Airborne Observations Constrain Heterogeneous Nitrogen and Halogen Chemistry on Tropospheric and Stratospheric Biomass Burning Aerosol

Abstract Heterogeneous chemical cycles of pyrogenic nitrogen and halides influence tropospheric ozone and affect the stratosphere during extreme Pyrocumulonimbus (PyroCB) events. We report field‐derived N 2 O 5 uptake coefficients, γ (N 2 O 5 ), and ClNO 2 yields, φ (ClNO 2 ), from two aircraft campaigns observing fresh smoke in the lower and mid troposphere and processed/aged smoke in the upper troposphere and lower stratosphere (UTLS). Derived φ (ClNO 2 ) varied across the full 0–1 range but was typically <0.5 and smallest in a PyroCB (<0.05). Derived γ (N 2 O 5 ) was low in agricultural smoke (0.2–3.6 × 10 −3 ), extremely low in mid‐tropospheric wildfire smoke (0.1 × 10 −3 ), but larger in PyroCB processed smoke (0.7–5.0 × 10 −3 ). Aged biomass burning aerosol in the UTLS had a higher γ (N 2 O 5 ) of 17 × 10 −3 that increased with sulfate and liquid water, but that was 1–2 orders of magnitude lower than values for aqueous sulfuric aerosol used in stratospheric models.