Assessing the Sources and Chemistry of Nitrogen Oxides in the Remote Oceanic Atmosphere

Atmospheric nitrogen oxides (NOx = NO + NO2) play a key part in controlling the abundance of OH and O3 in the atmosphere.
Long-term measurements of NOx at the Cape Verde Atmospheric Observatory have been used to investigate fundamental processes in the remote marine boundary layer (MBL). The measurements were conducted using a chemiluminescence detector with two different photolytic converters for NO2 conversion, which give comparable results when correcting for artefacts.
The photostationary state (PSS) of NO-NO2-O3 was used to evaluate our current understanding of the oxidation processes occurring in the MBL. Good agreement was observed between measured and PSS-derived NO2 calculated from measurements of NO, O3, and jNO2, modelled values of peroxy radicals (RO2 + HO2), and annually averaged halogen monoxides (IO + BrO) for air masses with [CO] < 90 ppbV. However, in air masses with [CO] > 100 ppbV, a missing oxidant converting NO into NO2 on the order of 18.5-104 pptV (assuming the same rate coefficient as CH3O2 with NO) was needed for the measured and PSS-derived NO2 to agree.
Formation of nitric acid (HNO3) has traditionally been seen as a sink of NOx, however, it has recently been suggested that photolysis of particulate nitrate (pNO3-) is up to orders of magnitude faster than photolysis of gas-phase HNO3, which could make it an important source of nitrous acid (HONO) and NOx in the MBL. Here, it is shown that the enhancement factor of particulate nitrate photolysis compared to gas-phase HNO3 decreases with increasing pNO3- concentration. This largely reconciles previous studies and can potentially be explained by a surface-enhanced mechanism described by the Langmuir isotherm.
Both the missing oxidants and the recycling of NOx through pNO3- can have important implications for atmospheric oxidants such as OH and O3 and their trends in both polluted and clean environments.