7.4 Reactive Gases and the Climate System
The atmospheric concentration of many reactive gases has increased substantially during the industrial era as a result of human activities. Some of these compounds (CH4, N2O, halocarbons, ozone, etc.) interact with longwave (infrared) solar radiation and, as a result, contribute to ‘greenhouse warming’. Ozone also efficiently absorbs shortwave (ultraviolet and visible) solar energy, so that it protects the biosphere (including humans) from harmful radiation and plays a key role in the energy budget of the middle atmosphere. Many atmospheric chemical species are emitted at the surface as a result of biological processes (soils, vegetation, oceans) or anthropogenic activities (fossil fuel consumption, land use changes) before being photochemically destroyed in the atmosphere and converted to compounds that are eventually removed by wet and dry deposition. The oxidizing power (or capacity) of the atmosphere is determined primarily by the atmospheric concentration of the OH radical (daytime) and to a lesser extent the concentrations of the nitrate radical (NO3; nighttime), ozone and hydrogen peroxide (H2O2). The coupling between chemical processes in the atmosphere and the climate system (Figure 7.15) are complex because they involve a large number of physical, chemical and biological processes that are not always very well quantified. An important issue is to determine to what extent predicted climate change could affect air quality (see Box 7.4). The goal of this section is assess recent progress made in the understanding of the two-way interactions between reactive gases and the climate system.
Box 7.4: Effects of Climate Change on Air Quality
Weather is a key variable affecting air quality. Surface air concentrations of pollutants are highly sensitive to boundary layer ventilation, winds, temperature, humidity and precipitation. Anomalously hot and stagnant conditions in the summer of 1988 were responsible for the highest ozone year on record in the north-eastern USA (Lin et al., 2001). The summer heat wave in Europe in 2003 was associated with exceptionally high ozone (Ordonez et al., 2005). Such high interannual variability of surface ozone correlated with temperature demonstrates the potential air quality implications of climate change over the next century.
A few GCM studies have investigated how air pollution meteorology might respond to future climate change. Rind et al. (2001) found that increased continental ventilation as a result of more vigorous convection should decrease surface concentrations, while Holzer and Boer (2001) found that weaker winds should result in slower dilution of pollution plumes and hence higher concentrations. A focused study by Mickley et al. (2004) for the eastern USA found an increase in the severity and persistence of regional pollution episodes due to the reduced frequency of ventilation by cyclones tracking across Canada. This effect more than offsets the dilution associated with the small rise in mixing depths. A decrease in cyclone frequency at northern mid-latitudes and a shift to higher latitudes has been noted in observations from the past few decades (McCabe et al., 2001). An urban air quality model study by Jacobson (1999) pointed out that decreasing soil moisture or increasing surface temperature would decrease mixing depths and reduce near-surface pollutant concentrations.
A number of studies in the USA have shown that summer daytime ozone concentrations correlate strongly with temperature (NRC, 1991). This correlation appears to reflect contributions of comparable magnitude from (1) temperature-dependent biogenic VOC emissions, (2) thermal decomposition of peroxyacetylnitrate, which acts as a reservoir for NOx and (3) association of high temperatures with regional stagnation (Jacob et al., 1993; Sillman and Samson, 1995; Hauglustaine et al., 2005). Empirical relationships between ozone air quality standard exceedances and temperature, as shown in Figure 1, integrate all of these effects and could be used to estimate how future regional changes in temperature would affect ozone air quality. Changes in the global ozone background would also have to be accounted for (Stevenson et al., 2005).
A few GCM studies have examined more specifically the effect of changing climate on regional ozone air quality, assuming constant emissions. Knowlton et al. (2004) use a GCM coupled to a Regional Climate Model (RCM) to investigate the impact of 2050 climate change (compared with 1990) on ozone concentrations in the New York City metropolitan area. They found a significant ozone increase that they translated into a 4.5% increase in ozone-related acute mortality. Langner et al. (2005) use an RCM driven by two different GCMs to examine changes in the Accumulated Ozone concentration above a Threshold of 40 ppb (AOT40) statistic (ozone-hours above 40 ppb) over Europe in 2050 to 2070 relative to the present. They found an increase in southern and central Europe and a decrease in northern Europe that they attributed to different regional trends in cloudiness and precipitation. Dentener et al. (2006) synthesise the results of 10 global model simulations for 2030 driven by future compared with present climate. They find that climate change caused mean decreases in surface ozone of 0.5 to 1 ppb over continents and 1 to 2 ppb over the oceans, although some continental regions such as the Eastern USA experienced slight increases.
There has been less work on the sensitivity of aerosols to meteorological conditions. Regional model simulations by Aw and Kleeman (2003) find that increasing temperatures should increase surface aerosol concentrations due to increased production of aerosol precursors (in particular semi-volatile organic compounds and HNO3) although this is partly compensated by the increasing vapour pressure of these compounds at higher temperatures. Perturbations of precipitation frequencies and patterns might be expected to have a major impact on aerosol concentrations, but the GCM study by Mickley et al. (2004) for 2000 to 2050 climate change finds little effect in the USA.