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

Andersen, Simone T.; Carpenter, Lucy J.; Nelson, Beth S.; Neves, Luis; Read, Katie A.; Reed, Chris; Ward, Martyn; Rowlinson, Matthew J.; Lee, James D.

Atmospheric nitrogen oxides (NO inline-formula+ NOinline-formula2inline-formula= NOinline-formulax) have been measured at the Cape Verde Atmospheric Observatory (CVAO) in the tropical Atlantic (16inline-formula51inline-formula N, 24inline-formula52inline-formula W) since October 2006. These measurements represent a unique time series of NOinline-formulax 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-formulax at very low levels. Data analysis methods for the NOinline-formulax 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-formulax 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.



Andersen, Simone T. / Carpenter, Lucy J. / Nelson, Beth S. / et al: Long-term NOx measurements in the remote marine tropical troposphere. 2021. Copernicus Publications.


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