Observational study of transport and photochemical formation of ozone over northern Europe

Seven years of ozone and total nitrate (the sum of gaseous HNO3 and particulate NO3-) data are analyzed to quantify the seasonal cycles of ozone production and transport in the boundary layer at 60 degrees N over the Baltic Sea. This analysis is based on the statistical correlations between ozone and total nitrate and the covariances between trace species and wind components. Ozone production efficiencies are highest near the summer solstice, decreasing to low values in March, April, and October. In winter, precursor emissions effectively deplete ozone. Horizontal average and transient fluxes of ozone and total nitrate in the boundary layer were estimated as the covariances 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 annual average, was observed. This is the sum of a northeastward flux by the average wind field and a northwestward flux by synoptic-scale transient eddies. 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 ozone concentrations. In winter, high winds advect background ozone into the European continent which then acts as a sink area. The average effect of European precursor emissions on the horizontal flux was estimated assuming that it is the sum of the transient eddy component and the estimated average upwind divergence component over Europe. The northward component of these fluxes 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 value of 44 mu g m(-2) s(-1) in December, resulting in a northward yearly average flux of 6 mu g m(-2) s(-1). The sum of the transient eddy flux component and the estimated average divergence flux component over Europe is estimated to affect monthly horizontal total ozone fluxes by 32-68% in the period April-September, but only by 3-23% during the winter months.