Evaluation of Novel Routes for NOx Formation in Remote Regions.

Photochemical cycling of nitrogen oxides (NOx) produces tropospheric ozone (O3), and NOx is traditionally considered to be directly emitted. The inability of current global models to accurately calculate NOx levels, and concurrently, difficulties in performing direct NOx measurements in low-NOx regimes (several pptv or several tens of pptv) globally introduce a large uncertainty in the modeling of O3 formation. Here, we use the near-explicit Master Chemical Mechanism (MCM v3.2) within a 0D box-model framework, to describe the chemistry of NOx and O3 in the remote marine boundary layer at Cape Verde. We explore the impact of a recently discovered NOx recycling route, namely photolysis of particulate nitrate, on the modeling of NOx abundance and O3 formation. The model is constrained to observations of long-lived species, meteorological parameters, and photolysis frequencies. Only a model with this novel NOx recycling route reproduces levels of gaseous nitrous acid, NO, and NO2 within the model and measurement uncertainty. O3 formation from NO oxidation is several times more efficient than previously considered. This study highlights the need for the inclusion of particulate nitrate photolysis in future models for O3 and for the photolysis rate of particulate nitrate to be quantified under variable atmospheric conditions.