Assessing the sources, chemistry and trends of nitrogen oxides in the remote oceanic atmosphere

Nitrogen oxides (NO and NO2, known as NOx) are key species in the ozone cycle and thus indirectly affect the atmosphere's oxidising capacity, as ozone photolysis produces the hydroxyl radical, OH. In the tropical marine boundary layer (MBL), low emissions, high relative humidities and photochemical activity result in high OH production rates. Whilst it is important to understand the chemical processes that take place in the tropical MBL, this region is remote therefore mixing ratios are low and measurements are challenging.

NOx has been measured at the Cabo Verde Atmospheric Observatory (CVAO) in the North Atlantic since 2006, using a high-sensitivity chemiluminescence instrument with low detection limits. The data processing steps used at the CVAO have been validated through comparison with those used at the Kennaook/Cape Grim Baseline Air Pollution Station in Tasmania. The long-term dataset from the CVAO was used to identify trends in NOx mixing ratios, particularly the effects different air masses reaching the site have on NOx values.

Photostationary state (PSS) analysis was performed on NO2 and HONO measurements from the CVAO, revealing that our understanding of the chemistry in the remote MBL is incomplete. The underestimation of PSS NO2 has been attributed to missing oxidants converting NO into NO2, with measured peroxy radicals at the CVAO not sufficient to reconcile measured and PSS values. HONO PSS analysis indicates that the photolysis of particulate nitrate (pNO3) is the main source of HONO in this environment, but more work needs to be done to determine the factors driving the enhancement of pNO3 photolysis compared to that of nitric acid.

Current NOx mixing ratios are low enough that net ozone destruction is seen in the MBL, but understanding NOx cycling over the oceans is necessary as increases in NOx could result in a shift to an ozone production regime.