Background

Episodic springtime destruction of ozone in the Arctic planetary boundary layer (PBL) was first reported in the early 1980's at both Barrow, Alaska (71.3$^{\circ }$N, 156.6$^{\circ }$W) (Oltmans, 1981) and at Alert, NWT (82.5$^{\circ }$N, 62.3$^{\circ }$W) (Bottenheim et al., 1986). At times, ozone was observed to decrease from 40 ppbv to below the detection limit of the instruments (<0.5 ppbv). These events coincided with strong temperature inversions which effectively isolated the PBL from air in the free troposphere. It was later observed that this destruction was strongly correlated with enhancements of filterable bromine (or f-Br), which is the sum of particulate and gaseous bromine (Barrie et al., 1988). It was suggested that free bromine atoms could destroy ozone catalytically through (Barrie et al., 1988),

$$\rm Br + O_3 \longrightarrow \rm BrO + O_2$$ (11.38)

where atomic bromine is regenerated through,

   

$$\rm BrO + BrO \longrightarrow \rm 2Br + O_2$$ (11.39)

$$\rm \longrightarrow \rm Br_2 + O_2$$ (11.40)

$$\rm Br_2 + h\nu \longrightarrow \rm 2Br.$$ (11.41)

Catalytic cycles involving atomic chlorine were also proposed after evidence supporting the presence of large amounts of gas-phase chlorine atoms (Jobson et al., 1994; Le Bras and Platt, 1995),

$$\rm Cl + O_3 \longrightarrow \rm ClO + O_2$$ (11.42)

where atomic chlorine is regenerated through the reaction of ClO of BrO as follows,

   

$$\rm ClO + BrO \longrightarrow \rm Br + Cl + O_2$$ (11.43)

$$\rm ClO + BrO \longrightarrow \rm BrCl + O_2$$ (11.44)

$$\rm BrCl + h\nu \longrightarrow \rm Br + Cl.$$ (11.45)

However, these gas-phase reaction sequences above cannot be responsible for the near-zero ozone amounts observed (McConnell et al., 1992; Brune et al., 1988) as reactions with hydrocarbons will rapidly tie up bromine and chlorine in the form of HBr, HCl, and other reservoir species. As a result, recycling of bromine through heterogeneous chemistry has been suggested (McConnell et al., 1992; Fan and Jacob, 1992; Mozurkewich, 1995; Tang and McConnell, 1996) although the exact reaction pathway remains unclear. Fan and Jacob (1992) proposed a mechanism whereby HBr is recycled through,

$$\rm HBr \stackrel{\rm aerosol} \longrightarrow \rm Br^- \sta... ...q)}} \longrightarrow \rm Br_{2(aq)} \longrightarrow Br_{2(g)}$$ (11.46)

and where the source of HOBr is BrONO₂. Even less is known about the chlorine recycling mechanism. Modeling of this and other possible mechanisms have established that about 40 pptv of BrO are required to account for the rapid destruction of ozone, $\tau_{\rm O_3}\sim$ 1 day (McConnell and Henderson, 1993).

The source of the bromine and chlorine atoms is still not certain. Emissions of halocarbons (such as CHBr₃) from the ocean have been suggested, but sea salt accumulation on the snowpack covering the sea ice seems to be the most probable (McConnell et al., 1992; Fan and Jacob, 1992; Tang and McConnell, 1996; Finleyson-Pitts et al., 1990). Recent observations of frost flowers, ice crystals which grow on the surface of young sea ice accompanied by the formation of a moist surface layer of high salinity, would seem to support this (Nghiem et al., 1997). Possible surface sites for heterogeneous chemistry include the snow surface, sulphate aerosols and the ubiquitous ice crystals. Also, direct heterogeneous destruction of O₃ on surfaces cannot be totally ruled out.

Two extensive measurement campaigns have been undertaken in recent years to investigate this phenomena: the Polar Sunrise Experiment (PSE) in 1992 at Alert, NWT, Canada (Journal of Geophysical Research, 99 (D12), 1994) and the Arctic Tropospheric Chemistry (ARCTOC) project in 1995 and 1996 at Ny-Ålesund/Spitsbergen (Tellus, 49B(5), 1997). Despite this, questions such as the spatial extent of the ozone depletion and enhanced BrO remain unknown although it appears to extend over the entire Arctic Ocean. In addition to Barrow and Alert, polar sunrise ozone depletion along with elevated amounts of BrO have also been observed at Kangerlussuaq, Greenland (67$^{\circ }$ N, 51$^{\circ }$ W) (Miller et al., 1997) and Ny-Alesund, Norway (78.9$^{\circ }$N, 11.9$^{\circ }$E) (Platt et al., 1998). There is also some evidence to suggest that this phenomena also occurs in the Antarctic (Kreher et al., 1997). Aircraft flights have also indicated that ozone depletion is extensive (Kieser et al., 1993; Leaitch et al., 1994). Due to the strong temperature inversion inside the PBL during the Arctic winter and spring, it has been generally assumed that this phenomena is restricted to the PBL.