Factors influencing the concentration of gas phase hydrogen peroxide during the summer at Niwot Ridge, Colorado

Measurements of H2O2 and several chemical and meteorological parameters were made during two summer studies at Niwot Ridge, Colorado, a rural site with low humidity, low SO2 concentrations, and low but varying levels of NOX. During two meteorological conditions, a strong nocturnal inversion and precipitation occurence, depositional processes play a major role in controlling the variability of H2O2. Selection of data from sunny, dry, and midday periods reduce these major effects and allow the influences of photochemistry to be discerned. Even during these selected periods, removal processes must be very rapid to balance the high production rates implied by measured and modeled levels of peroxy radicals. The relationships of the resulting near steady state concentrations of H2O2 with other parameters indicate that H2O2 is highest in air masses with low levels of NOX, highly aged NOY (as indicated by low NOX to NOY ratios), and high levels of O-3. These relationships also provide indirect evidence that isoprene is a significant precursor of H2O2. A significant negative correlation with relative humidity is found, which may reflect the dependence of the removal processes upon this parameter. The observed ratios of H2O2 to HNO3 indicate that the photochemical oxidant formation is NOX limited.