Impact of the South Asian monsoon outflow on atmospheric hydroperoxides in the upper troposphere

During the OMO (Oxidation Mechanism Observation) mission, trace gas measurements were performed onboard 15 the HALO (High Altitude LOng range) research aircraft in summer 2015 in order to investigate the outflow of the south Asian summer monsoon and its influence on the composition of the Asian Monsoon Anticyclone (AMA) in the upper troposphere over the eastern Mediterranean and the Arabian Peninsula. This study focuses on in situ observations of hydrogen peroxide (H2O2 ) and organic hydroperoxides (ROOH), as well as their precursors and loss processes. Observations are compared to photostationary state calculations (PSS) of H2O2 , methyl hydroperoxide (MHP) and 20 inferred unidentified hydroperoxide (UHP) mixing ratios. Measurements are also contrasted to simulations with the general circulation ECHAM/MESSy for Atmospheric Chemistry (EMAC) model. We observed enhanced mixing ratios of H2O2 obs (45%), MHP PSS (9%) and UHP (136%) in the AMA relative to the northern hemispheric background. Highest concentrations for H2O2 obs and MHP of 211 ppbv and 152 ppbv, respectively were found in the tropics outside the AMA, while for UHP, with 208 pptv highest concentrations were found within the AMA. In general, the observed concentrations 25 are higher than steady-state calculations and EMAC simulations by a factor of 3 and 2, respectively. Especially in the AMA, EMAC underestimates the H2O2 EMAC (medians: 71 pptv vs. 164 pptv) and ROOH EMAC (medians: 25 pptv vs. 278 pptv) mixing ratios. Longitudinal gradients indicate a pool of hydroperoxides towards the center of the AMA, most likely associated with upwind convection over India. This indicates main contributions of atmospheric transport to the local budgets of hydroperoxides along the flight track, explaining strong deviations from steady-state calculations which only account for 30 local photochemistry. Deviations between H2O2 observations and EMAC simulations are most likely due to uncertainties in the scavenging efficiencies for individual hydroperoxides in deep convective transport to the upper troposphere, corroborated by a sensitivity study. It seems that the observed excess UHP is excess MHP transported to the west from an upper tropospheric source related to convection in the summer monsoon over South-East Asia. 35


Introduction
The earth has an oxidizing atmosphere where OH functions as the main oxidizing agent (Levy, 1971). OH is formed by the photolysis of ozone (λ<320 nm) and subsequent reaction of the produced singlet D oxygen atom (O 1 D) with water vapor.
The main sinks of OH are also the main sources of peroxy radicals (HO 2 and RO 2 ) in the reactions with CO, CH 4 and volatile organic compounds (VOCs) and the reaction with nitrogen dioxide (NO 2 ) to form nitric acid (HNO 3 ). At low NO x 40 (NO+NO 2 ) concentrations, HO 2 reacts with itself to form H 2 O 2 or with RO 2 to form organic hydroperoxides (ROOH). Since HO 2 and RO 2 , especially CH 3 O 2 , react faster with NO than with HO 2 , peroxides are mainly produced in areas with low NO and high OH mixing ratios (Lee et al., 2000). H 2 O 2 is a strong oxidant in the aqueous phase, oxidizing for example SO 2 to H 2 SO 4 , and hence H 2 O 2 partially contributes to acid rain formation (e.g. Hoffmann and Edwards, 1975;Penkett et al., 1979; Robbin Martin and Damschen, 1981;Calvert et al., 1985). The major photochemical sinks of hydroperoxides are photolysis, 45 which recycles OH, and the reaction with OH forms HO 2 . Physical loss of hydroperoxides due to dry and wet deposition establishes an ultimate loss mechanism of HO x radicals. Thus H 2 O 2 and ROOH play a pivotal role to the HO x -budget and modulate the oxidation capacity of the atmosphere (Lelieveld and Crutzen, 1990;Crutzen et al., 1999).
The global distribution of hydroperoxides is affected by transport, physical removal by dry deposition and rainout as well as net photochemical production processes. With increasing altitude, and thus decreasing water vapor concentration, the 50 primary production of HO x decreases (Heikes et al., 1996) and leads to an increasing contribution of the photolysis of H 2 O 2 and ROOH to the HO x budget (Jaeglé et al., 1997;Jaeglé et al., 2000;Faloona et al., 2000;Faloona et al., 2004). In more polluted areas, especially in the boundary layer, the H 2 O 2 chemistry is more complex and leads to higher variabilities (Nunnermacker et al., 2008). Close to the surface dry deposition of H 2 O 2 forms a strong sink resulting in decreasing concentrations with decreasing altitude. This often leads to a local maximum of H 2 O 2 mixing ratios above the boundary layer 55 at 2-5 km of altitude (Daum et al., 1990;Heikes, 1992;Weinstein-Lloyd et al., 1998;Snow, 2003;Snow et al., 2007;Klippel et al., 2011). A similar but weaker maximum at 2-5 km was found for methyl hydroperoxide (MHP) (Weinstein-Lloyd et al., 1998;Snow, 2003;Snow et al., 2007). Due to its lower deposition velocity associated with less efficient uptake by solid and aqueous surfaces, MHP is not as sensitive to deposition processes as H 2 O 2 Kok, 1986, 1994), yielding rather constant mixing ratios with altitude within the boundary layer. Further, the mixing ratios of both species generally 60 decrease with increasing latitude in the free troposphere due to lower water vapor concentrations (Jacob and Klockow, 1992;Perros, 1993;Slemr and Tremmel, 1994;Snow, 2003;Snow et al., 2007;Klippel et al., 2011).
In spite of several in situ measurement campaigns of trace gases in the outflow of the Asian summer monsoon in the recent years, e.g. from the IAGOS-CARIBIC project (Ojha et al., 2016;, the IAGOS-MOZAIC project (Barret et al., 2016), the MINOS aircraft campaign (Lelieveld et al., 2002) and the PEM-WEST A mission (Heikes et al., 65 1996) our understanding of the physical and chemical processes within the Asian Monsoon Anticyclone (AMA) is limited.
So far we know that the updrafts of the summer monsoon deep convection can effectively transport insoluble pollutants from the surface to the upper troposphere and there these polluted air masses can be transported over a long distance (Lawrence and Lelieveld, 2010). Thus the Asian summer monsoon has a strong influence on the upper troposphere (UT) and the lower stratosphere (Randel et al., 2010;Gettelman et al., 2004) and it is important to study its physical and chemical properties in 70 greater detail.
The focus of the OMO (Oxidation Mechanism Observation) campaign was to investigate photochemical processes in the AMA in the UT. During the mission, HALO probed a large variety of air masses, ranging from clean northern hemispheric (NH) background air above the western Mediterranean, southern hemispheric (SH) background air over the northern Indian Ocean and air masses affected by the South Asian summer monsoon in the AMA over the Arabian 75 Peninsula. The main goals of the campaign were to analyze the influence of the AMA on the oxidizing power of the atmosphere and to determine the rates at which natural and human-made compounds are converted by oxidation processes in the atmosphere (Lelieveld et al., 2018).
The present study addresses the budgets of H 2 O 2 and organic hydroperoxides. Since the measurements of the sum of all organic hydroperoxides do not differentiate between different species, we estimate the contribution from MHP PSS based on 80 steady-state calculations. In former studies MHP was identified as the most abundant organic hydroperoxide in the free troposphere (Heikes et al., 1996;Jackson and Hewitt, 1996). Our goal was to investigate to what extent this is also the case for the outflow of the South Asian summer monsoon into the UT. In addition the in situ data were compared to results from the EMAC model (see 3.5) along the flight track for H 2 O 2 and individual ROOH mixing ratios. H 2 O 2 obs mixing ratios were also evaluated with steady-state calculations based on measured HO x and photolysis frequency measurements onboard of 85 HALO.

The OMO project
The OMO campaign took place from 21 st of July to 27 th of August 2015. During the campaign 17 flights with the HALO (High Altitude and LOng range) research aircraft were performed. The airports of Oberpfaffenhofen (Germany), Paphos (Cyprus), Gan (Maldives) and Bahrain served as bases for take-offs and landings. The flights were mainly performed over 90 the Arabian Peninsula, the Eastern Mediterranean and the Northern Indian Ocean (11.3-80.2°E and 0.2°S-48.1°N). In Figure   1 the tracks of all OMO flights are shown. The aircraft reached altitudes up to 15 km which corresponds to 130 hPa to study the chemistry of the UT.

Hydroperoxide measurements 95
The hydroperoxide data (H 2 O 2 obs and total organic hydroperoxides ROOH obs ) during OMO were obtained using a modified commercial instrument (AEROLASER, model AL2021, Garmisch-Partenkirchen, Germany) called HYPHOP (HYdrogen Peroxide and Higher Organic Peroxides monitor). The HYPHOP-instrument was installed in a 19'' rack together with the IR-laser absorption instrument TRISTAR (Tracer In Situ Tdlas for Atmospheric Research) mounted in the back of HALO. Air was sampled from the top of the aircraft fuselage through a forward-facing trace gas inlet (TGI) designed as a bypass, 100 consisting of a ½'' PFA (perfluoroalkoxy alkanes) tube inside the aircraft with an exit through a second TGI. From this bypass air was sampled at a flow rate of 2 slpm (standard liter per minute) through a ¼'' PFA tube and directed to HYPHOP.
To obtain constant pressure at the HYPHOP inlet a constant pressure inlet (CPI) consisting of a dual stage membrane pump (Vacuubrand MD1C VARIO SP, Wertheim, Germany) was used (Klippel et al., 2011).
HYPHOP relies on a dual enzyme detection method after transfer of gaseous hydroperoxides into a buffered solution 105 (potassium hydrogen phthalate/NaOH, pH 6) in a glass stripping coil Lazrus et al., 1986). This stripper also contains EDTA (ethylenediaminetetraacetic acid) to prevent the reaction of transition metal ions with the hydroperoxides. Additionally, formaldehyde (HCHO) is added to prevent the oxidation of dissolved SO 2 (in alkaline solutions HSO 3 -) by the hydroperoxides. Instead, HCHO and HSO 3 form hydroxymethyl sulfonate (HOCH 2 SO 3 -). After the stripping coil the hydroperoxide containing solution is divided into two channels. Catalase is added to one channel in order 110 to selectively destroy H 2 O 2 . This first channel thus measures only ROOH, while the second channel (without catalase) measures the sum of ROOH and H 2 O 2 . Since hydroperoxides cannot be detected by fluorescence directly, a second enzyme (horseradish peroxidase) and p-hydroxyphenylacetic acid (POPHA) are added to both channels. In a quantitative and selective reaction the enzyme catalyzes the oxidation of POPHA by hydroperoxides forming the fluorescent dye 6,6'dihydroxy-3,3'-biphenyldiacetid acid. After excitation at 326 nm with a Cd lamp, the fluorescence at 400-420 nm is 115 detected. To enlarge the fluorescence intensity sodium hydroxide is added.
In order to perform zero measurements, the sampled air is directed through a cylinder filled with hopcalite (MnO 2 and CuO) to eliminate H 2 O 2 , ROOH and Ozone. Since the efficiency of Hopkalit decreases with increased humidity, the air is dried beforehand with the help of orange gel (SiO 2 beads plus indicator).
To convert the detected signal into a concentration a 4-point calibration was performed before and after every flight. In the 120 first two steps a liquid standard of H 2 O 2 (1 μmol/L, freshly diluted from stock solution) followed by zero air is measured in both channels without catalase. Afterwards this is repeated with catalase in the ROOH channel for the last two steps. The sensitivity for both channels and the catalase efficiency are determined via this procedure. The concentration of the liquid standard is based on titration of the stock solution (10 mmol/L) with potassium permanganate.
To determine the stripping efficiency for H 2 O 2 , a gas phase standard based from a permeation source (Teflon tube filled with 125 30% H 2 O 2 in a temperature-controlled glass flask) is used at a constant flow rate of approximately 40 sccm diluted with synthetic air and measured with the instrument. The permeation rate of the source is quantified by collecting the output of the source into cooled water. The addition of hydrochloric titanium tetrachloride yields the formation of the yellow η 2peroxo complex [Ti(η 2 -O 2 )Cl 4 ] 2- (Pilz and Johann, 1974) whose concentration is determined via a UV photometer. The stripping efficiency of MHP was assumed to be 60% and that of H 2 O 2 100% (AEROLASER, 2006;Lee et al., 2000). 130 The inlet efficiency was determined with the help of the permeation source which was measured with and without the CPI.
In laboratory studies the inlet efficiency was determined to be 87% ± 3% decreasing during the campaign to 62.7% ± 0.8%, which is mainly due to the higher humidity.
The limits of detection (LOD) and precisions for H 2 O 2 and MHP (assuming total ROOH obs to be mainly MHP), respectively, have been calculated for each flight from the reproducibility (1σ standard deviation) of in-flight zero ( The total uncertainty calculated from statistical errors and uncertainties of liquid standard, inlet and stripping efficiency and ozone interference is 25% for H 2 O 2 and 40% for MHP.

Other in situ measurements 145
For this study CO, CH 4 , OH, HO 2 , O 3 , Acetone, NO, NO y , J H2O2 and J MHP data measured by other instruments have been used for data interpretation, steady-state calculations and interference corrections (see section 3.1). A complete list of all measured compounds can be found in Lelieveld et al., 2018. CO and CH 4 have been measured by the IR-quantum cascade laser absorption spectrometer TRISTAR (Schiller et al., 2008;Tadic et al., 2017). The measurements comprised an ambient air mode and in-flight calibrations. The latter were realized with secondary standards from pressurized bottles (6 L bottle, 150 Auer GmbH, Germany), which were calibrated against certified reference gases (Tomsche et al., 2019). With the help of the in-flight calibrations the in situ data are drift-corrected by interpolation between two calibrations (Tadic et al., 2017). The observed CO and CH 4 mixing ratios have a total uncertainty of 5.1% and 0.275%, respectively. The relatively high CO uncertainty reflects problems with the stability of the CO quantum cascade laser during the second half of OMO.
Laser induced fluorescence was the method utilized for HO x measurements (instrument name: HORUS, Faloona et al., 2004;155 Martinez et al., 2010). The accuracies of the measurements are 17.1% for OH and 17.6% for HO 2 . The limit of detection of the instrument does vary depending on altitude as this system has a sensitivity that depends on pressure. As altitude increases the LOD decreases from 0.1 ppt v to 0.02 ppt v for OH and 0.361 ppt v to 0.175 ppt v for HO 2 .
FAIRO (Fast AIRborne Ozone instrument) is a light-weight (14.5 kg) and accurate 2-sensor device for measuring O 3 . It combines two techniques, i.e. (a) a UV photometer that measures the light absorption by O 3 at a wavelength of λ = 250-160 260 nm emitted by a UV-LED and (b) a chemiluminescence detector that monitors the chemiluminescence generated by O 3 on the surface of an organic dye adsorbed on dry silica gel. These techniques are simultaneously applied in order to combine the high measurement accuracy of the UV photometry with the high measurement frequency of the chemiluminescence detection. The UV photometer has a 1-σ precision of 0.08 ppb v at a measurement frequency of 0.25 Hz (and a pressure of 1 bar) and an accuracy of 1.5% (determined by the uncertainty of the O 3 cross section). The chemiluminescence detector has 165 a precision of 0.05 ppb v at a measurement frequency of 12.5 Hz (Zahn et al., 2012). In post-processing the chemiluminescence detector data is calibrated using the UV photometer data.
Nitrogen oxide (NO) and total reactive nitrogen (NO y ) were measured using the AENEAS-atmospheric nitrogen oxides measuring system. The measurements were performed by a dual channel NO-chemiluminescence detector (CLD-SR 790, Eco Physics, Switzerland) in combination with a converter technique for the detection of total reactive nitrogen as NO. NO y 170 comprises among others NO, NO 2 , HNO 3 , NO 3 , N 2 O 5 , HNO 2 , HO 2 NO 2 , PAN and organic nitrates. The individual NO y species were detected after conversion to NO using a gold tube maintained at about 300 °C with H 2 as a reducing agent (Ziereis et al., 2000). Ambient air was sampled using a standard HALO trace gas inlet equipped with a heated (~ 40 °C) PFA inlet line. The time resolution of the measurements was about 1 s. The overall uncertainty of the NO and NO y measurements depends on its ambient concentrations and is about 8% (6.5%) for volume mixing ratios of 0.5 nmol/mol (1 nmol/mol), 175 respectively (Stratmann et al., 2016).
VOCs (e.g. acetone) were measured with a homebuilt light-weight (~55 kg without rack) proton-transfer-reaction mass spectrometer which uses a commercial quadrupole mass analyzer (Pfeiffer, QMA 410, Germany). A modular V25 micro computer system (MPI-C, Mainz, Germany) is applied for instrument control and data acquisition. A custom-built inlet system comprises a platinum/quartz wool scrubber (Shimadzu, High Sensitivity Catalyst) held at 300 °C and components for 180 flow and pressure control. The instrument was calibrated between flights with a dynamically diluted gas standard containing approximately 500 ppb v of VOCs (Apel-Riemer Environmental Inc., USA). The accuracy for acetone is typically ±10% and the detection limit is ~60 ppt v .
Photolysis frequencies were calculated from spectral actinic flux density spectra (280-650 nm) obtained from CCD spectroradiometer measurements on the top and bottom fuselage of the aircraft covering the upper and the lower hemisphere, 185 respectively (Bohn and Lohse, 2017). Recent recommendations of absorption cross sections and quantum yields were used in the calculations, as well as their temperature and pressure dependencies (if available) by taking into account measured static air temperatures and pressures. Radiometric uncertainties range around 5-6% under typical flight conditions. Additional uncertainties related to the molecular parameters are process specific. For H 2 O 2 in particular, recommended absorption cross sections and their temperature dependencies were applied and unity quantum yields were assumed (Burkholder et al., 2015). 190 However, the recommended H 2 O 2 absorption cross sections are confined to a wavelength range below 350 nm which is insufficient to capture atmospheric photolysis completely. Because measured cross sections decay exponentially over two orders of magnitude in the range 280-350 nm, this dependence was further extrapolated up to 370 nm where values drop well below 10 −22 cm 2 . Dependent on conditions this extrapolation increases atmospheric H 2 O 2 photolysis frequencies by 10-20%. For MHP the temperature dependence of the absorption cross sections is unknown. Therefore the recommended room 195 temperature data were used under all conditions as well as unity quantum yields (Burkholder et al., 2015). Combined total uncertainties of 15% and 25% are estimated for H 2 O 2 and MHP photolysis frequencies, respectively.
Latitude, longitude and altitude data as well as temperature and pressure were collected with the BAHAMAS (BAsic HALO Measurement And Sensor system) instrument. More detailed information about the installation of scientific instruments and mission flights can be found on http://www.halo.dlr.de/science/missions/omo/omo.html. 200

Photo-stationary state calculations
Since only the sum of organic hydroperoxides was measured we estimated the contribution of MHP using a photostationary-state (PSS) approximation relying on in situ measurements of HO 2 , OH, CO, CH 4 , NO, J MHP and J H2O2 (see 3.2) and rate coefficient data from Atkinson et al., 2004 andAtkinson et al., 2006).
In the free troposphere the production rate P of H 2 O 2 and MHP is due to the self-reaction of HO 2 and reaction of CH 3 O 2 with 205 HO 2 , respectively, and can be calculated from Eq. 1 and 2.
Photochemical loss rates L of H 2 O 2 and MHP are due to photolysis and reaction with OH according to Eq. 3 and 4.
For steady-state conditions the production and loss reactions are at equilibrium and the MHP PSS to H 2 O 2 obs ratio can be calculated from Eq. 5. [ Because individual peroxy radicals were not measured, the CH 3 O 2 to HO 2 ratio must be estimated from their production and 215 loss terms. This ratio can be deduced as written in Eq. 6.
Dominant loss processes for HO 2 and CH 3 O 2 are reactions with NO and the production of H 2 O 2 and MHP, respectively, neglecting the production of peroxy nitrates due to low NO 2 concentrations in the UT (Eq. 7 and 8). 220 The first terms on the right side of both equations are identical. The second terms are dominated by the rate coefficients of the reactions with NO and the NO concentration. For the calculations of the rate coefficients the mean temperature of 259.18 K, the mean altitude of 10,992.8 m and the mean pressure of 22,932.9 Pa were used. The resulting values are shown in Eq. 9-11. As the relative humidity is very low in the upper troposphere the water dependence in eq. 11 was neglected. 225 k HO 2 +NO = 3.45 • 10 −12 • exp 270 T = 9.78 • 10 −12 cm 3 molecule•s , This indicates that the reaction of HO 2 with NO is more than a factor of 3 faster than the self-reaction. The measured NO concentration is an order of magnitude larger than measured HO 2 , so that reaction with NO is the dominant process for both 230 HO 2 and CH 3 O 2 resulting in similar loss rates for both radicals in the UT. Thus, the ratio of CH 3 O 2 to HO 2 is dominated by their production rates (Eq. 12).
The combination of Eq. 5 and 12 yields in Eq. 13 which was used to calculate the MHP PSS concentrations out of the observed mixing ratios during OMO. 235 Please note that other sources of HO 2 and CH 3 O 2 , in particular the photolysis of formaldehyde (HCHO) and acetaldehyde, respectively have been neglected. This is justified by the generally low mixing ratios of these species at high altitudes.
Measurements of HCHO with the TRISTAR instrument yielded values below the detection limit of 30 pptv, and although acetaldehyde was not measured, we assume that its mixing ratio is within a factor of two of those for HCHO. 240 The total uncertainty of MHP PSS from the calculation according to equation 13 can be deduced from error propagation taking into account uncertainties in OH obs (17.1%), J H2O2 obs (15%), J MHP obs (25%), CH 4 obs (0.275%), CO obs (5.1%), H 2 O 2 obs (25%) and rate constants, to be of the order of 45% (1σ).
To estimate the composition of the organic hydroperoxides the calculated concentration of MHP PSS was subtracted from the measured sum of all organic hydroperoxides ROOH obs . This leads to a concentration of unidentified organic hydroperoxides 245 (UHP PSS ) (Eq. 14).

Other research tools
The EMAC (ECHAM/MESSy Atmospheric Chemistry) model comprises the 5 th generation of the European Center 255 HAMburg (ECHAM5; Roeckner et al., 2006;version 5.3.01) general circulation model and the Modular Earth Submodel System (MESSy; Jöckel et al., 2016;version 2.52, http://www.messy-interface.org/). For this study EMAC simulations (T42L90, 2.8° x 2.8° horizontal resolution, 90 vertical levels to 0.01 hPa, time resolution 12 min) were sampled along the OMO flights tracks. Detailed specifications and results have been published previously (Lelieveld et al., 2018;Tomsche et al., 2019). 260 Ten days back-trajectories were calculated along the flight path using FLEXPART to identify the air mass origin (Tomsche et al., 2019). Convective transport can be simulated in FLEXPART with the convection parameterization by Emanuel K. A. and Zivkovic-Rothman M., 1999. To represent moist convection realistically in models, the parametrization includes cloud microphysical processes, the physics of entrainment and mixing, as well as large scale control of ensemble convective activity. It builds on temperature and humidity fields to provide mass flux information (Stohl et al., 2005). The back 265 trajectories in the present paper are calculated with the convective parametrization. Further the Lagrangian particle dispersion model FLEXPART produces so called centroid trajectories, based on the analysis of a cluster of trajectories.
These trajectories are comparable to traditional trajectories, but include convection via the centroid of all particles per time differentiate between air masses influenced by the AMA and background air. A comparison of vertical profiles indicated that the air inside the AMA showed significantly higher CH 4 concentrations than outside. Thus a threshold of CH 4 ≥1879.8 ppb v was used to distinguish between air masses influenced by the AMA (CH 4 ≥1879.8 ppb v ), the SH background (CH 4 <1820 ppb v ) and the NH background (1820 ppb v ≤CH 4 <1879.8 ppb v ) (Tomsche et al., 2019).

Data processing
Data were collected from a merged data set given as 60-second-means (calculated from the original data set obtained at higher resolutions) in order to get the same time resolution for all compounds. The given time is the middle of the block mean.
For the histograms the concentrations of all species shown were binned into samples with a width of 10 ppt v , starting the 280 plots with the lowest bin. To compare the simulations from EMAC with measured and PSS calculated data, the corresponding values (out of the 60-second-means) were used at the given times from EMAC.  (Figure 6), with a slope of 0.19±0.02 (ppb v /ppb v ) and an offset of (-0.003±0.02) ppb v . The regression coefficient R 2 is very high (0.82). For H 2 O 2 obs and ROOH obs the correlation is not that strong with slopes of -0.02±0.02 (ppb v /ppb v ) and 0.13±0.03 (ppb v /ppb v ) respectively and offsets of (0.21±0.02) ppb v and (0.11±0.03) ppb v ( Figure   6). The relation between ROOH mixing ratios and an air mass age tracer based on the ratio between [NO] to [NO y ] shows 320 higher values of ROOH at smaller ratios representing older or more processed air masses (Figure 7), since highest ROOH mixing ratios (>200 ppt v ) are found at the lowest [NO]/[NO y ] ratios (all <0.19). Thus, most of the observed ROOH was measured in aged air masses transported within the anticyclone. The correlation with UHP PSS shows that this effect is mainly due to UHP PSS . For H 2 O 2 obs there are also some higher mixing ratios for high [NO] to [NO y ] mixing ratios and thus fresher air (Figure 7). 325

Results for the entire campaign
To extend the analysis to the entire campaign, Figure 8 shows all flight tracks in the UT during OMO. The color-code represents observed mixing ratios of H 2 O 2 obs , MHP PSS and UHP PSS varying from low (purple) to high values (red).
Histograms for the whole campaign of H 2 O 2 obs mixing ratios as well inferred MHP PSS and UHP PSS mixing ratios are presented in Figure 9. Here only data from the UT (<300 hPa which corresponds to altitudes >9 km) were included in the 330 analysis. Mixing ratios for all species were further differentiated by methane levels, such that data in air masses with CH 4 mixing ratios above the threshold of 1879.8 ppb v were classified as AMA influenced, while air masses with a CH 4 mixing ratios between 1820 ppb v and 1879.8 ppb v were classified as NH background and those with CH 4 <1820 ppb v as SH ratios, representing the oldest, i.e. chemically most processed air masses (Figure 11).

H 2 O 2 steady-state calculation
A scatter plot of the results from the H 2 O 2 PSS based on observed HO x data in the UT (eq. 15) is shown in Figure 12. The black dotted line shows the 1:1 line, the green dashed lines represent the 2:1 and 1:2 relations. It is obvious that the 360 comparison is affected by a rather large offset of approximately 350 ppt v in the observations that is not accounted for in the steady-state calculations. The regression coefficient R 2 is 0.26 with most of the H 2 O 2 PSS mixing ratios (75%) varying between 0 and 65 ppt v with a median value of 15 ppt v , while the H 2 O 2 obs extend over a larger range mainly between 10-210 ppt v with a median of 150 ppt v and thus 10 times higher than for steady-state, which can also be seen in the histograms in Figure 13.   In the NH background, the median UHP PSS mixing ratio from the observations is 70 ppt v higher than EMAC simulations (78 ppt v and 8 ppt v respectively). In the AMA the difference is even larger, with about 200 ppt v higher UHP PSS levels compared to the EMAC simulations. The smallest difference with a factor of four was found for the SH (Table 2). 390

Longitudinal gradients
So far discussions of different air masses have been based on measurements of methane, subdividing the observations in NH background, AMA and SH data. Tomsche et al., 2019) have shown that longitudinal gradients are found in the AMA over the Arabian Peninsula. Observations in the west are often near the edge of the anticyclone, while observations towards the east are closer to its center. In Figure 16 observations, steady-state calculations and EMAC simulations for upper 395 tropospheric (9-15 km) H 2 O 2 are displayed as a function of longitude from west to east (20-30 °N, 36-60 °E, according to the red box in Figure 15). To identify gradients, the data are subdivided into bins of 2° longitude. are based exclusively on observed concentrations of HO 2 and OH radicals and thus yield only the net photochemical production, while the EMAC simulations and the observations will also account for vertical and horizontal advection from up-wind source regions. Previous studies show inconsistent results. Snow et al. (2007) and Barth et al. (2016) for example both show that H 2 O 2 is depleted in convective outflow compared to background upper troposphere. In contrast, other studies found that deep convection can be a source of H 2 O 2 in the upper troposphere (e.g. Jaeglé et al., 1997;Prather and Jacob, 405 1997;Mari et al., 2003;Bozem et al., 2017). Similarly, convection over India during the summer monsoon is a potential source of excess H 2 O 2 in the upper troposphere. With a photochemical lifetime of several days, this excess in H 2 O 2 reaches the western AMA, giving rise to the observed and model simulated longitudinal gradients. Since the steady-state calculations do not account for transport this can explain the rather large deviation of 150 ppt v with the observations. Differences between observation and EMAC simulation could potentially arise due to uncertainties in the scavenging efficiency for H 2 O 2 , as the 410 chemistry does not seem to be a dominant cause of uncertainty.
Similar longitudinal gradients are also observed for measured total organic hydroperoxides (ROOH obs , green asterisks in Figure 17), inferred UHP PSS (black) as well as total ROOH EMAC (blue). Steady-state calculations of MHP PSS (pink) and simulations of MHP EMAC (yellow) show either no, or only weak longitudinal gradients. Assuming that MHP is also enhanced in the outflow of deep convection at least part of the enhancement in ROOH obs (and thus inferred UHP PSS ) could be due to 415 advected MHP.

Discussion
To our knowledge we present the first observations of H 2 O 2 and ROOH mixing ratios in the Asian Monsoon Anticyclone.
Previous studies have been mainly focused on the northern hemispheric upper troposphere. Several aircraft campaigns including peroxide measurements were performed over North America. They are summarized in Snow et al., 2007) (Table 2) shows that we found similar mixing ratios as in SONEX in the northern hemispheric 425 background of 115 ppt v and 64 ppt v for H 2 O 2 and MHP, respectively. Mixing ratios for both species reported for TOPSE and INTEX-NA are slightly higher than ours, which may be related to the lower altitude range of 6-10 km (in comparison to >9 km for OMO) in these studies. Previous observations have shown that H 2 O 2 and MHP show highest mixing ratios at altitudes between 2-5 km followed by a sharp decrease towards higher altitudes (see e.g. Daum et al., 1990;Heikes, 1992;Weinstein-Lloyd et al., 1998;Snow, 2003;Snow et al., 2007;Klippel et al., 2011). 430 Heikes et al. associated enhanced H 2 O 2 mixing ratios above 5 km in the North Pacific of the Asian coast (30 °N) with outflow from the typhoon Mireille (Heikes et al., 1996). These observations were made close to the source region for the AMA influenced air masses described here (see back trajectories in the case study of flight 17, Figure 2 or Tomsche et al., 2019). For MHP Heikes et al. (1996) found mixing ratios of 250-500 ppt v in the southern longitudinal section above 5 km, similar to median mixing ratios of 152 ppt v for MHP in SH air masses in the UT found in this study. 435 Although the mixing ratios observed during this study are similar to previous observations in the upper troposphere, one striking result is that a state-of-the-art global circulation model (EMAC) and a local steady-state calculation constrained by measured radical levels significantly underestimate H 2 O 2 mixing ratios in particular in the AMA. The general tendency is that the steady-state model produces the lowest values, with EMAC falling in between steady-state and observations (e.g. Figure 16). A comparison of the EMAC simulations for the two radicals that affect H 2 O 2 most strongly (OH and HO 2 ) yields 440 a rather good agreement. A scatter plot between modelled and observed HO 2 yields a slope of 0.72±0.01 (ppt v /ppt v ) and an offset of (4.30±0.09) ppt v with a regression coefficient R 2 of 0.58 (Figure 18 left). The OH data show more scatter with a tendency for EMAC to overestimate the mixing ratios (slope: 1.7±0.2 (ppt v /ppt v ); offset: (-0.1±0.1) ppt v ; regression coefficient R 2 : 0.09, Figure 18 right). Although there is rather good agreement between EMAC simulations and observations This is an indication that an additional H 2 O 2 source is accounted for in the global model and that the local photo-stationarystate assumption is not fulfilled. The additional source is attributed to transport associated with deep convection over India, yielding in an upwind source of H 2 O 2 that is significant throughout the western part of the AMA. In the AMA, clouds are absent, so that gas phase photochemical processes may determine the lifetime of H 2 O 2 . Based on observed OH obs levels and photolysis frequencies during OMO the H 2 O 2 lifetime in the upper troposphere is of the order of several days, sufficiently 450 long for the excess H 2 O 2 to reach the western parts of the AMA, producing the observed longitudinal H 2 O 2 gradient observed in both observations and EMAC simulations (Figure 16). The total amount of H 2 O 2 injected into the UT by convective outflow depends on the scavenging efficiency (Mari et al., 2000;Barth et al., 2016;Bozem et al., 2017).
Differences between H 2 O 2 obs and H 2 O 2 EMAC are most likely due to an overestimation of scavenging in the model as also pointed out by Klippel et al., 2011). To investigate this assumption we performed a sensitivity study with the wet scavenging 455 for all soluble species being switched-off globally. The result is shown in Figure 19. The H 2 O 2 mixing ratios significantly increase with longitude by a factor of 3-4 and thus to the level of H 2 O 2 obs .
There is a rather large uncertainty regarding the scavenging efficiency of MHP in deep convection (Barth et al., 2016). For the Trace A campaign Mari et al., 2000) found observed (modelled) enhancement ratios of postconvective to preconvective mixing ratios of 11 (9.5) for MHP and 1.9 (1.2) for H 2 O 2 . Such efficient transport in the Indian Summer Monsoon would 460 yield a strong source of upper tropospheric MHP explaining the large enhancement of ROOH obs in the AMA described here.
Please note that large enhancements of MHP EMAC and ROOH EMAC were not found in the sensitivity study with switched-off scavenging, indicating that the strong underestimation by the model of those species is not due to an overestimation of wet removal in convective clouds. It seems that a large part of the UHP PSS is actually MHP advected throughout the AMA after deep convective transport over India. In the EMAC simulations the transport of MHP is less efficient and thus MHP EMAC is 465 lower than MHP PSS and UHP PSS .

Conclusion
Hydrogen peroxide and organic hydroperoxides were measured during the OMO campaign in the upper troposphere in NH increasing values towards the center of the AMA. It is likely that at least a large part of UHP PSS is due to additional MHP from an up-wind source. A sensitivity study using EMAC with no scavenging tends to reproduce the observed longitudinal gradients in H 2 O 2 , although it does not increase the level of ROOH. The reasons for this different behavior are unclear. 485

Data availability
The data are available from the HALO database (https://halo-db.pa.op.dlr.de/, last access: 7 November 2019).

Author contributions
BH and SH were responsible for H 2 O 2 and ROOH measurements and data. BH conducted further data analysis and wrote the 490 original draft of the paper in close cooperation with HF. CH 4 and CO data were provided by LT, HO x data by DM, MM and HH, O 3 and acetone data by MN and AZ, photolysis frequencies by BB and NO and NO y data by HZ and GS. AP was responsible for the EMAC model simulations. JL was the principal investigator of the OMO mission. All authors were involved in the review and editing of the paper.