Fig. 2.

In the presence of NO₂ and O₃, BB emissions age rapidly under dark conditions, producing a similar amount of secondary aerosol as under photochemical conditions. Enhancement in (A) O:C ratio, (B) f₄₄/f₆₀, (C) OA, and (D) total (inorganic and organic) aerosol nitrate for the dark reference (experiment 1 in SI Appendix, Table S1) and UV reference (experiment 2) experiments as well as the dark aging experiments under dry (experiments 3 to 6) and high RH (experiments 7 to 9) conditions. The shaded region corresponds to the variability across all experiments (see SI Appendix, Table S1) due to differences in injected NO₂ and O₃ concentration (dark dry experiments) or RH (dark RH experiments), while the solid blue and red lines are the median across the experiments. Time is relative to the initiation of oxidation (at hour 0). The calculation of enhancement in OA and nitrate aerosol is normalized to sulfate to account for particle wall losses and collection efficiency.