Received: 19 Jan 2021 – Accepted for review: 24 Jan 2021 – Discussion started: 25 Jan 2021
Abstract. Mechanisms of tropospheric ozone (O3) formation are generally well understood. However, studies reporting on net ozone production rates (NOPRs) directly derived from in-situ observations are challenging, and are sparse in number. To analyze the role of nitric oxide (NO) in net ozone production in the upper tropical troposphere above the Atlantic Ocean and the West African continent, we present in situ trace gas observations obtained during the CAFE-Africa (Chemistry of the Atmosphere: Field Experiment in Africa) campaign in August and September 2018. The vertical profile of in situ measured NO along the flight tracks reveals lowest NO mixing ratios of less than 20 pptv between 2 and 8 km altitude and highest mixing ratios of 0.15–0.2 ppbv above 12 km altitude. Spatial distribution of tropospheric NO above 12 km altitude shows that the sporadically enhanced local mixing ratios (> 0.4 ppbv) occur over the West African continent, which we attribute to episodic lightning events. Measured O3 shows little variability in mixing ratios at 60–70 ppbv, with slightly decreasing and increasing tendencies towards the boundary layer and stratosphere, respectively. Concurrent measurements of CO, CH4, OH and HO2 and H2O enable calculations of NOPRs along the flight tracks and reveal net ozone destruction at −0.6 to −0.2 ppbv h−1 below 6 km altitude and balance of production and destruction around 7–8 km altitude. We report vertical average NOPRs of 0.2–0.4 ppbv h−1 above 12 km altitude with NOPRs occasionally larger than 0.5 ppbv h−1 over West Africa coincident with enhanced NO. We compare the observational results to simulated data retrieved from the general circulation ECHAM/MESSy Atmospheric Chemistry (EMAC) model. Although the comparison of mean vertical profiles of NO and O3 indicates good agreement, local deviations between measured and modelled NO are substantial. The vertical tendencies in NOPRs calculated from simulated data largely reproduce those from in situ experimental data. However, the simulation results do not agree well with NOPRs over the West African continent. Both measurements and simulations indicate that ozone formation in the upper tropical troposphere is NOx-limited.
concerning the measurement techniques, I'd like to add the info that the chemiluminescence data is calibrated by the data from the UV photometer.
So on p.6, we should change l.169 to read as "O3 was quantified with a chemiluminescence detector calibrated by a UV photometer (Fast AIRborne Ozone Instrument; [...]"
...and in table 1 (p.7, l.184ff), in the "Technique / method" column, that would be a change to "UV photometry / chemiluminescence".
Thanks to Andreas Zahn for pointing me to info missing. The reference Zahn et al. 2012 only addresses the chemiluminescence detector and not the whole FAIRO instrument.
Central role of nitric oxide in ozone production in the upper tropical troposphere over the Atlantic Ocean and West AfricaIvan Tadic and Horst Fischer https://doi.org/10.5281/zenodo.4442616
Ivan Tadic et al.
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Although mechanisms of tropospheric ozone (O3) formation are well understood, studies reporting on ozone formation derived from field measurements are challenging and remain sparse in number. We use airborne measurements to quantify nitric oxide (NO) and O3 distributions in the upper troposphere over the Atlantic Ocean and West Africa and compare our measurements to model simulations. Our results show that NO and ozone formation are greatest over the tropical areas of West Africa.
Although mechanisms of tropospheric ozone (O3) formation are well understood, studies reporting...