Aircraft measurements of nitrous acid in excess of model predictions in the boundary layer and free troposphere

Middle and long-term  photo-chemical effects of local and regional pollution are not well quantified and are an area of active study. NO_x (here defined as NO, NO₂, and HONO) is a regional pollutant, which influences atmospheric oxidation capacity and ozone formation. Airborne measurements of atmospheric trace gases from the HALO (High Altitude Long Range) aircraft, particularly of NO, NO₂, and HONO were performed as part of the EMeRGe (Effect of Megacities on the Transport and Transformation of Pollutants on the Regional to Global Scales) campaign over continental Europe and southeast Asia in July 2017 and April 2018, respectively. NO (and NO_Y), O₃, and the photolysis frequencies of NO₂ and HONO were measured in-situ. NO₂ and HONO were inferred from Limb measurements of the mini-DOAS (Differential Optical Absorption Spectroscopy) instrument, using the novel scaling method (Hüneke et al., 2017). These measurements were compared with simulations of the MECO/EMAC models. In relatively polluted air-masses in the boundary layer and free troposphere, HONO measured in excess of model predictions (and previous measurements) suggests an in-situ formation and a significant source of OH as well as a pathway for re-noxification. Aerosol composition simultaneously measured  by the C-Tof-AMS instrument may reveal potential reaction mechanisms to explain the discrepancy.