Long-term NOx measurements in the remote marine tropical troposphere.

Atmospheric nitrogen oxides (NO + NO 2 = NO x) 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 x in the background remote troposphere. Nitrogen dioxide (NO 2) 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 NO 2 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 NO 2 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 2 artefact from the original BLC, the two NO 2 converters are shown to give comparable NO 2 mixing ratios (BLC = 0.99 × PLC + 0.7 ppt, linear least-squares regression), giving confidence in the quantitative measurement of NO x at very low levels. Data analysis methods for the NO 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 ∼ 0 ppt during the night. NO 2 shows a fairly flat diurnal signal, although a small increase in daytime NO x is evident in some months. Monthly average mixing ratios of NO 2 vary between 5 and 30 ppt depending on the season. Clear seasonal trends in NO and NO 2 levels can be observed with a maximum in autumn and winter and a minimum in spring and summer. [ABSTRACT FROM AUTHOR]