Isotopic studies of ice core nitrate and atmospheric nitrogen oxides in polar regions.

摘要

Atmospheric nitrogen oxides regulate concentrations of natural and anthropogenic trace gases through interactions with tropospheric oxidants. Understanding past and present changes in atmospheric NOx (NO + NO2) is possible through measurements of nitrate (NO3- or nitric acid, HNO3) in polar ice cores. This dissertation is comprised of four studies which contribute towards understanding the controls on nitrate isotopes preserved in polar ice.;Box modeling of local photochemistry at Summit, Greenland show that the delta 15N and delta18O of HNO3 are influenced by several factors, including isotope fractionation associated with NO-NO 2 cycling and seasonal changes in HNO3 formation chemistry and in NOx sources. A technique for the capture of atmospheric NO2 in remote regions for later isotopic analysis is described. First measurements of the delta15N of NO2 at Summit show little difference with the delta15N of HNO3, indicating that isotope fractionation associated with the oxidation of NO 2 to HNO3 is small.;The role of post-depositional processing on nitrate isotopes in the Summit snowpack is explored through isotopic measurements of gas-phase HNO3 , surface snow nitrate, and snowpit nitrate. These measurements indicate that NOx emitted from the snow following nitrate photolysis quickly recombines with local oxidants to produce HNO3 prior to recycling back to the snow. This photolytic loss and recycling has a small influence on nitrate isotopes preserved in ice at Summit.;Measurements of nitrate isotopes in an ice core from South Pole, Antarctica show evidence of active post-depositional recycling and loss of nitrate. A large near-surface trend in the delta15N of nitrate is attributed to post-depositional losses, while the delta18O of nitrate indicates that oxygen isotope fractionation associated with post-depositional loss is overwhelmed by the influence of local oxidants on nitrate recycling.;The concentration and delta15N of nitrate in an ice core from Summit, Greenland exhibit trends which are strongly correlated with recent changes in global NOx emissions. The Greenland delta 15N record indicates that the delta15N of recent NOx emissions must be isotopically light, which is consistent with the combustion of fossil fuels. This shows that the Greenland delta 15N record preserves changes in source emissions of atmospheric NO x.