IMPROVE Algorithm for Estimating Light Extinction Draft Recommendations to the IMPROVE Steering Committee

Current IMPROVE Algorithm Assumptions include that Six particle component terms plus a constant Rayleigh scattering term are sufficient for a good estimate of light extinction; Constant dry extinction efficiency terms for each of the six particle components works adequately for all locations and times; and Light extinction by the individual particle components can be adequately estimated as separate terms (like externally mixed components).

Reasons for Revision Regional Haze Rule utilizes the IMPROVE algorithm for calculating the RHR index –Haze index is in deciview (a logarithmic transformation of light extinction) –Calls for improvement in 20% worst haze to natural levels by 2064 –Rate of progress for each 10-year SIP period is linear between current and natural haze levels –So haze estimates need to be good at the upper and lower extremes, should have no fixable biases and should include all important contributions even if exclusively from natural sources

Recommended Revised IMPROVE Algorithm (Changed terms are in Bold Font) where and nitrate and organic are split using the same process

Split Component Extinction Efficiency Model for Sulfate, Nitrate, and Organic components Composite dry extinction efficiency varies linearly from 2.2 to 4.8m 2 /g Small: D g = 0.2μm &  g = 2.2 Large: D g = 0.5μm &  g = 1.5 Different size distributions dictate different water grow function, f(RH) Small size dominates at low concentrations, while large size dominates at high concentrations Small size is likely young aerosol produce by homogenous gas to particle processes. Large size is likely aged aerosol involving cloud processing.

Other New or Revised Terms Organic Compound Mass to Organic Carbon Mass Ratio changed from 1.4 to 1.8 Sea Salt = 1.8 x [Chlorine] and has its own water growth term f SS (RH). Important at coastal locations Rayleigh Scattering is calculated for the monitoring site elevation and annual mean temperature and integer rounded. Ranges from 8Mm -1 at 10,000’ to 12Mm -1 at sea level NO 2 light absorption in the visible is included for sites that have such data (not routinely available at IMPROVE sites)

Bias = (estimated – measured)/measured

Implementation Steps for Regional Haze Rule Application IMPROVE Steering Committee approval – 11/05 Calculation of water growth functions for monthly & annual averaged conditions for each monitoring site – SAIC task-1 to 2 months Recalculation of current (5-year baseline) and natural haze levels – VIEWS-1 month EPA modifies the regional haze guidance, so states can choose – 6 to 12 months VIEWS will support both versions of the algorithm for the foreseeable future