June 3, 2020 - EPA prepared this memo and associated technical addendum to communicate recommendations on ambient data completeness criteria for the Regional Haze program.  Referencing the 2017 Regional Haze Rule, this memo confirms that the baseline period for tracking progress is 2000-2004 and that the data completeness criteria contained in the 2003 Guidance for Tracking Progress Under the Regional Haze Rule EPA continues to be appropriate.  The memo also states that the EPA believes that both data patching and substitution are appropriate for use in the current IMPROVE ambient data calculations for the second implementation period.  The technical addendum to the memo contains a summary of the baseline, most recent (through 2018), and natural visibility condition estimates on the 20% clearest and 20% most impaired days for each IMPROVE site representing a Class I area in the Regional Haze Program.

• Recommendation for the Use of Patched and Substituted Data and Clarification of Data Completeness for Tracking Visibility Progress for the Second Implementation Period of the Regional Haze Program (pdf) (335.86 KB, June 3, 2020)
• Technical addendum including updated visibility data through 2018 for the memo titled “Recommendation for the Use of Patched and Substituted Data and Clarification of Data Completeness for Tracking Visibility Progress for the Second Implementation Period (pdf) (12.15 MB, June 3, 2020)