Measurements of gaseous peroxides near the Grand Canyon—Implication for summertime visibility impairment from aqueous-phase secondary sulfate formation

Warm cloud processes in which dissolved peroxides and ozone oxidize SO 2 to sulfate are important contributors to acidic sulfate deposition, and may also contribute to visibility impairment if the clouds evaporate prior to rainfall, e.g. under summer monsoonal conditions in the Southwest U.S.A. However, data from summer season gaseous peroxide measurements are sparse for this region, specifically in the area of the Grand Canyon National Park (GCNP). Results are reported herein from a gaseous peroxide measurement campaign at Meadview, AZ (between GCNP and various sources of S02 to the west and south) during July–August 1992. Total peroxide and H 2O 2 concentrations were measured by the concurrent coil collection/enzyme catalyzed fluorescent technique (Lazrus et al., 1986, Anal. Chem. 58, 594–597 Total gaseous peroxide levels observed were generally in the range of 1.0–5 ppbv. Results from measurements of peroxides are used with other preliminary data for ozone, SO 2, temperature and humidity, and from other observations to estimate the potential contribution of in-cloud oxidation processes to visibility degradation due to increased sulfate levels. The data show that peroxide concentrations and estimated H 2O 2 concentrations nearly always exceeded SO 2 levels at Meadview, indicating the absence of “oxidant limited” conditions in air in which clouds might be formed. Only modest quantities of aerosol sulfate ( < 1 μg m −3) can be formed in clouds that evaporate, for calculated rates of conversion at the usually observed ambient SO 2 concentrations of 100–500 pptv, when these rates are averaged over mean boundary layer thicknesses. Estimated initial rates of in-cloud reaction of ozone with S(IV) are comparable to rates for peroxides when cloudwater pH values exceed about 6. The self-quenching of the ozone-S(IV) reaction makes this process unimportant for typical summer conditions at Meadview with cloud liquid water contents (LWC) of about 0.1, but the process may become important at higher LWC and pH.