Measurements of H2O2 along with a variety of other photochemically active compounds were made during the summers of 1990 and 1992 at a rural site in Alabama. The chemical composition in the air sampled at this site provides an interesting contrast to earlier measurements made at a site in the Colorado mountains. Even though the ozone levels were slightly smaller, the H2O2 levels were a factor of 2 to 3 higher at the Alabama site. These measurements serve to further test our understanding of the meteorological and chemical parameters that determine the concentration of H2O2 in the rural troposphere. Deposition beneath a shallow nocturnal inversion layer as well as precipitation events played a major role in the loss of H2O2 and had a strong effect on the diurnal trend. When these major influences were eliminated through selection of dry, high sunlight and midmorning to late afternoon periods only, several relationships were apparent. A strong positive correlation between H2O2 and temperature was probably due to an increase in the concentration of isoprene with temperature. Isoprene serves as a major source of HO2 radicals, the precursors of H2O2.

At the Alabama site, higher temperatures lead to much higher isoprene levels and, consequently, the higher H2O2 levels compared to the Colorado site. At the Alabama site the relative humidity was anticorrelated with the concentration of H2O2, probably due to the effect of the humidity on deposition, evidently the dominant loss process. H2O2 exhibited only a weak positive correlation with O3 and no correlation with H2O. The degree of photochemical processing of the air mass was positively related to the concentration of H2O2. An increase in the concentration of NOx had a small but significant negative effect on the level of H2O2, and the diurnal correlation of H2O2 closely reflected the variation of the ratio of the concentrations of isoprene to NOx, which again reflects the important role isoprene may play as a photochemical precursor for H2O2 and the inverse role that NOx plays in curtailing the photochemical production of H2O2. Finally, the H2O2 to HNO3 ratio indicates that the photochemistry is strongly NOx limited at this Alabama site. ¿ American Geophysical Union 1995