The mysteries of reactive nitrogen species above polar snow

Emission of NOx and HONO from the snowpack can have a significant impact on the oxidising capacity of the boundary layer in the polar regions. This thesis presents work that furthers our understanding of snow nitrate photolysis and HONO production. The HONO amount fraction was measured at Halley Research Station, and was between 10 times larger contribution to the HONO budget than gas phase HONO formation. Isotopic analysis of atmospheric HONO, nitric acid and particulate nitrate samples from Halley was attempted in this study, but was affected by large field blanks and low sample concentrations. Analysis of snow nitrate concentration and isotopic composition depth profiles showed that post-depositional processing occurs at Halley: an increase in δ(15N, NO3 – ) and decrease in nitrate concentration with depth was observed in the top 30 cm of the snowpits. Preservation of seasonal trends in δ( 15N, NO3 – ) was also found to occur. An experimental setup for measuring HONO and NOx production rates, from snow samples irradiated using a UV lamp in the lab, is presented. Both the NO2 and HONO production rates decreased as the snow temperature decreased. HONO production made up 47 and 54 % of total HONO and NO2 production at −29.5 and −18.5 ◦C. The measured production rates were used to calculate a quantum yield for NO2 production from nitrate photolysis, specific to Halley snow (Φ(NO2) = 0.0012 to 0.0029), and predict diurnal profiles of the NO2 and HONO flux densities at Halley.

Details