OH and HO₂ chemistry in the North Atlantic free troposphere

Abstract

Interactions between atmospheric hydrogen oxides and aircraft nitrogen oxides determine the impact of aircraft exhaust on atmospheric chemistry. To study these interactions, the Subsonic Assessment: Ozone and Nitrogen Oxide Experiment (SONEX) assembled the most complete measurement complement to date for studying HO x (OH and HO 2 ) chemistry in the free troposphere. Observed and modeled HO x agree on average to within experimental uncertainties (±40%). However, significant discrepancies occur as a function of NO and at solar zenith angles >70°. Some discrepancies appear to be removed by model adjustments to HO x ‐NO x chemistry, particularly by reducing HO 2 NO 2 (PNA) and by including heterogeneous reactions on aerosols and cirrus clouds.