The levels of oxidants and precursors have incidentally been monitored
for a long period in Croatia and systematically since 1989 in the cad
re of the EUROTRAC subproject TOR (Tropospheric Ozone Research). The r
esults of measurements at different locations indicate that the diurna
l behaviour of ozone concentrations is dependent on local air pollutio
n. Urban sites show diurnal variations of a factor 10 or more, rural s
ites of a factor 2 to 5 and less than a factor of 2 is observed above
the boundary layer.