Biomass-burning influence on tropospheric ozone over New Guinea and South America

Using NIMBUS-7 version-7 Total Ozone Mapping Spectrometer (TOMS) ozone-column measurements, we derive lower-tropospheric ozone amounts over western South America and over the western Pacific Ocean near New Guinea from the column difference between two nearby regions with a topographic contrast, mountain and sea level. The seasonal variation of lower tropospheric-column ozone from ozonesondes at Natal, Brazil, agrees well with the seasonal variation of lower-tropospheric ozone over areas east and west of the Andes at the Natal latitude. This result suggests that, in the presence of the persistent easterly winds throughout the year over equatorial South America: favorable conditions exist for transporting biomass-burning products from burning regions to western South America. The seasonal variation of lower-tropospheric ozone over New Guinea shows a distinguishable annual cycle with a maximum in July-September and a minimum in January-February. Because the ozone monthly variation is well anticorrelated with monthly rainfall (the maximum ozone episode occurs during the dry season), biomass burning appears to be a likely precursor of the elevated ozone in a mechanism similar to the mechanism observed over South America and Africa. Overall, we observed less lower-tropospheric ozone over western New Guinea (6.6+/-2.2 Dobson units (DU) annual mean and monthly extrema) than over the plains east of the Andes (9.0 +3.4/-2.0 DU). The tropospheric-ozone linear trend derived from a regression of the deseasonalized monthly averaged lower-tropospheric ozone amount versus time east of New Guinea (upwind of the biomass-burning areas) shows no significant trend; however, west of New Guinea (downwind of the biomass burning regions) we find a statistically significant increase of 0.06 +/- 0.04 [DU yr(-1)] (95% confidence interval (c.i.)), corresponding to 1.0 +/- 0.6 [% yr(1)] (95% c. i.). The magnitude of this increasing trend is similar to the magnitude of the trend in the lower-troposphere ozone in western South America.