What controls ozone sensitivity in the upper tropical troposphere?

. Ozone is after water vapor, the second most important contributor to the radiative energy budget of the upper troposphere (UT). Therefore, observing and understanding the processes contributing to ozone production are important for monitoring the progression of climate change. Nitrogen oxides (NO x ≡ NO + NO 2 ) and volatile organic compounds (VOC) are two main tropospheric precursors to ozone formation. Depending on their abundances, ozone production can be sensitive to changes in 5 either of these two precursors. Here, we focus on processes contributing to ozone chemistry in the upper tropical troposphere between 30 ◦ S and 30 ◦ N latitude, where changes in ozone have a relatively large impact on anthropogenic radiative forcing. Based on modeled trace gas mixing ratios and meteorological parameters simulated by the EMAC atmospheric chemistry - general circulation model, we analyze a variety of commonly applied metrics including ozone production rates (P(O 3 )), the formaldehyde (HCHO) to NO 2 ratio and the share of methyl peroxyradicals (CH 3 O 2 ) forming HCHO ( α (CH 3 O 2 )), for their 10 ability to describe the chemical regime. We show that the distribution of trace gases in the tropical UT is strongly influenced by the varying locations of deep convection throughout the year, and we observe peak values for NO x and P(O 3 ) over the continental areas of South America and Africa where lightning is frequent. We find that P(O 3 ) and its response to NO is unsuitable for determining the dominant regime in the upper troposphere. Instead, α (CH 3 O 2 ) and the HCHO/NO 2 ratio in combination with ambient NO levels perform well as metrics to indicate whether NO x or VOC sensitivity is prevalent. A 15 sensitivity study with halving, doubling and excluding lightning NO x demonstrates that lightning and its distribution in the tropics are the major determinants


Introduction
Ozone (O 3 ) is abundant in the stratosphere and makes life on earth possible by absorbing highly energetic UV radiation emitted by the sun (Rowland, 1991;Staehelin et al., 2001).In the troposphere, on the other hand, high O 3 levels have adverse effects on human health, plant growth and climate (Ainsworth et al., 2012;Cooper et al., 2014;Nuvolone et al., 2018).Groundlevel tropospheric ozone has received particular attention due to its role in causing cardiovascular and respiratory diseases (Nuvolone et al., 2018).Additionally, ozone can be detrimental to plants through limiting stomatal conductance and therefore the capability of to perform plants photosynthesis (Ainsworth et al., 2012;Mills et al., 2018).Ozone in the free troposphere is subject to particular focus due to its radiative forcing efficiency as a greenhouse gas and its contribution to global warming and climate change.In the upper troposphere (UT), ozone is the second most important greenhouse gas after water vapor and https://doi.org/10.5194/egusphere-2023-816Preprint.Discussion started: 4 May 2023 c Author(s) 2023.CC BY 4.0 License. et al., 1999;Tonnesen and Dennis, 2000a;Tadic et al., 2021).Other studies considered the so-called ozone production efficiency (OPE), which evaluates how many ozone molecules are formed by NO x before it is removed to reaction products such as HNO 3 or PAN (peroxyacetyl nitrate) (Liu et al., 1987;Trainer et al., 1993;Wang et al., 2018a).Low OPEs indicate a VOC-sensitive and high OPEs a NO x -sensitive regime.Similar approaches such as the ratio of O 3 and reactive nitrogen species (NO y ) or NO z (≡ NO y -NO x ) have also been reported (Milford et al., 1994;Sillman, 1995;Fischer et al., 2003;Peralta et al., 2021;Wang et al., 2022).A common method for determining the dominant regime in urban environments is the weekend ozone effect, where the response of O 3 levels to decreasing NO x mixing ratios on weekends is monitored (e.g., Fujita et al. (2003); Pusede and Cohen (2012); Nussbaumer and Cohen (2020); Sicard et al. (2020); Gough and Anderson (2022)).Another indicator is the ratio between formaldeyhde (HCHO) and NO 2 .Sillman (1995) originally suggested the ratio HCHO/NO y (NO y ≡ NO x + HNO 3 + organic nitrates) as a metric, which was later adjusted to the HCHO/NO 2 ratio.This metric evaluates the reaction of OH radicals with VOCs (ultimately leading to HCHO as a reaction intermediate) enhancing O 3 production in competition with the reaction of OH radicals with NO 2 , which decelerates O 3 formation (Tonnesen and Dennis, 2000b).The HCHO/NO 2 ratio has been widely applied in the literature based on ground-based measurements and satellite observations (e.g., Duncan et al. (2010); Jin et al. (2020); Xue et al. (2022)).The ratio of hydrogen peroxide (H 2 O 2 ) to HNO 3 is another metric used for regime analysis.Also initially suggested by Sillman (1995), it compares the HO 2 self-reaction (forming H 2 O 2 ) with the reaction of OH and NO 2 , both leading to termination of the HO x cycle.While the HO 2 self-reaction dominates over the formation of HNO 3 as a termination reaction, O 3 increases linearly with NO x .Recent studies using H 2 O 2 /HNO 3 include Wang et al. (2018b), Vermeuel et al. (2019) and Liu et al. (2021).The HCHO/NO 2 and H 2 O 2 /HNO 3 ratios, as well as the OPE require absolute values as reference points to determine the regime, which can vary depending on the ambient conditions; background mixing ratios are a major drawback of these metrics.Dyson et al. (2022) recently analyzed the dominant regime in Beijing using a method that considers the loss of OH, HO 2 and RO 2 radicals via reaction with NO x in comparison to the overall production of these radical species.The production thereby equals the overall radical loss via reaction with NO x , self-reaction and aerosol uptake, an idea which has been previously described by Sakamoto et al. (2019).Within a VOC-sensitive regime, HO 2 is predominantly lost via the reaction with NO, while in a NO x -sensitive regime, aerosol uptake plays a significant role in HO 2 loss.Dyson et al. (2022) found the transition to occur around 0.1 ppbv of NO.Cazorla and Brune (2009) and Hao et al. (2023) reported direct measurements of P(O 3 ) in a reaction chamber through observing changes in O x in a certain time interval.This technique can be used to determine the dominant chemical regime when correlated with ambient NO mixing ratios.Some studies (including Jaeglé et al. (1998), Wennberg et al. (1998) and Jaeglé et al. (1999)) have analyzed the dominant chemical regime in the UT.These studies focus on the U.S. and the North Atlantic and consistently report a linear correlation between P(O 3 ) and NO x based on aircraft observations, deducing a NO x -sensitive regime, while model simulations predict a P(O 3 ) decrease with high NO x .One explanation for these observations could be that these studies, published around 25 years ago, overestimated the NO x loss.We know today that the reaction rate of NO 2 and OH is much lower than previously assumed (Mollner et al., 2010;Henderson et al., 2012;Nault et al., 2016).The loss reaction of NO 2 with OH to HNO 3 does not play a significant role under the conditions in the upper troposphere (in contrast to low-altitude) so that the typical burden was observed with increasing lightning NO x .We suggest that this observed anti-correlation might not result from increased NO x loss as applicable for surface conditions, and might instead be an outcome of decreasing HO 2 with increasing NO.Pickering et al. (1990) reported a VOC-sensitive regime over the U.S. at 11 km altitude based on measurements in June 1985 and model simulations.A study by Dahlmann et al. (2011) indicates increasing P(O 3 ) with increasing NO at 250 hPa over Europe, implying a NO x -sensitive regime following the common definition.Shah et al. (2023) analyzed the relationship between the NO y /NO ratio and O 3 mixing ratios and assumed an NO x -sensitive regime over the Central U.S. based on a flight during the DC3 research campaign in 2012.Liang et al. (2011) analyzed changes in net ozone production with NO x in the Arctic troposphere and found a proportional relationship up to 10 ppbv NO x based on box model calculations and observations.While all studies have briefly touched upon the dominant chemical regime in the upper troposphere, a thorough analysis and a definition that is valid throughout the troposphere have not yet been reported.In view of ozone's major implications for the earth's radiative energy budget and climate change (particularly in the UT), O 3 sensitivity is highly relevant for understanding and monitoring which precursors and processes are most important for the O 3 budget at high altitudes in the troposphere.
In Nussbaumer et al. (2021a), we introduced a new metric α(CH 3 O 2 ) for determining the dominant regime, which presents the ratio of methyl peroxyradicals (CH 3 O 2 ) forming HCHO with NO versus the reaction of CH 3 O 2 with HO 2 .We have applied this metric to ground-based observations at three different sites in Europe and for aircraft observations during the 2022 BLUESKY research campaign in the upper troposphere over Europe (Nussbaumer et al., 2022).We found a change at high altitudes from a VOC-to a NO x -sensitive regime over the past two decades up to 2020, promoted by emission reductions during the COVID-19 pandemic.
In this study, we use α(CH 3 O 2 ) to analyze the dominant regime in the upper tropical troposphere between 30 • S and 30 • N latitude based on modeled trace-gas mixing ratios and meteorological parameters by the EMAC atmospheric chemistrygeneral circulation model.We additionally investigate the effects of NO x produced by lightning in six different tropical areas: the Pacific Ocean, South America, the Atlantic Ocean, Africa, the Indian Ocean and South East Asia.Finally, we provide a new definition for NO x -and VOC-sensitive regimes, which is valid throughout the troposphere.

Methods
2.1 Calculations of ozone production (P(O 3 )) and loss (L(O 3 )) rates The calculation of ozone production (P(O 3 )) and loss (L(O 3 )) rates was performed as presented in Section 2.1 of Nussbaumer et al. (2022) The resulting net ozone production rate (NOPR) is then calculated by subtracting ozone loss from its production as shown in Equation (4).
represents the share of methyl peroxyradicals forming HCHO with NO and OH versus the reaction with HO 2 yielding CH 3 OOH and is calculated as shown in Equation (5). (5) We demonstrated in previous studies that α(CH 3 O 2 ) can be used as a metric to determine the dominant chemical regime (Nussbaumer et al., 2021a(Nussbaumer et al., , 2022))  with NO and therefore the amount of O 3 formed is enhanced.This regime is referred to as NO x -sensitive.In comparison, for higher NO mixing ratios (here > 0.1 ppbv), α(CH 3 O 2 ) only shows minor changes with increasing NO and is almost constant.
The resulting slope is 0.16 ± 0.01 ppbv −1 .In this range, NO is so abundant that CH 3 O 2 reacts primarily with NO and changes in NO have almost no impact on the reaction.The amount of O 3 formed is limited by the abundance of CH 3 O 2 , which itself is formed by a precursor VOC and no longer increases with increasing NO.This regime is referred to as VOC-sensitive.
Depending on where in this graph the data points from specific areas are located, it is possible to identify if a NO x -or a VOC-sensitive regime is dominant.

Modeling study
The data analyzed in this study were produced by model simulations using the ECHAM5 (fifth generation European Centre Hamburg general circulation model, version 5.3.02)/MESSy2(second-generation Modular Earth Submodel System, version 2.54.0)Atmospheric Chemistry (EMAC) model.Details on the EMAC model can be found in Jöckel et al. (2016).We applied EMAC in the T63L47MA-resolution, i.e., with a spherical truncation of T63 (corresponding to a quadratic Gaussian grid of 1.875 by 1.875 degrees in latitude and longitude) with 47 vertical hybrid pressure levels up to 0.01 hPa.Roughly 22 levels are included in the troposphere depending on the latitude, and the model has a time step of 6 minutes.The dynamics of the EMAC model have been weakly nudged in the troposphere (Jeuken et al., 1996) towards the ERA5 meteorological reanalysis data (Hersbach et al., 2020) of the European Centre for Medium-Range Weather Forecasts (ECMWF) to represent the actual day-to-day meteorology in the troposphere.The set-up adopted here is similar to the one presented in Reifenberg et al. (2022), using the anthropogenic emissions CAMS-GLOB-ANTv4.2(Granier et al., 2019), with varying monthly values for the period 2000-2019.The model has been extensively evaluated for ozone (e.g., Jöckel et al., 2016), showing a systematic though minor overestimation of the model compared to observations, which is a common feature in chemistry general circulation models of this complexity (Young et al., 2013).Comparison of the model results against numerous field campaigns (e.g., Lelieveld et al., 2018;Tadic et al., 2021;Nussbaumer et al., 2022)  For detailed analysis, six different areas are defined and their geographic extent is shown in Figure 2.These areas refer to the Pacific Ocean (cyan), South America (blue), the Atlantic Ocean (pink), Africa (red), the Indian Ocean (yellow) and South East Asia (green).
3 Results and Discussion

Development of trace gases over time
The analyzed trace gases do not show statistically significant trends over time from 2000 to 2019 at 200 hPa, which we show in Figure S1 of the Supplement.We find small global increases of some trace gases, e.g., average NO and HO 2 mixing ratios increase by ∼ 5 % and average NO 2 and O 3 mixing ratios by up to 10 % from 2000 to 2019.Global mean temperature increases by approximately 1 • C over the 20 year period.Even though slight trends can be detected, the variability is high and the 1 σ standard deviation (grey shaded) is significantly larger than the variation over time and we therefore used a daily climatology from the 20-year period in order to simplify the calculations.June-August (JJA) and September-November (SON).Each grid cell extends over 1.875During JJA, peak NO mixing ratios are found north of the equator over Central America and North Africa.Average NO mixing ratios are 0.14 ± 0.07 ppbv in the northern and 0.09 ± 0.02 ppbv in the southern tropical hemisphere.The distribution therefore changes drastically compared to DJF.During SON, NO mixing ratios are similar to MAM and peak over South America and Central Africa.The highest NO mixing ratios are found in the locations of predominant deep convection, which vary throughout the year.During DJF, deep convection dominates in the southern hemisphere, and during JJA it is most prevalent in the northern hemisphere.In July, deep convection is highest over Central America, North Africa and South Asia (northern India).
In January, it is predominant over South America, Central to South Africa and North Australia (Yan, 2005).The areas where these convective processes are prevailing define the ITCZ where north-and southeasterly trade winds converge.Increased thunderstorm activity explains the occurrence of peak NO mixing ratios.Various studies have reported significantly increased lightning over land compared to the ocean, which is in line with the distribution of NO as shown in Figure 3 (Christian et al., 2003;Rudlosky and Virts, 2021;Nussbaumer et al., 2021b).South East Asia is often referred to as the "maritime" continent.This region experiences frequent cumulonimbus activity, but the convective available potential energy (CAPE) is less compared with that over the South American and in particular the African land masses.This region therefore shows lower NO mixing ratios throughout the year.The relative distribution of NO 2 is very similar to NO, which we show in Figure S2 of the Supplement.On average, NO 2 mixing ratios are around a factor of 7 lower compared to NO.

NOPR
The tropical UT distribution of net ozone production rates is closely related to the distribution of NO (Figure 3).We show the model-calculated results for each period in Figure S5  in the southern tropical hemisphere and more than twice as high in the northern tropical hemisphere with 0.36 ± 0.15 ppbv h −1 .
The production of O 3 outweighs its loss by a factor of 8 on average for the studied conditions.The difference is larger in regions with peak NOPRs, e.g., over South America with a factor of 11, and smaller in regions with low NOPRs, e.g., over the Pacific Ocean with a factor of around 7. We show the distribution of both P(O 3 ) and L(O 3 ) in Figures S6 and S7 of the Supplement.
These results are in line with findings by Apel et al. (2015), who reported enhanced ozone production for high lightning NO x over the U.S. during a research flight in June 2012 as part of the DC3 campaign.have no or only little effect.We will discuss the implications of α(CH 3 O 2 ) for the dominant chemical regime in the tropical UT and specific regions in the following section.for NO x sensitivity and HCHO/NO 2 < 1 for VOC sensitivity (Duncan et al., 2010).These threshold values are not valid in the upper troposphere due to the vertical gradients of the trace gases.However, the HCHO/NO 2 ratio can also indicate the transition from a NO x -to a VOC-sensitive regime when binned to NO mixing ratios, which does not require any absolute threshold values.Within the NO x -sensitive regime, the HCHO/NO 2 strongly decreases with small increases in NO, and within the VOC-sensitive regime it is mostly unresponsive to changes in NO.Depending on where in these plots a specific data point or an average of several data points is located, it is possible to derive the dominant chemical regime.

α(CH
As explained earlier, it is not possible to determine the dominant chemical regime from ozone formation rates P(O 3 ) in the upper troposphere, as the formation of HNO 3 plays a minor role at UT altitudes and therefore does not lead to a decrease in P(O 3 ), which in theory indicates the dominance of VOC over NO x sensitivity.In fact, P(O 3 ) does decrease for NO mixing ratios above around 0.7 ppbv, but for a different reason, as shown in Figure S10   for NO vs. the HCHO/NO 2 ratio.For South East Asia, the Indian Ocean and the Pacific Ocean, NO mixing ratios are below 0.1 ppbv and HCHO/NO 2 ratios are high with values of 6.3, 8.5 and 10.9 ppbv ppbv −1 , respectively.For the Atlantic Ocean and Africa, the average NO mixing ratios are higher and the HCHO/NO 2 ratios are lower with values of 4.2 and 4.7 ppbv ppbv −1 , respectively.NO mixing ratios over South America are even higher, but the HCHO/NO 2 ratio is also higher with a value of 7.4 ppbv ppbv −1 .This underlines the limitation of using absolute threshold values for determining the dominant chemical regime.If a threshold for the regime transition were to be set to, e.g., 5 ppbv ppbv −1 , the South American UT would be characterized as NO x -sensitive, while it clearly shows VOC sensitivity.It is therefore important to consider the metrics used in relation to ambient NO mixing ratios, and it is best to use them in combination with other metrics.Figure S11 of the Supplement shows the mean values of the specified areas for P(O 3 ) vs. NO.While the computational tools presented above allow for a clear distinction between the regimes depending on location and time of the year, this indicator shows no differences.According to the surface-oriented definition for chemical regimes, all data points would be located in the NO x -sensitive regime.
Since NO and HO 2 mixing ratios, as well as NOPRs change throughout the year, the varying locations of deep convection also affect the dominant chemical regime.Areas with deep convection are potentially associated with lightning activity, resulting in higher NO mixing ratios that lead to VOC sensitivity.The continental areas of South America and especially Africa experience most lightning and therefore show the most VOC-sensitive regimes (Williams and Sátori, 2004).As the cumulonimbus clouds in South East Asia are mostly formed in maritime conditions, the region experiences significantly less lightning and therefore shows NO x sensitivity all year round.Ozone formation over the oceans is either NO x -sensitive or in the transition regime as lightning strikes are significantly less frequent in maritime compared to continental areas.Figure 7 presents a geographical distribution of the tropical UT colored by the slopes of the NO vs α(CH 3 O 2 ) data in each individual grid region to illustrate the dominating chemical regimes.Here, we present a map for MAM data.In Figure S12 of the Supplement, we show them for all periods.High values for the slopes and red colors (i.e.values well above 1) represent the predominantly While we focus on the upper troposphere in this study, α(CH 3 O 2 ) remains a suitable indicator for the dominant chemical regime at all altitudes.In Figure 8 we present the slopes of NO vs α(CH 3 O 2 ) by pressure altitude and longitudes, as an example for MAM data close to the equator (1 • N). White areas at the bottom (between 800 and 1000 hPa) result from the local surface topography (mountains).For the free troposphere, we find strong NO x sensitivity with a maximum of 38 ppbv −1 .In the upper troposphere lower stratosphere at pressure altitudes between 300 and 100 hPa, we observe the transitioning to a VOC-sensitive regime.For latitudes with strong lightning activity, including areas such as continental South America (-80 to -60 • longitude) and Africa (5 to 30 • longitude), the transition occurs in the upper troposphere, corresponding to pressure altitudes of 250-300 hPa.For latitudes with low lightning activity, for example, between 130 and 160 • longitude (South East Asia), the regime

Sensitivity study: lightning NO x
Three additional model runs were performed in order to investigate the impact of lightning NO x .First, lightning NO x was completely omitted.In second and third runs, NO x from lightning was halved and doubled, respectively, compared to the baseline scenario.The emissions of global lightning NO x in the baseline scenario amount to 6.2 Tg/year (estimated from the climatological data) in agreement with the work of Miyazaki et al. (2014).
Figure 9 shows the three previously discussed metrics (a) α(CH 3 O 2 ), (b) O 3 and (c) the HCHO/NO 2 ratio binned to NO mixing ratios for the modeling scenario excluding lightning NO x .As there are no significant differences between the periods, we show all-year data here.In Figure S16 and S17, we present the three metrics α(CH 3 O 2 ), O 3 and the HCHO/NO 2 ratio binned to NO mixing ratios for all periods and locations for halved and doubled lightning NO x , respectively.The transition region between the regimes occurs around 0.1 ppbv and is therefore not meaningfully different from that in the baseline scenario, but the distinction between the regimes is more conspicuous with increasing lightning NO x emissions.Figure S18 shows the average α(CH 3 O 2 ) vs.
NO for each considered location and for all modeled lightning NO x scenarios.As expected, in each location the data points shift to higher values for both NO and α(CH 3 O 2 ) with increasing influence of lightning NO x .When excluding lightning, the dominant regime changes to NO x -sensitive in all locations.Removing lightning also shows that lightning is by far the dominant source of NO x in the upper tropical troposphere.In maritime regions where lightning is relatively infrequent, NO x depends more strongly on advection from continental regions, formation from HNO 3 and aircraft emissions.A model run excluding NO x emissions from aircraft does not lead to significant differences compared to the baseline scenario, which we present by the black crosses in Figure S18.Excluding lightning shows that NO x mixing ratios also decrease significantly in maritime environments, including South East Asia where NO x mixing ratios drop from 90 pptv to 17 pptv on average.This illustrates that in maritime locations in the tropics, i.e., apart from South America and Central Africa, NO x mixing ratios are largely dependent on transported lightning NO x .For halved lightning NO x , NO x sensitivity also prevails in most locations.Only Africa and South America show a transition regime for the periods of the year with maximum lightning.For doubled lightning NO x , the qualitative regime observations are similar to the baseline scenario.The UT over Central Africa and South America is mostly VOC-sensitive, over South East Asia and the Indian Ocean it is NO x -sensitive and this layer is in the transition regime over the Pacific and Atlantic Ocean.Therefore, regions with frequent lightning are VOC-sensitive in the baseline scenario while the doubling of lightning NO x does not have a large impact in regions where lightning is generally infrequent.In accordance with our prior analysis, O 3 does not increase significantly from the doubling of lightning NO x .In the VOC-sensitive regime, increases in locations where it is currently sparse or if lightning decreases in areas where it is presently frequent.

Conclusions
We have investigated the dominance of NO x and VOC sensitivity in the upper tropical troposphere (200 hPa) between 30 • S and 30 • N latitude.The analyzed trace gas mixing ratios and meteorological parameters are calculated with the EMAC atmospheric chemistry -general circulation model for a 1.875 • x 1.875 • horizontal resolution and the years 2000-2019.One model run considers a baseline scenario and three additional ones with halved, doubled and excluded lightning NO x emissions, respectively.We find that the mixing ratios of the considered trace gases have not changed significantly in the upper troposphere over the past two decades and we therefore evaluate the average of the data which benefits from a higher statistical significance.
The distribution of the analyzed trace gases varies with the time of the year and the changing areas of deep convection, confined within the ITCZ.During DJF, maximum convection occurs over South America, Central to South Africa and North Australia and during JJA, over Central America, North Africa and continental Asia.As a consequence, NO x mixing ratios and net ozone production rates peak over South America, South Africa and North Australia during DJF, over South America and Central Africa during MAM and SON, and over Central America and North Africa during JJA, as deep convection brings increased thunderstorm and lightning activity, particularly over continental areas.The distribution of HO 2 mostly differs from NO x due to enhanced mixing ratios over South East Asia, where NO x is low year around.
We analyzed several commonly applied metrics for their potential to determine the dominant chemical regime in the upper troposphere, including ozone production rates P(O 3 ), the fraction of methyl peroxyradicals forming formaldehyde α(CH 3 O 2 ), and the ratio of HCHO to NO 2 .We show that α(CH We conclude that lightning is the major driver of the dominant ozone chemistry in the upper tropical troposphere.While it is still not fully understood how lightning activity will evolve in the future, it remains important to monitor and understand ozone production in the upper tropical troposphere, a process which has a major impact on the radiative energy budget, and in turn on global warming. https://doi.org/10.5194/egusphere-2023-816Preprint.Discussion started: 4 May 2023 c Author(s) 2023.CC BY 4.0 License.definition for a VOC-sensitive regime where O 3 production decreases with increasing NO x does not apply anymore.Khodayari et al. (2018) reported an NO x -saturated (VOC-sensitive) regime based on a modeling sensitivity study, where a decreasing O 3

Figure 1 .
Figure 1.Demonstration of how α(CH3O2) can be used as a metric to determine the dominant chemical regime.The black line shows the tropical background α(CH3O2) binned to NO mixing ratios.The grey error shades show the 1σ standard deviation.The red and blue dashed lines represent a NOx-sensitive and a VOC-sensitive regime, respectively.

Figure 2 .
Figure 2. Overview of the defined areas in the tropics between 30 • S and 30 • N latitude: Pacific Ocean (cyan), South America (blue), Atlantic Ocean (pink), Africa (red), Indian Ocean (yellow) and South East Asia (green).
reveals a good agreement between observations and model results of NO x and VOCs for locations in the UT.The reference simulation covers the time period 2000-2019 with hourly output of trace gas mixing ratios of O 3 , NO, NO 2 , OH, HO 2 , CH 3 O 2 , HCHO, CO, CH 4 and H 2 O, as well as the photolysis rates j(NO 2 ) and j(O 1 D) and meteorological parameters such as temperature and pressure, necessary for calculating net ozone production rates and α(CH 3 O 2 ).The data were post-processed to obtain daily values at local noon time and calculated for 200 hPa (upper troposphere) using bilinear interpolation between the hybrid pressure model levels.

Figure 3
Figure3shows the distribution of NO in the upper tropical troposphere (at 200 hPa).We find large changes throughout the year, which are related to the seasonality of deep convection and the location of the intertropical convergence zone (ITCZ).In order to illustrate the differences, we subdivide the data into four periods, December-February (DJF), March-May (MAM), https://doi.org/10.5194/egusphere-2023-816Preprint.Discussion started: 4 May 2023 c Author(s) 2023.CC BY 4.0 License.

Figure 4
Figure4shows the DJF and JJA distributions of HO 2 .An overview of all four periods can be found in FigureS3of the Supplement.Similar to NO, the spatial DJF distributions of HO 2 mixing ratios show peak values between 15 and 20 pptv over South America and South Africa.While NO shows minimum values over South East Asia and the Indian Ocean, HO 2 mixing ratios are elevated with average values of 13 ± 1 pptv and 12 ± 1 pptv, respectively.Mixing ratios are lower over the Pacific and the Atlantic Oceans with 8-9 ± 1 pptv on average and north of ∼ 20 • N.During JJA, HO 2 mixing ratios are elevated over 225

Figure 4 .
Figure 4. Distribution of HO2 in the tropical UT between 30 • S and 30 • N during DJF (top panel) and JJA (bottom panel).
of the Supplement.During DJF, NOPRs peak over South America with average values of 0.77 ± 0.20 ppbv h −1 and maximum values above 1 ppbv h −1 over southern Africa and northern Australia.In the course of the year and the local variations in deep convection, NOPR peaks move northwards, reaching the northernmost point in JJA, and moving southwards again in SON and DJF.During DJF, NOPRs are 0.36 ± 0.21 ppbv h −1 and 0.23 ± 0.09 ppbv h −1 in the southern and northern tropical hemisphere, respectively.During JJA, NOPRs are 0.17 ± 0.06 ppbv h −1

3 O 2 )Figure 5
Figure 5 shows the distribution of α(CH 3 O 2 ) in the tropical UT during DJF and JJA.We show all periods in Figure S8 of the Supplement.During DJF, α(CH 3 O 2 ) ranges from 0.77 to 0.95 with lowest values over South East Asia and highest values over South Africa and Australia.During JJA, lowest values are obtained over South East Asia, the Indian Ocean and over the Pacific and Atlantic Oceans around 10 • N latitude.Maximum values of up to 0.97 are reached over North Africa and the ArabianPeninsula.Therefore, as expected, α(CH 3 O 2 ) is proportional to NOPR and NO x mixing ratios and is anti-proportional to HO 2 mixing ratios.At low NO x /HO 2 ratios, increases in NO enhance α(CH 3 O 2 ), while at high NO x /HO 2 ratios, changes in NO

Figure 6
Figure 6 presents α(CH 3 O 2 ), O 3 and the HCHO/NO 2 ratio binned to NO mixing ratios during DJF, MAM, JJA and SON.The graphs show NO mixing ratios up to 0.5 ppbv, which includes 99.6 % of all data points.The frequency distribution of the NO data can be seen in Figure S9 of the Supplement.The black lines and the grey shades represent the average of all data points binned to NO and the associated 1 σ standard deviation.The colored data points show the average of the individual areas as shown in Figure 2. The error bars represent the 1 σ variability.Data for the Pacific Ocean are shown in cyan, for South America in blue, for the Atlantic Ocean in magenta, for Africa in red, for the Indian Ocean in yellow and for South East Asia in green.α(CH 3 O 2 ) (left column: (a), (d), (g) and (j)) increases strongly with NO for mixing ratios below 0.1 ppbv with a slope of 3.75 ± 0.44 ppbv −1 .For example, an average increase of α(CH 3 O 2 ) by 0.1 results from an increase of ambient NO by around 27 pptv.This characterizes the NO x -sensitive regime.In contrast, for NO mixing ratios higher than 0.1 ppbv NO, increasing NO has only a minor effect on α(CH 3 O 2 ) (slope = 0.16 ± 0.01 ppbv −1 ), which represents the VOC-sensitive regime.To reach an increase of α(CH 3 O 2 ) by 0.1, ambient NO needs to increase by 625 pptv, a factor of >20 higher compared to the low-NO x regime.Within the NO x -sensitive regime, predominantly CH 3 O 2 reacts with NO, forming O 3 , as well as with HO 2 , which does not result in formation of O 3 .With increasing NO, the share of the reaction with NO (compared to the reaction with HO 2 ) increases, which in turn enhances O 3 .In contrast, within the VOC-sensitive regime CH 3 O 2 radicals mostly react with NO in any case and increases in NO do not affect O 3 .This is illustrated in the middle column of Figure6((b), (e), (h) and (k)): O 3 increases with NO for low NO mixing ratios and reaches a plateau for high NO mixing ratios.While the shift from the NO x -

Figure 6 .
Figure 6.Different metrics to determine the dominant chemical regime.Left column: α(CH3O2), middle column: O3 and right column: HCHO/NO2 ratio, binned to NO mixing ratios for (a)-(c) DJF (December-February), (d)-(f) MAM (March-May), (g)-(i) JJA (June-August) and (j)-(l) SON (September-November). Black lines show averages of all data points and grey shades present the 1 σ standard deviation.Colored data points show the averages for the indicated areas and the 1 σ variability.
of the Supplement.Panel (a) presents P(O 3 ) binned to NO, which increases for low NO, reaches a plateau around 0.6-0.7 ppbv NO and decreases at higherNO.Panel (b)    shows NO x loss (L(NO x )) rates via OH, HO 2 and CH 3 O 2 , which are negligible compared to P(O 3 ) rates as shown in panel (a).Even though L(NO x ) increases with increasing NO, it is still only 6 % of the ozone production at 1 ppbv NO.The decrease in P(O 3 ) is therefore not associated with the formation of HNO 3 (as it is in the lower troposphere) but reflects the decrease of HO 2 with increasing NO (see panels (c)-(d)).The peak in P(O 3 ), therefore, does not provide an indication for a regime change.

Figure 6
Figure 6 (a) shows NO vs α(CH 3 O 2 ) during DJF.The tropical UT over the Indian Ocean and South East Asia is characterized by NO x sensitivity with NO mixing ratios between 80 and 90 pptv and an average α of 0.84 and 0.82, respectively.Ozone formation over South America is VOC-sensitive with an average NO mixing ratio of 222 pptv and an α of 0.90.The data points for the Pacific Ocean, the Atlantic Ocean and Africa are close to the transition point of the two regimes, with a tendency of the Pacific Ocean towards NO x and of the Atlantic Ocean and Africa towards VOC sensitivity.This is in line with Figure 6 (b) which presents NO vs. O 3 mixing ratios.The data points for South East Asia, the Indian Ocean and the Pacific Ocean are located mostly in the upsloping part of the curve, where O 3 strongly increases with increasing NO.The averages for the Atlantic Ocean, Africa and South America are located towards the flattening of the curve.Figure 6 (c) shows the DJF averages

Figure 6
Figure 6 (d) shows α(CH 3 O 2 ) binned to NO for MAM data.The UT over South East Asia is NO x -sensitive with values similar to DJF.Over the Indian Ocean, both the average NO mixing ratio and α(CH 3 O 2 ) increase to 130 pptv and 0.90, respectively, being located in the transition regime, together with the Pacific Ocean and the Indian Ocean.Minor changes from DJF to MAM occur over South America, which is still VOC-sensitive.A strong VOC sensitivity is calculated for the UT

Figure 7 .
Figure 7. Map of the tropical UT between 30 • S and 30 • N colored by the slopes of NO vs α(CH3O2) of the data in model grid regions.Red colors indicate NOx and blue colors VOC sensitivity.For gray areas the R 2 of the fit is below 30 %.

Figure 6
Figure 6 (g), (h) and (i) show similar graphs for JJA (June-August), indicating NO x sensitivity for the UT over South East Asia and the Indian Ocean, a transition regime for the Pacific Ocean, the Atlantic Ocean and South America and VOC sensitivity for Africa.During SON (September-November), as shown in Figure 6 (j), (k) and (l), South America shifts back to VOC sensitivity.All other regimes remain unchanged between JJA and SON.

Figure 8 .
Figure 8. Slopes of NO vs α(CH3O2) by pressure altitude and longitude on a log scale during the period March-May (MAM) and close to the equator (1 • N).Red colors indicate NOx and blue colors VOC sensitivity.White areas between 800 and 1000 hPa result from the local surface topography.
https://doi.org/10.5194/egusphere-2023-816Preprint.Discussion started: 4 May 2023 c Author(s) 2023.CC BY 4.0 License.change only occurs at the transition to the lower stratosphere -at a pressure altitude of around 150 hPa -which is characterized by strong NO x saturation.In Figure S13 of the Supplement we additionally show the dominant chemical regime, indicated by NO vs α(CH 3 O 2 ), on a global scale near the surface as the annual average.As we would expect, α(CH 3 O 2 ) indicates VOC sensitivity at the surface for all urbanized and industrialized regions characterized by high NO x emissions and NO x sensitivity for remote regions.Shipping routes, which are closer to the transition regime, can be distinguished from the pronounced maritime NO x sensitivity.
Figure S14 of the Supplement shows the subdivision into DJF, MAM, JJA and SON.The black lines representing the average of all data points show a similar course compared to the baseline scenario including lightning NO x , but the distinction between the regimes is less pronounced.Figure 9 (a) presents NO vs α(CH 3 O 2 ).At low NO mixing ratios, α(CH 3 O 2 ) increases with NO, indicating NO x sensitivity and for higher NO mixing ratios, α(CH 3 O 2 ) is only marginally affected by changes in NO, indicating VOC sensitivity.The tropical UT over all selected areas is clearly located within the NO x -sensitive chemical regime.The average NO mixing ratios range from 17 pptv over South East Asia to 33 pptv over Africa.Compared to the baseline scenario, excluding lightning NO x leads to a decrease of ambient NO levels by up to one order of magnitude.The average α(CH 3 O 2 ) ranges from 0.49 to 0.68 over South East Asia and Africa, respectively.The abundance of HO 2 in comparison to NO is therefore high, and a significant amount of CH 3 O 2 undergoes reaction with HO 2 , next to NO. Figure9(b) shows O 3 mixing ratios as a function of NO.O 3 mixing ratios first increase as a function of NO, reach a peak at around 0.05 ppbv NO and 85 ppbv O 3 and subsequently change little at higher NO levels.Note that the number of data points decreases rapidly for high NO mixing ratios.Only around 5.5 % of the data points represent NO values of > 0.05 ppbv.We show the associated frequency distribution in FigureS15of the Supplement.As expected, the data points of all selected areas are located at low NO and O 3 levels, at average O 3 mixing ratios ranging from 30 ppbv over South East Asia to 50 ppbv over Africa.Figure9(c) shows the HCHO/NO 2 ratio binned to NO mixing ratios.The tendencies of the average values (black line) are again similar to the one for the baseline scenario, but the absolute values for the HCHO/NO 2 ratio are higher.The highest average value occurs over South East Asia with 35.2 ppbv ppbv −1 and the lowest over the Atlantic Ocean with 13.2 ppbv ppbv −1 .All data points are therefore clearly located within the NO x -sensitive regime, which is in line with the findings from the correlation between NO and α(CH 3 O 2 ) from Figure9(a).

Figure 9 .
Figure 9. Determination of the dominant chemical regime in the tropical UT in the "no lightning" scenario via (a) α(CH3O2), (b) O3 and (c) the HCHO/NO2 ratio, binned to NO mixing ratios.Black lines show averages of all data points and grey shades present the 1 σ standard deviation.Colored data points show the averages for the indicated areas with the 1 σ variability.
the black curve representing the average of all data points of NO vs O 3 levels off at around 90 ppbv compared to 80 ppbv for the baseline scenario.This aids our understanding of NO x and VOC sensitivity in the upper troposphere, as all available HO 2 and CH 3 O 2 radicals react with NO within the VOC-sensitive regime and changes in NO x therefore do not affect changes in O 3 .https://doi.org/10.5194/egusphere-2023-816Preprint.Discussion started: 4 May 2023 c Author(s) 2023.CC BY 4.0 License.The sensitivity study of lightning NO x emphasizes two major aspects.First, lightning is the predominant source of NO x in the upper tropical troposphere as the mixing ratios drop to near zero when excluding it, and a model run excluding aircraft NO x does not show significant differences compared to the baseline scenario.Second, lightning and its distribution in the tropics, which is affected by the partitioning of continental and maritime areas and the varying locations of deep convection throughout the year, are the most important determinants of the dominant chemical regime in the UT.Our results additionally indicate that any future changes in lightning will only significantly affect O 3 levels in the upper troposphere if lightning substantially 3 O 2 ) and the HCHO/NO 2 are good indicators for the chemical regime in the upper troposphere, while P(O 3 ) is unsuitable.At the surface, NO x sensitivity is generally defined by increasing P(O 3 ) with NO, and VOC sensitivity by decreasing P(O 3 ) with NO.In the upper troposphere, this indicator is no longer valid as the reaction of NO 2 with OH does not play a significant role.Instead, under conditions of NO x sensitivity CH 3 O 2 undergoes reaction with both HO 2 and NO, and increasing NO leads to an enhancement of O 3 .For VOC-sensitive conditions, CH 3 O 2 predominantly reacts with NO and as the latter is present in excess it does not influence O 3 mixing ratios.In this case, ozone formation changes are governed by those in VOC, controlling the availability of peroxy radicals.The transition point can be read from the course of α(CH 3 O 2 ) and the HCHO/NO 2 ratio as a function of NO abundance.This definition of chemical regimes in terms of NO x and VOC sensitivity is valid throughout the entire troposphere.When assessing O 3 sensitivity in the upper troposphere based on trace gas measurements, α(CH 3 O 2 ) is to be preferred over the HCHO/NO 2 ratio as it can be https://doi.org/10.5194/egusphere-2023-816Preprint.Discussion started: 4 May 2023 c Author(s) 2023.CC BY 4.0 License.more easily determined from in-situ data.While NO and HO x measurements are commonly performed on research aircraft, for example, NO 2 measurements tend to suffer from the unselective detection or artifacts from reservoir species, which makes accurate quantification challenging.In the ITCZ over continental areas, ozone chemistry is mostly VOC-sensitive.The UT over South America and Africa is therefore VOC-sensitive apart from JJA and DJF, respectively, where chemistry moves towards the transition area.Over maritime areas, including South East Asia, ozone chemistry is mostly NO x -sensitive or in the transition regime depending on the time of the year.The metrics which are found to be good indicators for the UT, α(CH 3 O 2 ), O 3 mixing ratios and the HCHO/NO 2 ratio as a function of NO, show that the transition between a NO x -and a VOC-sensitive regime occurs around 0.1 ppbv NO.When decreasing or excluding lightning NO x , the considered areas are mostly dominated by a NO x -sensitive regime.
1) Ozone loss L(O 3 ) is calculated as shown in Equation (2) via the reaction of O 3 with HO 2 and OH and via photolysis.The latter only yields an effective ozone loss if O( 1 D) (resulting from O 3 photolytic cleavage) reacts with H 2 O instead of colliding with O 2 or N 2 (and reforming O 3 ).This share is represented by α O( 1 D) in Equation (3).
. While the formation of HCHO from CH 3 O 2 enhances O 3 formation, the reaction of CH 3 O 2 It therefore describes the background behavior of NO vs α(CH 3 O 2 ) for all data used in this study.The grey error shades show the 1 σ standard deviation resulting from the averaging.At low NO mixing ratios (here < 0.1 ppbv), α(CH 3 O 2 ) changes rapidly even with small changes in NO.The resulting slope of the linear fit of the data is 3.75 ± 0.44 ppbv −1 .In this range, CH 3 O 2 reacts both with NO and with HO 2 (and with itself).With increasing availability of NO, the reaction of CH 3 O 2 • x 1.875 • latitude and longitude and represents a 20-year average of the respective period.During DJF, NO mixing ratios are highest over South America, southern Africa and northern Australia with average peak values between 0.3 and 0.4 ppbv.Over the other tropical regions, NO mixing ratios are much lower with average values of 0.09 ± 0.01 ppbv over the Pacific and the Indian Oceans, 0.12 ± 0.02 ppbv over the Atlantic Ocean, 0.10 ± 0.03 ppbv over North Africa and 0.08 ± 0.01 ppbv over South East Asia.Generally, the mixing ratios over land are much higher than those over the ocean, and the mixing ratios north of the equator are lower with an average value of 0.09 ± 0.03 ppbv compared to south of the equator with 0.14 ± 0.06 ppbv.During MAM, NO mixing ratios over South America are similar to those during DJF with average values of 0.21 ± 0.05 ppbv.NO mixing ratios over Africa are much higher compared to DJF with 0.28 ± 0.11 ppbv on average and peak values of 0.53 ppbv.The relatively high mixing ratios relocate from South to Central Africa from DJF to MAM and also over the Arabian Peninsula and South Asia, including India.Mixing ratios over Australia are around 0.15 ppbv and therefore approximately half of those during DJF and are similar over South East Asia.