Long-term NO _x measurements in the remote marine tropical troposphere

Atmospheric nitrogen oxides (NO inline-formula+ NOinline-formula₂ inline-formula= NOinline-formula_x) have been measured at the Cape Verde Atmospheric Observatory (CVAO) in the tropical Atlantic (16inline-formula^∘51inline-formula^′ N, 24inline-formula^∘52inline-formula^′ W) since October 2006. These measurements represent a unique time series of NOinline-formula_x in the background remote troposphere. Nitrogen dioxide (NOinline-formula₂) is measured via photolytic conversion to nitric oxide (NO) by ultraviolet light-emitting diode arrays followed by chemiluminescence detection. Since the measurements began, a blue light converter (BLC) has been used for NOinline-formula₂ photolysis, with a maximum spectral output of 395 nm from 2006 to 2015 and of 385 nm from 2015 onwards. The original BLC used was constructed with a Teflon-like material and appeared to cause an overestimation of NOinline-formula₂ 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 NOinline-formula₂ artefact from the original BLC, the two NOinline-formula₂ converters are shown to give comparable NOinline-formula₂ mixing ratios (BLC inline-formula= 0.99 inline-formula× PLC inline-formula+ 0.7 ppt, linear least-squares regression), giving confidence in the quantitative measurement of NOinline-formula_x at very low levels. Data analysis methods for the NOinline-formula_x 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 ppt during the day depending on the season and inline-formula∼ 0 ppt during the night. NOinline-formula₂ shows a fairly flat diurnal signal, although a small increase in daytime NOinline-formula_x is evident in some months. Monthly average mixing ratios of NOinline-formula₂ vary between 5 and 30 ppt depending on the season. Clear seasonal trends in NO and NOinline-formula₂ levels can be observed with a maximum in autumn and winter and a minimum in spring and summer.