Water-insoluble (a) and -soluble (b) oxidative potential distributions on a per PM mass basis at a roadside site (RS, left panels, March 28–April 4, 2016) and a representative urban site (GT, right panels, March 16–23, 2016) in Atlanta, GA, USA. Average OP per mass for three modes, quasi-ultrafine (<0.18 μm), accumulation (0.18 ≤ Dp ≤ 3.2 μm), and coarse (3.2 < Dp < 10 μm) is also shown (diamonds with line). PM mass on each MOUDI stage was estimated from the sum of measured chemical components. The dotted line is aerodynamic diameter Dp = 2.5 μm.