Central role of nitric oxide in ozone production in the upper tropical troposphere over the Atlantic Ocean and West Africa

Abstract

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.

Comment on acp-2021-79

“…In contrast, MF12 showed clear signs of lightning, particularly when passing the identified convective area in the morning. The measured NO concentrations showed characteristic spikes with a maximum of 459 ppt v which is by more than one order of magnitude higher than the detected signals during MF14.NO concentrations for MF12 outside the convective areas were 169 ±21 ppt v which is very similar to NO levels during MF10.Backward trajectories for MF10 and MF12 (FigureS2of the Supplement) show that the air originated from the African con-Assuming dominant NO x -limited chemistry as suggested byTadic et al (2021), low NO background levels induce low O 3 levels. In the convective areas during MF14 with particular low NO concentrations, an O 3 destruction regime might have been present.…”

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“…An overview of the flight tracks is presented in Figure 2. More details on the campaign can be found in Tadic et al (2021).…”

Comment on acp-2021-79

“…14 measurement flights (MF) were performed with the HALO (High Altitude Long Range) research aircraft starting from the campaign base in Sal on Cape Verde (16.75 • N, 22.95 • W). A detailed description of the campaign is provided by Tadic et al (2021). In this paper, we report observations based on three measurement flights -MF10, MF12 and MF14.…”