The influence of changing UVB radiation in near-surface ozone time series

UVB radiation plays an important role in tropospheric photochemistry since it determines the rate of ozone photolysis J((OD)-D-1) and subsequent forma tion of OH radicals. Consequently, changes of UVB radiation, for example du e to changes of the stratospheric ozone amount, could alter the concentrati on of reactive tropospheric gases including ozone. An observation-based att empt is made to quantify the effect of changing UVB radiation on surface oz one peaks on a day-to-day scale using a time series of measurements at a Sw iss mountain site. Seven years data of ozone, NO, NOx, and meteorological m easurements from Chaumont (1140 m above sea level (asl)), total ozone and U VB measurements from Arosa (1847 m asl), and surface albedo from satellite observations are investigated. The study is restricted to fair weather days with moderately high NOx concentrations. Multiple regression analysis is p erformed using chemical, meteorological, and UV dependent variables to pred ict afternoon ozone peaks. From autumn to spring, positive deviations of oz one peaks are clearly connected with positive UVB deviations. The relation is statistically significant only in part of the seasonal data subsets; how ever, it is consistent with model studies. The estimated net effect on ozon e peaks is normally within a range of 4 ppb, a range of about B ppb is pred icted for large UVB changes. Applying the coefficients for the large intera nnual variability of the stratospheric ozone layer observed in spring in th e last 10 years results in a range of variation of at most 1 to 1.5 ppb for monthly mean ozone peaks. For trends of J((OD)-D-1) from 1970 to 1990, a t rend bias of surface ozone peaks on polluted fair weather days of less than 0.12 ppb/yr is calculated Although the numbers are rather small, they may play a role in certain circumstances.