Surface ozone at rural sites in the latrobe valley and Cape Grim, Australia

Abstract Ozone and other air quality data from five rural sites in the industrialized Latrobe Valley, Victoria, have been subject to statistical analyses including linear regression modelling. The behaviour of O3 in the Latrobe Valley is explained largely in terms of natural background atmospheric processes as observed at Cape Grim, Tasmania. The maximum 1-h average concentration of naturally occurring O3 (obtained from a 6-year record at Cape Grim) is less than 40 ppb (v/v). In contrast the industrialized Latrobe Valley sites show O3 values exceeding 40 ppb between 1% and 3% of the time. These higher concentrations occur in conditions consistent with local photochemical production of O3 via ‘smog’ type processes and appear preferentially at low NOx concentrations (3–4 ppb) during the afternoon (13–18 h) and at high temperatures (above 25°C). A comparison of observations from an elevated station (750 m) with those from the valley floor shows systematic differences in seasonal and diurnal O3 variations and the time of day of occurrence of elevated O3 concentrations which can be explained in terms of the diurnal cycle of convective mixing and mountain/valley winds. A linear regression model incorporating this understanding has accounted for between 43% and 64% of the variance of O3 concentration at the elevated and rural stations. The statistical model incorporates temperature, time of day, month of year, wind speed, O3 concentration 24-h earlier and NOx concentration as variables in the regression equation, with temperature being the dominant variable. The standard deviation of the residual O3 values (observed minus fitted) is around 5 ppb. Auto and cross correlations are used to show that perhaps half of the unexplained variance is coherent from site to site and hence potentially could be modelled.