<p>Atmospheric nitrogen oxides (NO + NO<sub>2</sub> = NO<sub>x</sub>) have been measured at the Cape Verde Atmospheric Observatory (CVAO) in the tropical Atlantic (16° 51' N, 24° 52' W) since October 2006. These measurements represent a unique time series of NO<sub>x</sub> in the background remote troposphere. Nitrogen dioxide (NO<sub>2</sub>) is measured via photolytic conversion to nitric oxide (NO) by ultra violet light emitting diode arrays followed by chemiluminescence detection. Since the measurements began, a <q>blue light converter</q> (BLC) has been used for NO<sub>2</sub> photolysis, with a maximum spectral output of 395 nm from 2006–2015 and of 385 nm from 2015. The original BLC used was constructed with a Teflon-like material and appeared to cause an overestimation of NO<sub>2</sub> when illuminated. To avoid such interferences, a new additional photolytic converter (PLC) with a quartz photolysis cell (maximum spectral output also 385 nm) was implemented in March 2017. Once corrections are made for the NO<sub>2</sub> artefact from the original BLC, the two NO<sub>2</sub> converters are shown to give comparable NO<sub>2</sub> mixing ratios (PLC = 0.92 × BLC, R<sup>2</sup> = 0.92), giving confidence in the quantitative measurement of NO<sub>x</sub> at very low levels. Data analysis methods for the NO<sub>x</sub> measurements made at CVAO have been developed and applied to the entire time series to produce an internally consistent and high quality long-term data set. NO has a clear diurnal pattern with a maximum mixing ratio of 2–10 pptV during the day depending on the season and ~0 pptV during the night. NO<sub>2</sub> shows a fairly flat diurnal signal, although a small increase in daytime NO<sub>x</sub> is evident in some months. Monthly average mixing ratios of NO<sub>2</sub> vary between 5 and 30 pptV depending on the season. Clear seasonal trends in NO and NO<sub>2</sub> levels can be observed with a maximum in autumn/winter and a minimum in spring/summer.</p>