Correction Factors for Covariance between Concentration and Deposition Velocity on CASTNes HNO3 Deposition Estimates

The covariance between hourly concentration (C) and deposition velocity (V) for various atmospheric species may act to bias the dry deposition (D) computed from the product of the weekly average C and V. This is a potential problem for the CASTNet filter pack (FP) species, nitric acid (HNO sub(3)). Using ozone (O sub(3)) behavior as a surrogate for HNO sub(3), correction factors (CF) are developed to estimate this bias. Weekly CF for O sub(3) depend on both site and season, and seasonal average weekly CF for O sub(3) at a given site may be as high as 1.25. The seasonal magnitude of this CF is generally largest in summer and is ordered: summer greater than or equal to fall greater than or equal to spring > winter. The CF is driven to a large extent by the diurnal correlation between C and V (i.e., both are generally higher during the day and lower at night). However, since the diurnal C profile at elevated sites is relatively constant, the resulting correlation between C and V is small, and the CF at montane sites is generally negligibly small. The sampling protocol using daytime integrated sampling for a week and nighttime integrated sampling for a week captures the diurnal correlation between C and V adequately and may be used to aggregate relatively unbiased weekly D estimates. Day-night CF for O sub(3) are close to unity, and limited results suggest similar behavior for HNO sub(3). Using these limited FP results, the site- and seasonally-specific weekly CF for O sub(3) are refined to estimate the corresponding CF for HNO sub(3). Worst-case adjustments for HNO sub(3) as high as 30% are indicated for summer periods at a given site.