Relative contributions of tropospheric and stratospheric sources to nitrate in Antarctic snow

The nitrate contents of nearly 500 firn samples collected at various sites on the Antarctic ice sheet are reported. Additional chemical measurements of all major ions, in particular protons, have also been performed so that new conclusions regarding the most probable form and origin of nitrate in the Antarctic can now be drawn.

It is shown that nitrate is deposited as gaseous HNO₃ and that tropospheric sources dominate in this remote region. Continental and anthropogenic nitrates are not significant contributors. It is suggested that lightning at tropical and/or mid latitudes is the most likely source of Antarctic nitrate. The formation of HNO₃ (or its precursors) in the stratosphere is discussed and the possible evidence for this contribution in several profiles is carefully investigated. The absence of a convincing correlation between solar factors and nitrate concentrations in snow confirms that past solar activity fluctuations cannot be reconstructed from polar ice cores. The spatial and temporal variations observed in this study are, however, not fully explained. Finally, emphasis is placed on the necessity of undertaking HNO₃ measurements in the Antarctic atmosphere in order to elucidate the deposition mechanism of this major component of atmospheric chemistry.