The remote marine boundary layer at the tropics represents a baseline condition for the Earth's atmosphere. Air in these regions has typically not encountered emissions for days to weeks and thus has been subject only to processing; that is degradation and reaction of species in the air. Understanding the most basic chemistry occurring in the remote marine boundary layer is fundamental to our overall understanding of the atmosphere as a whole.
The source and manifestation of nitrogen oxides, NOx, in the remote marine boundary layer at the Cape Verde observatory and elsewhere has been intriguing. NOx observations could often not be reconciled easily with known chemistry. This, when coupled to the difficulty in measuring the very low concentrations present has led to various speculative theories which are thus-far unsubstantiated.
By carefully characterising the NO2 photolysis/NO chemiluminescence technique for artefacts, interferences, and uncertainties the applicability and limitations of the technique were established. It was found that peroxyacyl nitrates cause an interference of ~5% which can lead to significant errors in NO2 determination, especially in cold or high altitude
environments but are insignificant in the remote marine boundary layer at Cape Verde.
Using two years of data collected at Cape Verde and a 0-D model of tropospheric chemistry the budget of NOx was estimated. Evidence that nitric acid deposited on aerosol, previously thought to be the terminating step in the cycle of NOx, is able to be released back to the atmosphere as NO2 and HONO has been shown. This `renoxification' process likely represents the dominant source of NOx in the tropical marine boundary layer, counter to previous theory of long range transport of reactive nitrogen species.
The implication of these findings changes our fundamental understanding of NOx sources and sinks in the remote troposphere. Moreover, by understanding the limitations of NO2 measurement techniques allows for easier interpretation of sometimes puzzling data without the need for unprovable mechanisms.
Additionally, a new technique for quantifying atmospheric HONO was developed by exploiting the limitations of photolytic NO2 measurement and photolysing differentially at two different wavelengths.
