The Environmental Protection Agency (EPA) is taking direct final action to make a technical correction to an equation in an appendix in the National Ambient Air Quality Standards (NAAQS) for Particle Pollution. Equation 2 describes an intermediate step in the calculation of the design value for the annual PM<INF>2.5</INF> (particles with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers) NAAQS. This action corrects a scrivener's error in one of the equations used to calculate an annual mean PM<INF>2.5</INF> concentration, to properly account for cases where a site does not have four complete quarters of data and passes one of two substitution tests. This change accurately reflects the intended calculation of the annual mean PM<INF>2.5</INF> design value and is consistent with the text elsewhere in the appendix.