Airborne remote sensing of nitrous acid in the troposphere: potential sources of excess HONO

Abstract. The photolysis of nitrous acid (HONO) produces hydroxyl radicals (OH), the most important cleaning agent of the troposphere. For decades, HONO has been measured in concentrations which exceed the photo-stationary concentration arising from its gas phase formation via the reaction NO + OH and destruction by photolysis. Several heterogeneous formation mechanisms as well as the photolysis of particulate nitrate have been proposed which may explain this excess HONO. This study reports on airborne remote sensing measurements of the mini-DOAS instrument over continental Europe, Southeast Asia, and the tropical Atlantic. The observations form a C-shaped profile in the troposphere with maximum volume mixing ratios of approximately 150 ppt in the planetary boundary layer, about 10 ppt in the free troposphere and up to 100 ppt in the tropical upper troposphere. These measurements of HONO throughout the troposphere exceed model predictions by up to an order of magnitude. Together with a host of other measured species and parameters, various formation mechanisms are explored to investigate in situ HONO sources. Although a precise formation mechanism in the polluted boundary layer remains elusive, the photolysis of particulate nitrate may explain excess HONO in the marine boundary layer. The excess HONO observed in the upper troposphere requires a gas phase source with a formation rate of up to 300 ppt h^-1. The possible role of peroxynitrous acid (HOONO), formed by the reactions NO + HO₂+ M and NO₂+ OH + M, and further oxidation by reactions with NO or O₃, is explored.