Potential sources of excess nitrous acid in the troposphere inferred from airborne remote sensing

The photolysis of nitrous acid (HONO) produces the hydroxyl radical (OH). For decades. HONO measurements have exceeded model predictions, which are often based on gas phase chemistry, and various mechanisms have been proposed as sources of this excess HONO. We report here on airborne remote sensing observations from the mini-DOAS instrument onboard the HALO aircraft during several research missions from various regions (Europe, Asia, the Atlantic) at altitudes up to 15 km. HONO slant column densities detected from limb scattered skylight in the ultraviolet wavelength range using the DOAS technique are converted to volume mixing ratios with the O3/O4 scaling method. These observations form a C-shaped profile in the troposphere which exceed model (EMAC/MECO(n)) predictions by up to an order of magnitude, with volume mixing ratios up to 150 ppt in the boundary layer and more than 100 ppt in the upper troposphere. Together with a plethora of atmospheric parameters and trace gases measured simultaneously onboard HALO, various formation mechanisms are explored to investigate in situ HONO sources. While the photolysis of particulate nitrate can explain HONO in the marine boundary layer over the remote Atlantic, HONO formation in the polluted lower troposphere remains difficult to explain quantitatively. In the cold upper troposphere of the tropics, the aerosol loading was not sufficient to explain the necessary HONO source with heterogeneous chemistry, and a novel gas phase formation mechanism is proposed. The potential formation, lifetime, and oxidation of peroxynitrous acid in the upper troposphere is investigated in some detail.