Abstract
Abstract. A range of solar radiation management (SRM) techniques has been proposed to counter anthropogenic climate change. Here, we examine the potential effects of stratospheric sulfate aerosols and solar insolation reduction on tropospheric ozone and ozone at Earth's surface. Ozone is a key air pollutant, which can produce respiratory diseases and crop damage. Using a version of the Community Earth System Model from the National Center for Atmospheric Research that includes comprehensive tropospheric and stratospheric chemistry, we model both stratospheric sulfur injection and solar irradiance reduction schemes, with the aim of achieving equal levels of surface cooling relative to the Representative Concentration Pathway 6.0 scenario. This allows us to compare the impacts of sulfate aerosols and solar dimming on atmospheric ozone concentrations. Despite nearly identical global mean surface temperatures for the two SRM approaches, solar insolation reduction increases global average surface ozone concentrations, while sulfate injection decreases it. A fundamental difference between the two geoengineering schemes is the importance of heterogeneous reactions in the photochemical ozone balance with larger stratospheric sulfate abundance, resulting in increased ozone depletion in mid- and high latitudes. This reduces the net transport of stratospheric ozone into the troposphere and thus is a key driver of the overall decrease in surface ozone. At the same time, the change in stratospheric ozone alters the tropospheric photochemical environment due to enhanced ultraviolet radiation. A shared factor among both SRM scenarios is decreased chemical ozone loss due to reduced tropospheric humidity. Under insolation reduction, this is the dominant factor giving rise to the global surface ozone increase. Regionally, both surface ozone increases and decreases are found for both scenarios; that is, SRM would affect regions of the world differently in terms of air pollution. In conclusion, surface ozone and tropospheric chemistry would likely be affected by SRM, but the overall effect is strongly dependent on the SRM scheme. Due to the health and economic impacts of surface ozone, all these impacts should be taken into account in evaluations of possible consequences of SRM.
Highlights
1.1 Atmospheric ozoneIt is well known that sulfate aerosols in the stratosphere enhance heterogeneous chemical reactions that lead to enhanced ozone depletion after larger volcanic eruptions (Solomon, 1999)
One of the most important differences between the two scenarios is that if a permanently enhanced stratospheric aerosol layer is artificially created in an attempt to reduce anthropogenic global warming, the resulting strong ozone depletion, in particular in mid- and high latitudes, would have serious impacts on the biosphere, similar to the effects observed after large volcanic eruptions described above (Crutzen, 2006; Rasch et al, 2008a, b; Tilmes et al, 2008, 2009, 2012)
As a consequence of the same net all-sky TOA solar flux reduction in G4 Specified Stratospheric Aerosol (G4SSA) and G4SSA-S (Fig. 1a), the two scenarios show approximately the same global mean surface temperature reduction of 0.8 K compared with RCP6.0 (Fig. 2a)
Summary
It is well known that sulfate aerosols in the stratosphere enhance heterogeneous chemical reactions that lead to enhanced ozone depletion after larger volcanic eruptions (Solomon, 1999). With present-day anthropogenic halogen loading, the aerosols provide additional surface area for heterogeneous reactions that activate halogens and increase catalytic ozone destruction, especially in high latitudes (Tie and Brasseur, 1995). This has been modeled and observed following the large 1982 El Chichón and 1991 Pinatubo volcanic eruptions (Tie and Brasseur, 1995; Portman et al, 1996). Differences in future tropospheric ozone concentrations will be strongly dependent on the emission pathway followed (Stevenson et al, 2006), which will determine both in situ tropospheric chemical production of ozone and transport from the ozone-rich stratosphere (Collins et al, 2003; Wild et al, 2012; Neu et al, 2014)
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