this in order to create a database suitable for the proper validation of models used to predict global sulphate aerosol distribution, and its present and future influence on climate.
Volcanos and SO2
Volcanic eruptions can lower mean global temperatures. It was thought for many years that the greatest volcanic contribution of the haze effect was from the suspended ash particles in the upper atmosphere that would block out solar radiation. However, these ideas changed in the 1982 after the eruption of the Mexican volcano, El Chichon. Although the 1980 eruption of Mt. St. Helens lowered global temperatures by 0.1C, the much smaller eruption of El Chichon lowered global temperatures three to five times as much. Although the Mt. St. Helens blast emitted a greater amount of ash in the stratosphere, the El Chichon eruption emitted a much greater volume of sulfur-rich gases (40x more). It appears that the volume of pyroclastic debris emitted during a blast is not the best criteria to measure its effects on the atmosphere. The amount of sulfur-rich gases appears to be more important. Sulfur combines with water vapor in the stratosphere to form dense clouds of tiny sulfuric acid droplets. These droplets take several years to settle out and they are capable to decreasing the troposphere temperatures because they absorb solar radiation and scatter it back to space.
Molecular hydrogen (H2) is considered by many to be one of the most important fuels of the future, notably for mobile use. The benefits of a hydrogen fuel economy are reduced urban pollution (the emissions from H2 combustion consist simply of water vapour) and, if H2 can be produced from non-fossil fuel dependent sources, reduced CO2 emissions from the transport sector [Schultz et al., 2003; Tromp et al., 2003].
Large scale use of H2 fuel would inevitably lead to increased atmospheric concentrations of this gas due to leakage during production and handling. The atmospheric residence time of H2 in the troposphere has been estimated to be 1.4±0.2 years [Xiao et al., 2007]. The soil sink of H2 may also be subject to climatic or land use changes, with consequential positive or negative effects on atmospheric concentrations. While H2 has no direct effect on the atmospheric radiation budget, it does have in indirect effect through its reactivity with hydroxyl radical (OH). If this were to lead to a reduced OH abundance then the lifetime of many gases of environmental interest, for example methane, would become longer [Schultz et al., 2003; Tromp et al., 2003]. In one modelled scenario a 120% increase in global H2 burden resulted in a 10% increase in methane lifetime [Schultz et al., 2003]. Increased H2 would also increase water vapour in the stratosphere, potentially leading to stratospheric cooling, and possibly increased ozone loss through enhanced heterogenous activation of chlorine.
Present day global average concentrations of H2 are between about 500 and 550 ppb (slightly lower in the Northern Hemisphere due to the larger soil sink there). These levels are believed to be supported principally by emissions from fossil fuel and biomass burning, and from the atmospheric oxidation of methane and other hydrocarbons, balanced primarily by uptake by soils and secondarily reaction with atmospheric OH.
In the past rather little attention has been paid to measurements of atmospheric H2. Long term measurements have been made by NOAA ESRL, and by CSIRO Marine and Atmospheric Research (CMAR) in cooperation with AGAGE, yet there is disagreement even on the sign of the long term trend [Langenfelds et al., 2002; Novelli et al., 1999]. There has been no concerted effort to date to coordinate global measurement activities. A recent development has been the funding by the EU of a coordinated network “EUROHYDROS”. This network has yet to become operational, but will comprise 12 continuous measurement and 7 flask sampling sites in Europe, and 6 global flask sampling sites. It will make efforts to ensure consistency of calibration and data quality across the network. Clearly there is a need to bring the various measurement groups together to stimulate further measurement activities, and encourage consistency in measurement standards.
Houghton, J. T., et al. (Eds.) (2001), Third Assessment Report - Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), 944 pp., Cambridge University Press, UK.
Langenfelds, R. L., et al. (2002), Interannual growth rate variations of atmospheric CO2 and its delta C-13, H-2, CH4, and CO between 1992 and 1999 linked to biomass burning, Global Biogeochem Cy, 16, doi:10.1029/2001GB001466.
Novelli, P. C., et al. (1999), Molecular hydrogen in the troposphere: Global distribution and budget, J. Geophys. Res.-Atmos., 104, 30427-30444.
Oltmans, S. J., et al. (2006), Long-term changes in tropospheric ozone, Atmos Environ, 40, 3156-3173.
Schultz, M. G., et al. (2003), Air pollution and climate-forcing impacts of a global hydrogen economy, Science, 302, 624-627.
WMO (2007), A WMO/GAW Expert Workshop on Global Long-term Measurements of Volatile Organic Compounds (Geneva, Switzerland, 30 January-1 February 2006) GAW Report No. 171 (WMO TD No. 1373) World Meteorological Organization, Geneva, Switzerland
Xiao, X., et al. (2007), Optimal estimation of the soil uptake rate of molecular hydrogen from the Advanced Global Atmospheric Gases Experiment and other measurements, J. Geophys. Res., 112, doi:10.1029/2006JD007241.
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