Given that these measurements were made in the Arctic, which can be subject to anthropogenic pollutants, the next question was – are these natural or man-made effects on ozone levels? The answer lay in Antarctic measurements, which showed that these features of rapid ozone loss, now known as Ozone Depletion Events (ODEs) were also measured at research stations in coastal Antarctica (see Figure 1). These data confirmed that ODEs were a natural phenomenon.
Ongoing research in both polar regions progressively built the now familiar picture of ODEs. They occur only during the spring; sometimes ozone concentrations drop from normal amounts to near-zero within the space of a few minutes; sometimes ozone loss can be less complete, and can occur more gradually; ozone can remain suppressed for several days; vertical profiling using balloons has shown that the ozone loss can extend up to ~1500 m in altitude and that they are contained within the planetary boundary layer by a capping inversion. An important observation was that ODEs are associated with transport of air masses over sea ice.
The mechanism that is driving surface ozone loss is known as the “Bromine Explosion”, and is autocatalytic in bromine in the sense that one gaseous bromine going into the system results in two gas-phase bromines coming out (Figure 2). Bromide is liberated from the quasi-liquid layer on some sort of sea ice surface (exactly which is still open to debate). The possible candidates include sea salt aerosol, sea salt deposited onto the snowpack, or frost flowers – delicate dendritic ice crystals which are highly saline and grow as new sea ice forms. Interestingly, elevated BrO has also been measured in other locations where large areas of exposed salt exist, such as salt pans and the Dead Sea.
ODEs have been significant for atmospheric chemists for a number of reasons. They highlighted new reaction pathways that are important for halogen chemistry in the boundary layer. They have a potential radiative role that could be relevant on a regional scale, if air that is low in ozone is mixed to higher altitudes where ozone is particularly radiatively important. Finally, they were completely unanticipated, and so provide a nice example that the natural world still holds surprises for us!
Photochemical reactions in snow
As outlined above, our initial view of the polar troposphere was that it would be relatively uninteresting from a chemical point of view. Low mixing ratios of the reactive trace gases such as OH, HO2, NO, NO2 were expected, and the snow was anticipated to be important for albedo and as a barrier to surface exchange; it was not anticipated to be chemically active. The assumption that snow is a chemically-inert substrate has since been turned completely on its head. In order to explore the new subject of snow photochemistry, I will describe the story of NOx production from snow.
In the 1990s, a group of atmospheric scientists were working at Summit, Greenland. They had instrumentation to make an integrated measurement of all oxidised nitrogen trace gases referred to as NOy, (= NO + NO2 + HNO3 + HONO + ….), as well as NOx (= NO + NO2). Given that NOx has relatively few natural sources, the majority in the present-day troposphere being anthropogenic, the scientists were anticipating low mixing ratios of NOx and a lot more NOy. What they found were extraordinarily high mixing ratios of NOx, and a high ratio of NOx to NOy, suggesting that there had to be a highly significant local source of NOx. They turned their attention to the snow. Snow is not a solid medium, but is porous to a greater or lesser extent. The instruments measuring NOy and NOx sucked their air sample through a piece of Teflon tubing. The tubing was now inserted into the snowpack to allow the scientists to sample the interstitial air and to compare it with ambient. What they found was quite astonishing: both NOy and NOx that was sampled from within the snow was present at mixing ratios many times higher than in the ambient air. There appeared to be a major source of NOx from within the snowpack. The scientists measured for roughly 1½ days, and found that their results were robust throughout this time period. They also found a signal of a diurnal variation, and suggested that this was due to variation in the amount of solar radiation reaching the snow surface.
Figure 3: Results from an experiment whereby air was sampled through a block of snow and compared with ambient. NO and NO2 measurements provide evidence of production by the snowblock, and that that production varied over the course of a day. The bottom panel shows UV irradiance and air temperature, both of which vary over the course of a day
I heard about this research at the American Geophysical Union conference in December 1996. A month later I travelled to Antarctica as part of a collaborative project between the British Antarctic Survey and the German Alfred Wegener Institute for Polar and Marine Research to study oxidised nitrogen chemistry. The fieldwork was carried out at the German research station, Neumayer, using instrumentation to measure NOy, NO, and NO2. We decided to try to replicate the Greenland work to see if these processes operated in Antarctica, and also to try to quantify the production of these species. We found that ambient air was processed as it passed through the snow, and that both NO and NO2 were produced. The diurnal variation was also present, and we tested to see if this was driven by changing solar radiation or by temperature; it was the former (Figure 3 and Figure 4).
So, an extremely interesting new phenomenon had been found in both polar regions, but to have an influence on the atmosphere, the NOx had to be emitted from the snowpack and into the overlying boundary layer. For three days at Neumayer we measured gradients of NOx above the snowpack, and using coincident meteorological data, calculated fluxes of NOx out of the snow. So, NOx was both being produced within the snowpack and being released into the overlying boundary layer. It was highly significant to find a local source of this important trace gas in these remote locations.
Production of NOx from snow has now been measured at every polar location where it has been studied. The NOx is produced by the photolysis of nitrate impurities within the snow, and as nitrate is a ubiquitous impurity, this appears to be a process occurring wherever sunlight shines on snow (see Box 1).
The effect on the polar troposphere varies considerably with geographical location; at the coastal Antarctic stations, for example, NOx in the boundary layer is present only at low mixing ratios, albeit higher than would occur without emissions from the snowpack. The most extreme case is the South Pole station, where NO mixing ratios are routinely over 500 pptv. The exceedingly high mixing ratios are driven by several factors including a very shallow and stable boundary layer into which emissions are concentrated, high concentrations of nitrate impurities in the top layers of the snowpack, intense solar radiation at such a high elevation (South Pole is at 2830 m above sea level) and the fact that South Pole is downslope of the polar plateau, and therefore receives air that has been exposed to much snow.
The importance of NOx emissions for polar boundary layer chemistry lies in the response of other trace gases, whose mixing ratios are significantly altered compared to those in a low-NOx environment. For example, the OH:HO2 ratio is shifted towards OH as a result of the reaction:
NO + HO2 → NO2 + OH
The result is that OH has been measured at concentrations of the order 106 molecules cm-3, more typical of tropical regions. Another example is ozone, whose only chemical source in the troposphere arises from the photolysis of NO2:
NO2 + hν → NO + O (3P)
O(3P) + O2 → O3
Figure 4: This figure shows results of a shading experiment, where we alternatively shaded and unshaded the block of snow while measuring air from within the block. Data points are averages over the measurement period. The plot shows that when the showblock was exposed to sunlight, the amount of NO and NO2 measured was significantly higher than when the block was shaded – i.e. the production of NOx was driven by the sun