Using High Time Resolution Aerosol and Number Size Distribution Measurements to Estimate Atmospheric Extinction

Abstract

Rocky Mountain National Park is experiencing reduced visibility and changes in ecosystem function due to increasing levels of oxidized and reduced nitrogen. The Rocky Mountain Atmospheric Nitrogen and Sulfur (Ro-MANS) study was initiated to better understand the origins of sulfur and nitrogen species as well as the complex chemistry occurring during transport from source to receptor. As part of the study, a monitoring program was initiated for two 1-month time periods—one during the spring and the other during late summer/fall. The monitoring program included intensive high time resolution concentration measurements of aerosol number size distribution, inorganic anions, and cations, and 24-hr time resolution of PM_2.5 and PM₁₀ mass, sulfate, nitrate, car bon, and soil-related elements concentrations. These data are combined to estimate high time resolution concentrations of PM_2.5 and PM₁₀ aerosol mass and fine mass species estimates of ammoniated sulfate, nitrate, and organic and elemental carbon. Hour-by-hour extinction budgets are calculated by using these species concentration estimates and measurements of size distribution and assuming internal and external particle mixtures. Summer extinction was on average about 3 times higher than spring extinction. During spring months, sulfates, nitrates, carbon mass, and PM₁₀ – PM_2.5 mass contributed approximately equal amounts of extinction, whereas during the summer months, carbonaceous material extinction was 2–3 times higher than other species.