Ozone and Related Species (CM-O3)

Principal Participants: Strong, Drummond, Manson, Sica, Walker, Hocking, Manney, Rochon, Tarasick; Stakeholders: Chipperfield, De Mazière, Hannigan, Kreher, Manney, Van Roozendael; Canadian Gov't Departments: EC, CSA; International Orgs: NDACC; HQP: 25% RA + 1 graduate student.

Rationale The overarching goal of this project is to address the question of “What are the processes determining the chemical composition of the Arctic atmosphere and its evolution with time?” While the Montreal Protocol and its amendments have reduced the production of ozone-depleting chlorofluorocarbons (CFCs), their long lifetimes mean that stratospheric ozone depletion will continue for several decades. At the same time, the impact of climate change on ozone is increasing, and there is considerable interest in the processes coupling stratospheric chemistry and climate, in the Arctic and globally. While increasing concentrations of CO2 are cooling the stratosphere and thereby reducing global ozone loss rates, this cooling may generate more polar stratospheric clouds (PSCs) and enhance ozone depletion in the Arctic (Eyring et al., 2010). Chemistry-climate models predict that springtime Arctic ozone will recover to 1980 values between 2020 and 2035 (WMO, 2011). However, these models underestimate current Arctic ozone loss due to their inability to reproduce observed low temperatures, and so these predictions may be biased toward an early recovery. Measurements are essential to verify or refute such model predictions. Globally, total ozone columns are about 3.5% lower than the 1964-1980 average (WMO, 2011). In the Arctic, there continues to be springtime ozone depletion, averaging 10-15% since 1979. Record Arctic ozone depletion was observed during spring 2011, with about 40% of the ozone column destroyed due to the stable vortex and very low temperatures (Manney et al., 2011; Adams et al., 2012; Lindenmaier et al., 2012). Interannual variability in Arctic ozone is largely driven by atmospheric dynamics, transport, and temperature. Knowledge of the meridional wind-field is especially important for understanding mixing events between different latitudes. Due to the absence of solar radiation in the High Arctic winter, almost all polar winter-time ozone arises as a result of transport from the equator and mid-latitudes in autumn; the development of the winter polar vortex during late autumn, and the continued existence of a stable winter vortex, ensures the retention of ozone within it but can also create conditions for chemical ozone depletion. Understanding mean and planetary-wave motions is thus important to understanding of Arctic ozone, giving this project strong synergy with PN-PV.

Arctic tropospheric ozone is greatly affected by severe ozone depletion events (ODEs), first observed at Alert (Bottenheim et al., 1986; Barrie et al., 1988). These events have since been linked to extremely high concentrations of BrO in bromine explosion events (BEEs), but tropospheric ODEs are still not fully understood. Observational evidence suggests that the frequency of ODEs is increasing (Tarasick and Bottenheim, 2002, Tarasick et al., 2012). If the Arctic becomes ice-free in spring and summer, Voulgarakis et al. (2009) suggest that tropospheric ozone will increase by up to 50-60% in the Arctic and 20% at mid-latitudes due to the elimination of bromine chemistry. Conversely, Simpson et al. (2007) suggest that more first-year sea ice will enhance the occurrence of BEEs. Bromine explosions are connected to the bromine budgets of the free troposphere and the stratosphere. McElroy et al. (1999) reported significant amounts of BrO in both the boundary layer and the free troposphere from high-altitude aircraft measurements, and suggested that convective transport over large Arctic ice leads injects BrO into the free troposphere. Satellite observations of enhanced column BrO during spring have typically been associated with the surface release of bromine and ODEs (e.g., Chance, 1998; Richter et al., 1998; Wagner et al., 2001). However, recent aircraft measurements during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) and ARCPAC (Aerosol, Radiation and Cloud Processes affecting Arctic Climate) field campaigns found little correlation between BrO in the boundary layer and satellite observations of column BrO ‘hotspots’, suggesting that the stratosphere may be the source of enhanced BrO columns when the tropopause is low (Salawitch et al., 2010). Given the large uncertainties in the altitude sensitivity of satellite BrO total column measurements, as the tropopause height and boundary-layer bromine explosions both contribute significantly to the BrO total column, ground-based measurements of stratospheric and tropospheric BrO in the Arctic are essential for the interpretation of satellite total columns.