Calibration data collected with the MP-IV.  Open red squares and solid red fit represent our preferred method of calibration, with nitrogen dioxide measured simultaneously with the PAS and CRD; this agrees within 5% of theoretical calculations of the expected absorption based on absorption cross sections (solid red squares and dotted red line).  The black line and colored circles represent measurements of nigrosin, a particle calibrant, and also agree with NO2.  The solid purple line and triangles represent measurements of ozone in a matrix of 100% O2, which is roughly 20% lower than NO2; the dotted purple line and inverted triangles represent measurements of ozone in 100% N2, and are clearly different from other methods of calibration.  There is discrepancy about this in the literature.