Abstract
Abstract. Nitrogen oxides (NOx≡NO+NO2) are centrally involved in the photochemical processes taking place in the Earth's atmosphere. Measurements of NO2, particularly in remote areas where concentrations are of the order of parts per trillion by volume (pptv), are still a challenge and subject to extensive research. In this study, we present NO2 measurements via photolysis–chemiluminescence during the research aircraft campaign CAFE Africa (Chemistry of the Atmosphere – Field Experiment in Africa) 2018 around Cabo Verde and the results of laboratory experiments to characterize the photolytic converter used. We find the NO2 reservoir species MPN (methyl peroxy nitrate) to produce the only relevant thermal interference in the converter under the operating conditions during CAFE Africa. We identify a memory effect within the conventional photolytic converter (type 1) associated with high NO concentrations and rapidly changing water vapor concentrations, accompanying changes in altitude during aircraft measurements, which is due to the porous structure of the converter material. As a result, NO2 artifacts, which are amplified by low conversion efficiencies, and a varying instrumental background adversely affect the NO2 measurements. We test and characterize an alternative photolytic converter (type 2) made from quartz glass, which improves the reliability of NO2 measurements in laboratory and field studies.
Highlights
Nitrogen oxides (NOx) represent the sum of NO and NO2, which can rapidly interconvert in the atmosphere in the presence of sunlight and O3 as shown in Reactions (R1) and (R2) (Jacob, 1999).NO + O3 → NO2 + O2 (R1)NO2 + hν → NO + O(3P) O(3 P) O2 −→ O3 (R2)Considering only these two reactions in atmospheric NOx chemistry, the so-called Leighton ratio represents NO2, NO and O3 in the photostationary state (PSS) as shown in Eq (1) (Leighton, 1961). kNO+O3 is the rate coefficient of Reaction (R1), and jNO2 is the photolysis frequency for NO2 in Reaction (R2)
We recommend the implementation of a monitoring system for both temperature and pressure within the photolytic converter, which is difficult to implement in the type 1 blue light converter but allows for a more accurate calculation of the decomposing share and a reliable correction of the NO2 signal
We have investigated a modified conventional blue light converter with a highly reflective and porous inner surface made from optical PTFE regarding its application during the research aircraft campaign CAFE Africa, which took place in August and September 2018 around Cabo Verde
Summary
Considering only these two reactions in atmospheric NOx chemistry, the so-called Leighton ratio represents NO2, NO and O3 in the photostationary state (PSS) as shown in Eq (1) (Leighton, 1961). kNO+O3 is the rate coefficient of Reaction (R1), and jNO2 is the photolysis frequency for NO2 in Reaction (R2)
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