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The EPA modified an equation used when calculating the level of particulate matter (PM2.5) in the air, namely Equation 2 in appendix N to part 50, section 4.4(b). Original equation 2 was not appropriate as written and did not accurately reflect the intended calculation of the annual mean PM2.5 concentration. Equation 2 was modified to include calculation of the annual mean PM2.5 concentration in cases where a site does not have four complete quarters of data, but passes one of two substitution tests described in sections 4.1(c)(i) and 4.1(c)(ii). This adjustment to Equation 2 is a currently used calculation of the PM2.5 annual design value, is consistent with the text of section 4.1 within appendix N to Part 50, and does not affect the calculation of annual mean PM2.5 concentrations when four complete quarters of data are available.