Central role of nitric oxide in ozone production in the upper tropical troposphere over the Atlantic Ocean and western 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 western Africa, 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 western Africa, 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, 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 western Africa coincident with enhanced NO. We compare the observational results to simulated data retrieved from the general circulation model ECHAM/MESSy Atmospheric Chemistry (EMAC). Although the comparison of mean vertical profiles of NO and O3 indicates good agreement, local deviations between measured and modeled 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 western Africa. Both measurements and simulations indicate that ozone formation in the upper tropical troposphere is NOx limited.

Modification of a conventional photolytic converter for improving aircraft measurements of NO<sub>2</sub> via chemiluminescence

“…We linearly interpolated these instrumental background and calibration measurements to each entire flight. The NO data were processed as described by Tadic et al (2021) (5 pptv detection limit at 1 min integration time and 6 % relative uncertainty, 1σ ). Please note that the instrumental background for the NO data was determined at 5.0±5.3 pptv during a nighttime measurement on 26 August 2018 of NO as presented by Tadic et al (2021) and previously described by Lee et al (2009).…”

“…The NO data were processed as described by Tadic et al (2021) (5 pptv detection limit at 1 min integration time and 6 % relative uncertainty, 1σ ). Please note that the instrumental background for the NO data was determined at 5.0±5.3 pptv during a nighttime measurement on 26 August 2018 of NO as presented by Tadic et al (2021) and previously described by Lee et al (2009). The instrumental background determined via zero air measurement was similar with 4 ± 7 pptv (measured four to six times per MF).…”

“…For calculating the photostationary-state NO 2 concentrations during CAFE Africa, we assume that NO 2 production occurs through reaction of NO with O 3 (Reaction R1), HO 2 (Reaction R3) and RO 2 (Reaction R4). Tadic et al (2021) showed that RO 2 is well-represented by CH 3 O 2 during CAFE Africa via model simulations (80 % at 200 hPa of altitude and up to 90 % below), which we therefore use as surrogate for describing all organic peroxy radicals. In analogy to Leighton (1961), we describe NO 2 loss by photodissociation as shown in Reaction (R2).…”

“…A bypass line provided the instruments with air from the aircraft inlet for which the residence time depended on the ambient pressure level (for high altitudes < 0.1 s). The sample line temperature was approximately 25 • C. More details on the campaign can be found inTadic et al (2021).…”

Tropospheric ozone production and chemical regime analysis during the COVID-19 lockdown over Europe

“…We assume R z O 2 (the sum of all peroxy radicals) to be represented by CH 3 O 2 which was found to be a reasonable approximation for the comparable, though independent aircraft campaign CAFE Africa in 2018 as described in Tadic et al (2021).…”