A seasonally resolved alpine ice core record of nitrate: Comparison with anthropogenic inventories and estimation of preindustrial emissions of NO in Europe

Abstract

Continuous high‐resolution records from Col du Dôme (CDD, 4250 m above sea level, French Alps) ice cores were used to investigate the impact of growing NO emissions on the midtroposphere nitrate levels over Europe from 1925 to 1997. The large snow accumulation rate at the CDD site largely minimized diffusional and depositional perturbations of the nitrate record as commonly encountered in ice cores. That permits for the first time examination of the winter and summer trends separately with a high degree of confidence. Being close to 100 ng g−1 in 1925, summer levels increased at a mean rate of 2 ng g−1 per year from 1925 to 1960 and 8.5 ng g−1 yr−1 from 1960 to 1980. These summer nitrate changes follow rather well the course of growing NO emissions from source regions (France, Italy, Spain, and Switzerland, denoted WE4) located 1000 km around the Alps as estimated by NMI and EDGAR‐HYDE inventories. After 1980, the summer nitrate levels continue to increase at a slightly weaker rate than during the 1960–1980 time period. Such a lasting increase of nitrate levels is inconsistent with NO emission estimates, which indicate a decrease after 1993. The nitrate record shows a weaker increase (1 ng g−1 yr−1 from 1930 to 1990) in winter than in summer, corresponding to a lower contamination of the European wide midtroposphere as preferentially recorded in CDD winter samples. Using ice core and emission data, we calculate a preindustrial NO emission (including agricultural and natural emissions) of 0.8 ± 0.2 Mt NO2 per year for WE4 countries, which represents 15–20% of the recent NO emissions. These data would help to reduce existing uncertainties in NO inventories which represent key information to assess past and future ozone changes over Europe.