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Letters to the Editor

In Lowenthal and Kumar1 we examined data analysis protocols for assessing improvement in visibility under the U.S. Environmental Protection Agency (EPA) Regional Haze Rule.2 Equation 3 of Lowenthal and Kumar1 is the “IMPROVE” formula for estimating light extinction (Bext) from aerosol chemical measurements. The f(RH) in this equation are hygroscopic growth factors that account for the enhancement of light scattering (Bsp) due to water absorption as a function of relative humidity (RH). The Haze Rule protocol relies on “climatological” f(RH) [FRHC] that were derived from historical RH data from across the United States. In that process, hourly RH values greater than 95% were “rolled back” or set to 95%.2 To compare measured and estimated Bsp, it was necessary to eliminate periods when RH was greater than 95%. RH measurements are highly uncertain above 95% and Bsp measurements in this range can be high-biased by precipitation, fog drops, or hygroscopic growth, which becomes asymptotic above 95% RH. For this reason, f(RH) based on “actual” RH ( 95%) [FRHA] are most appropriate for comparing estimated and measured Bsp. Table 4 in Lowenthal and Kumar1 shows that Bsp was severely overestimated with FRHC compared with FRHA. However, this comparison is not entirely appropriate because while conditions with RH95% are represented in the FRHC (i.e., as data values rolled back to 95%), they are necessarily omitted from the measured Bsp and FRHA. This introduces a potential bias in the comparison of Bsp estimated with FRHC and FRHA at the very highest RH. However, this does not affect the conclusion drawn from Figure 3 in Lowenthal and Kumar1 that the FRHC were higher than the FRHA at low RH. If the FRHA were calculated in the same manner as the FRHC (i.e., by rolling back RH95% to 95% rather than eliminating them), Figure 3 would appear the same. If we use “rolled back” FRHA [FRHAr] to estimate Bsp, there is also no apparent difference in Figure 4 in Lowenthal and Kumar,1 which compares measured Bsp and Bsp estimated with FRHC and FRHA at Great Smoky Mountains National Park. Table 5 in Lowenthal and Kumar1 presents the contribution of sulfates to estimated Bext using various combinations of FRHC, FRHA, dry sulfate, and nitrate scattering efficiencies, and the ratio of organic mass/organic carbon (OCM/OC). On average, sulfates accounted for 32, 27, 24, and 22% of Bext with the following assumptions: (1) FRHC, dry efficiencies 3 m/g, OCM/OC 1.4; (2) FRHA, dry efficiencies 3 m/g, OCM/OC 1.4; (3) FRHA, dry efficiencies 2.5 m/g, OCM/OC 1.4; and (4) FRHA, dry efficiencies 2.5 m/g, OCM/OC 2.1. If we substitute FRHAr for FRHA, the corresponding average sulfate contributions to Bext are 32, 29, 26, and 24%, respectively. These observations show that the FRHC still overestimate Bsp and the contribution of sulfates to Bext with respect to f(RH) based on actual ambient RH, although somewhat less than previously reported.1 This effect is most pronounced for IMPROVE sites that experience high RH, but is not relevant to drier locales.