A European inventory of soil nitric oxide emissions and the effect of these emissions on the photochemical formation of ozone

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An inventory of soil nitric oxide (NO) emissions for Europe was developed. The emission of NO was parameterized using empirical relationships with type of landuse, fertilization rate of agricultural areas and soil temperature. For the year 1994, it was estimated that annual soil NO emissions in the inventoried area amounted to 535 kt NON yr −1, i.e. 8% of the emissions from combustion processes. On a hot summer day, this fraction increased to 27%. The uncertainty of these emission estimates, however, is high. They are accurate only within a factor of three to four. As the major fraction (in summer 81 %) of the emissions came from arable land, soil NO emissions have to be considered as largely anthropogenic. Large regional differences existed in the soil NO emission rates with the highest emission rates found in southern and western Europe, intermediate rates in eastern Europe and the lowest rates in Scandinavia and northern Russia. The effect of the soil NO emissions on the photochemical formation of ozone (0 3) was investigated with a one-dimensional photochemical model. When soil NO emissions were considered in the model simulations, computed mean daily maximum 0 3 concentrations over Europe from June to August 1994 were 4 ppb (uncertainty range: 1.4–9.6 ppb) higher than without these emissions. Reductions of pyrogenic NO x , emissions were less efficient in reducing O 3 concentrations when soil NO emissions were taken into account.