ALBERT

Description: Ozone (O3) is an important atmospheric oxidant, a greenhouse gas, and a hazard to human health and agriculture. Here we describe airborne in situ measurements and model simulations of O3 and its precursors during tropical and extratropical field campaigns over South America and Europe, respectively. Using the measurements, net ozone formation/destruction tendencies are calculated and compared to 3-D chemistry–transport model simulations. In general, observation-based net ozone tendencies are positive in the continental boundary layer and the upper troposphere at altitudes above  ∼  6 km in both environments. On the other hand, in the marine boundary layer and the middle troposphere, from the top of the boundary layer to about 6–8 km altitude, net O3 destruction prevails. The ozone tendencies are controlled by ambient concentrations of nitrogen oxides (NOx). In regions with net ozone destruction the available NOx is below the threshold value at which production and destruction of O3 balance. While threshold NO values increase with altitude, in the upper troposphere NOx concentrations are generally higher due to the integral effect of convective precursor transport from the boundary layer, downward transport from the stratosphere and NOx produced by lightning. Two case studies indicate that in fresh convective outflow of electrified thunderstorms net ozone production is enhanced by a factor 5–6 compared to the undisturbed upper tropospheric background. The chemistry–transport model MATCH-MPIC generally reproduces the pattern of observation-based net ozone tendencies but mostly underestimates the magnitude of the net tendency (for both net ozone production and destruction).

Description: Discrepancies between expected and observed NO-NO〈sub〉2〈/sub〉 ratios in the upper troposphere suggest the presence of an unknown NO〈sub〉X〈/sub〉 reservoir. We report on airborne remote sensing limb observations from the mini-DOAS instrument on board the HALO (High Altitude Long Range) aircraft during the CAFÉ-Africa (Chemistry of the Atmosphere Field Experiment) campaign in 2018. Nitrous acid (HONO) slant column densities in limb scattered sunlight in the ultraviolet wavelength range retrieved by DOAS (Differential Optical Absorption Spectroscopy) are converted to volume mixing ratios using the O〈sub〉3〈/sub〉 / O〈sub〉4〈/sub〉 scaling method. Over the tropical Atlantic Ocean, in the cold upper troposphere, HONO is found in excess of what may be expected from known gas phase formation mechanisms or is predicted by the ECHAM/MESSy Atmospheric Chemistry (EMAC) model. At these altitudes (10-15 km), heterogeneous sources of the excess HONO are inefficient and thus unlikely. Therefore, we investigate the possibility of a gas phase HONO source, namely the oxidation of peroxynitrous acid (HOONO) formed in the reactions NO + HO〈sub〉2〈/sub〉 and OH + NO〈sub〉2〈/sub〉. Since there are no reported atmospheric measurements of HOONO, we use complementary, simultaneous in situ measurements of OH, NO, HO〈sub〉2〈/sub〉, NO〈sub〉2〈/sub〉, O〈sub〉3〈/sub〉 and photolysis frequencies from onboard HALO to make steady state arguments and quantify reaction rate coefficients for both formation pathways and destruction of HOONO by O〈sub〉3〈/sub〉, OH, and NO, the last of which may form HONO and NO〈sub〉2〈/sub〉.