Observational study of transport and photochemical formation of ozone overnorthern Europe

Seven years of ozone and total nitrate (the sum of gaseous HNO3 and particu late NO3-) data are analyzed to quantify the seasonal cycles of ozone produ ction and transport in the boundary layer at 60 degrees N over the Baltic S ea. This analysis is based on the statistical correlations between ozone an d total nitrate and the covariances between trace species and wind componen ts. Ozone production efficiencies are highest near the summer solstice, dec reasing to low values in March, April, and October. In winter, precursor em issions effectively deplete ozone. Horizontal average and transient fluxes of ozone and total nitrate in the boundary layer were estimated as the cova riances between trace gas concentrations and horizontal wind components. A year-round northward flux of total nitrate, 2.0 mu g(N) m(-2) s(-1) as annu al average, was observed. This is the sum of a northeastward flux by the av erage wind field and a northwestward flux by synoptic-scale transient eddie s. Embedded in an east-northeastward flux of background ozone, the sums of the components of the transient and estimated average flux divergence over Europe show large northward fluxes in summer and southward fluxes in winter . In summer, these fluxes are due to photochemically produced elevated ozon e concentrations. In winter, high winds advect background ozone into the Eu ropean continent which then acts as a sink area. The average effect of Euro pean precursor emissions on the horizontal flux was estimated assuming that it is the sum of the transient eddy component and the estimated average up wind divergence component over Europe. The northward component of these flu xes is higher in summer, with a highest monthly average in July of 49 mu g m(-2) s(-1), compared to the southward fluxes in winter, with a highest val ue of 44 mu g m(-2) s(-1) in December, resulting in a northward yearly aver age flux of 6 mu g m(-2) s(-1). The sum of the transient eddy flux componen t and the estimated average divergence flux component over Europe is estima ted to affect monthly horizontal total ozone fluxes by 32-68% in the period April-September, but only by 3-23% during the winter months.