# On the relative role of convection, chemistry and transport over the south pacific convergence zone during pem-tropics B: A case study

Céline Mari, Carine Saüt, Daniel Jacob, Amanda Staudt, Melody Avery, William Brune, Ian Faloona, Brian Heikes, Glen Sachse, Scott Sandholm, et al.

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Céline Mari, Carine Saüt, Daniel Jacob, Amanda Staudt, Melody Avery, et al. On the relative role of convection, chemistry and transport over the south pacific convergence zone during pem-tropics B: A case study. Journal of Geophysical Research, 2003, 107 (D2), pp.PEM 4-1-PEM 4-16. 10.1029/2001JD001466. hal-00138895

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# On the relative role of convection, chemistry, and transport over the South Pacific Convergence Zone during PEM-Tropics B: A case study

Céline Mari,<fnref n="1" /> Carine Saüt,<fnref n="1" /> Daniel J. Jacob,<fnref n="2" /> Amanda Staudt,<fnref n="2" /> Melody A. Avery,<fnref n="3" /> William H. Brune,<fnref n="4" /> Ian Faloona,<fnref n="5" /> Brian G. Heikes,<fnref n="6" /> Glen W. Sachse,<fnref n="7" /> Scott T. Sandholm,<fnref n="8" /> Hanwant B. Singh,<fnref n="9" /> and David Tan<fnref n="8" />

Received 6 November 2001; revised 24 April 2002; accepted 24 April 2002; published 16 November 2002.

[1] A mesoscale 3D model (Meso-NH) is used to assess the relative importance of convection (transport and scavenging), chemistry, and advection in the vertical redistribution of HOₓ and their precursors in the upper tropical troposphere. The study is focused on marine deep convection over the South Pacific Convergence Zone (SPCZ) during the PEM-Tropics B Flight 10 aircraft mission. The model reproduces well the HOₓ mixing ratios. Vertical variations and the contrast between north and south of the SPCZ for O₃ are captured. Convection uplifted O₃-poor air at higher altitude, creating a minimum in the 9–12 km region, in both modeled and observed profiles. The model captured 60% of the observed HCHO variance but fails to reproduce a peak of HCHO mixing ratio at 300 hPa sampled during the northern spirals. Simulated HCHO mixing ratios underestimate observations in the marine boundary layer. In the model, convection is not an efficient process to increase upper tropospheric HCHO, and HCHO is unlikely to serve as a primary source of HOₓ. Convection plays an important role in the vertical distribution of CH₃OOH with efficient vertical transport from the boundary layer to the 10–15 km region where it can act as a primary source of HOₓ. The SPCZ region acts as a barrier to mixing of tropical and subtropical air at the surface and at high altitudes (above 250 hPa). The 400–270 hPa region over the convergence zone was more permeable, allowing subtropical air masses from the Southern Hemisphere to mix with tropical air from NE of the SPCZ and to be entrained in the SPCZ-related convection. In this altitude range, exchange of subtropical and tropical air also occurs via airflow, bypassing the convective region SW and proceeding toward the north of the SPCZ.
**INDEX TERMS:** 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 3314 Meteorology and Atmospheric Dynamics: Convective processes; **KEYWORDS:** convection, SPCZ, PEM-Tropics, mesoscale model, HOₓ

**Citation:** Mari, C., et al., On the relative role of convection, chemistry, and transport over the South Pacific Convergence Zone during PEM-Tropics B: A case study, *J. Geophys. Res.*, 107, 8232, doi:10.1029/2001JD001466, 2002. [printed 108(D2), 2003]

## 1. Introduction

[2] Deep convection plays a critical role in determining the vertical structure of the atmosphere. From a large-scale point of view, convection is an essential dynamical process to uplift air from the boundary layer to higher altitudes, seeding the upper troposphere with pollutants. At these high altitudes, air more likely experiences long-range transport. Deep convection is thus a major actor for intercontinental and interhemispheric transport in the troposphere [Collins et al., 1999; Cooper et al., 2001]. At local scale, rapid convective injection yields air with a much different chemical signature than slow ascent [Pickering et al., 1996; Prather and Jacob, 1997; Gettelman et al., 2001]. However, assessing the chemical signatures of convection is a nontrivial exercise. Convection can have both a direct effect by uplifting poor or rich boundary

<footnote n="1">¹Laboratoire d'Aérologie, UMR CNRS/Université Paul Sabatier, Toulouse, France.</footnote>

<footnote n="2">²Division of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts, USA.</footnote>

<footnote n="3">³Department of Physics, Hampton University, Hampton, Virginia, USA.</footnote>

<footnote n="4">⁴Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania, USA.</footnote>

<footnote n="5">⁵Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA.</footnote>

<footnote n="6">⁶Center for Atmospheric Chemistry Studies, University of Rhodes Island, Narragansett, Rhodes Island, USA.</footnote>

<footnote n="7">⁷Aerospace Electronic Research Division, NASA Langley Research Center, Hampton, Virginia, USA.</footnote>

<footnote n="8">⁸Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA.</footnote>

<footnote n="9">⁹NASA Ames Research Center, Moffett Field, California, USA.</footnote>
layer material to the upper troposphere, and an indirect (sometimes counteracted) effect by replenishing upper troposphere with precursors or consumptors [Folkins et al., 2002]. In terms of the supply of hydrogen oxide radicals ($HO_x = OH + HO_2$), recent studies have shown that the upper troposphere is more photochemically active than previously thought due to the convective transport of $HO_x$ precursors [McKeen et al., 1997; Wennberg et al., 1998, 1999; Jaeglé et al., 1997, 2001]. An unresolved issue is the relative contribution of advection, convection and local chemistry to the in the upper troposphere.

[3] In this study, we present an analysis of the South Pacific Convergence Zone (SPCZ) during the PEM-Tropics B Flight 10 in March 1999 [Raper et al., 2001]. The SPCZ is a persistent, well defined convective cloud band and acts as a barrier between tropical and subtropical air masses of the western South Pacific ocean, particularly at lower tropospheric levels [Gregory et al., 1999; Vincent, 1994]. Thus, SPCZ is particularly well suited for a study of the vertical transport and scavenging of chemical species by convection with marked gradients of chemical concentrations both sides of the cloud band.

[4] Flight 10 has been well documented in two previous papers. Pickering et al. [2001] used a two-dimensional model to quantify the convective mixing, lightning $NO_x$ production and wet scavenging of soluble species during this flight. The authors found that little NO were produced by electrical discharges within the SPCZ convection. 90% of and $HNO_3$ and $H_2O_2$ were removed by convective precipitation during cloud transport in the 2D model. Convective transport of low NO marine boundary layer to the upper troposphere reduced ozone production by 60% compared to air undisturbed by convection. Mari et al. [2002] used a zero-dimensional model to contrast the relative contributions of different sources of upper tropospheric $HO_x$ during that flight. The conclusion was that the primary sources $HO_x$ of varied significantly along the flight track in correlation with the supply of water vapor. Production from acetone became significant in drier region south of 20°S. Over the SPCZ region, in the cloud outflow, $CH_3OOH$ transported by convection accounts for 22–64% of the total primary source. In this work, our approach is based on a three-dimensional model coupled on-line with chemistry and an entraining/detraining plume model. Our goal is to estimate the relative importance of the advection, convection, and in situ chemical processes in determining the vertical structure and evolution of $HO_x$ and their precursors ($H_2O_2$, $CH_3OOH$, and HCHO). The three-dimensional mesoscale model provides an integrative tool to study the interplay between the dynamical and physicochemical processes at regional scale.

## 2. Model Description

[5] The model used in this study is a nonhydrostatic mesoscale model, Meso-NH, coupled on-line with chemistry. A detailed description of the dynamics and chemistry of the model can be found in the literature (http://www.aeroobs-mip.fr/mesonh) [Lafore et al., 1998; Tulet et al., 2002; Mari et al., 1999; Suhre et al., 2000]. The on-line coupling more easily enables for a maximum coherence

between transport schemes (advection, convection, prognostic turbulence mixing) and the treatment of the chemical components. Model horizontal resolution is 40 × 40 km. Vertical resolution is 50 m in the boundary layer and 300 in the free troposphere up to 20 km, with a total of 62 levels. Time step is 50 s for dynamics. The chemical mechanism is integrated in the model with the QSSA solver [Hesstvedt et al., 1978]. TUV model is used to compute photolysis rates as a function of solar zenith angle [Madronich and Weller, 1990] (R. E. Shetter et al., Comparison of photolysis frequencies determined by scanning actinic flux spectroradiometry (SAFS) with the NCAR radiative transfer model (TUV) during the PEM-Tropics B mission, submitted to Journal of Geophysical Research, 2001) and a correction of the rate below and above cloud is applied [Chang et al., 1987]. A reduced chemical scheme is used [Crassier et al., 1999], including 39 prognostic species and 128 reactions. Scavenging by convective precipitation has been implemented following the work of Mari et al. [2000]. Large-scale forcing for dynamical parameters are provided by the ECMWF analysis. Initialization and boundary conditions of chemical species are provided by the Harvard/Florida State University (FSU) CTM model [Staudt et al., 2001] for the PEM-Tropics B meteorological period. Acetone and $CH_3OOH$ are not simulated well by that model for the PEM-Tropics B environment [Singh et al., 2001]. Therefore we imposed initial and boundary conditions for these species on the basis of the aircraft observations.

## 3. Meteorological Conditions

### 3.1. Background Meteorology During Flight 10

[6] Flight 10 was dedicated to the study of SPCZ convection [Mari et al., 2002; Pickering et al., 2001]. The flight traversed the extensive and strong band of convection, sampling air masses on both sides of the SPCZ and in the convective outflow (Figure 1). Two spiral points were located on either side of the SPCZ area. The flight started from Fiji on 20 March at 2230 UTC (20 March at 0930 Local Solar time) and ended on 21 March at 0615 UTC (20 March at 1715 Local Solar Time).

[7] The meteorology of the SPCZ is described in detail by Fuelberg et al. [2001] and Vincent [1994]. Here, we summarize the main features of the meteorological situation during Flight 10 and discuss the model's capability to simulate the dynamics. The SPCZ region is characterized by a diagonal band of convective clouds oriented NW-SE as illustrated by the satellite image (Figure 1). The SPCZ lies in a region of low-level moisture convergence, between the predominantly northeasterly flow and the cooler predominantly southeasterly flow at higher latitude. This extensive and persistent cloud bands is well reproduced by the model as shown in Figure 2. South of the SPCZ is characterized by a large-scale subsidence associated with the jet stream. The core of the upper tropospheric subtropical jet was located near 30°S and the jet streak was around 13 km altitude. Wind speed and direction, potential temperature, and water vapor mixing ratio are well reproduced by the model as shown in Figures 3 and 4. In particular, as the flight track crossed the SPCZ region at 10 and 11.3 km, sudden change in wind speed and direction were well captured by the
<figure><img src="image_2.png" /><figcaption>**Figure 1.** GEOS-10 IR satellite image for 21 March at 0300 UTC with track of Flight 10 during PEM-Tropics B experiment. Flight 10 started on 20 March at 2230 UTC from Fiji and returned on 21 March at 0615 UTC.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** (a) Simulated cloud height (in km) on 21 March at 0300 UTC. The square represents the area of budget calculations and is commensurate with the SPCZ-related convection traversed by the flight. (b) Simulated wind speed (in m/s) at 330 K with the subtropical jet stream south of the SPCZ. (c) Simulated cross sections along the flight track of relative humidity. (d) Same as (c), but for updraft convective mass flux (in kg/s). The dotted lines represent the pressure (in Pa).</figcaption></figure>

model. South of the SPCZ was characterized by SW elevated strong wind speed (18 m/s) whereas north of the SPCZ has lower wind speed (around 10 m/s) and NE flow. It is worth noting that in areas of convection, the winds tend to be less reliable because of the influence of small-scale motions. The measured winds contain an influence from the convection, and are not simply describing the large-scale environmental flow. In Figure 4, observed and simulated water vapor mixing ratios in the upper troposphere in the SPCZ and ITCZ regions were relatively high (median values respectively 296 and 354 ppmv) compared to the region south of the SPCZ (median value 86 ppmv) These results confirm the capability of the model to reproduce the

important meteorological features observed during PEM-Tropics B Flight 10.

## 3.2. Advection and Convection in the SPCZ Region

[8] Figure 5 shows the airflow at three different altitudes together with the accumulated ground convective precipitation. Ground precipitation is plotted here to serve as an indicator of the convergence zone. Near the surface, boundary layer air north of the SPCZ is entrained in the convective clouds. It is worth noting that near the surface (950 hPa) and at very low pressure (200 hPa), the SPCZ acts as a barrier to mixing of tropical and subtropical air. On the contrary, in the 400–270 hPa region, the conver-
<figure><img src="image_4.png" /><figcaption>**Figure 3.** (a) Observed wind speed obtained from the aircraft (crosses) and simulated wind speed (solid line) (in m/s) during Flight 10 as a function of time. Model results are linearly interpolated along the flight track. The entire flight took place during daytime, crossing the SPCZ twice during the outbound and returning part of the flight. (b) Same for wind direction (in degrees).</figcaption></figure>

gence zone becomes porous allowing subtropical air masses from the Southern Hemisphere to mix with tropical air from NE of the SPCZ. These results extend the hypothesis of Pickering et al. [2001]. The authors stated that mixing in tropical air from NE of the SPCZ with Southern Hemisphere subtropical air from SW of the SPCZ occurred in the upper troposphere, above and near the cloud band top. The present study suggests that air south of the SPCZ can also traverse the SPCZ at midtropospheric levels where it is entrained in the convective clouds. In addition to this mixing over the cloudy region, airflow at 400 hPa, also reveals a significant flow counterclockwise going round the convective zone, SW of the SPCZ. This flow brings Southern Hemisphere air to the north of the

SPCZ, therefore favoring air mass exchange between the two hemispheres. Figure 2d shows updraft convective mass fluxes along the flight track. Convective updraft mass fluxes in Figure 2d have parabolic profiles consistent with entrainment of air in the 900–270 hPa region. Convective cloud outflow in the SPCZ and ITCZ regions occurs in the 250–200 hPa region. Air entrained in the 250–200 hPa region is then advected toward the E-SE according to Figure 5c.

## 4. Chemical Signatures in the SPCZ Region

[9] In the following sections, the chemical signatures of the different air masses in the SPCZ region are presented.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** (a) Same as Figure 3, but for potential temperature (in K). (b) Same as (a), but for water vapor mixing ratio (in ppmv).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Accumulated ground precipitation (in mm) on 21 March at 0300 UTC (after 21 hours of simulation) and wind at (a) 200 hPa, (b) 400 hPa, and (c) 950 hPa.</figcaption></figure>

The capability of the model to reproduce these chemical signatures is discussed. Budget calculations in the model are performed to assess the relative impact of convection, advection, and chemistry on HOₓ precursors over the SPCZ-related convection area.

**4.1. Observed and Simulated Chemical Signature**

[10] Chemical variations observed across the SPCZ at 200-300 hPa on both the outbound and the inbound flight

segments reflected the transition between subtropical and tropical air masses as well as strong convective influence within the SPCZ itself [Mari et al., 2002].

[11] In the southern part of the track (south of 20°S), relatively high concentrations of ozone (mean value 32 ppbv), NO (mean value 29 pptv) and small enhancements of CO (up to 58 ppbv) were measured suggesting some continental influence from Australia [Pickering et al., 2001]. Observed HCHO and CH₃OOH mixing ratios
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Simulated cross sections along the flight track (a) on 21 March at 0300 UTC for (b) CO (in ppbv), (c) O₃ (in ppbv), (d) CH₃OOH (in ppbv), (e) H₂O₂ (in ppbv), and (f) HCHO (in ppbv). Double arrows represent the locations of the SPCZ and ITCZ.</figcaption></figure>

in the upper troposphere, south of the SPCZ, were relatively low (median values 80 and 70 pptv, respectively). In this region, because of the strong subsidence associated with the upper tropospheric subtropical jet, water vapor concentrations were low (median value 86 ppmv). In contrast, for the region north of the SPCZ, influenced by convection, observed upper tropospheric CH₃OOH median mixing ratio was 110 pptv. Median mixing ratio was 110 pptv between 20°S and 17°S, lower in the cloud free region between the two convergence zones (median value 96 pptv) and higher in the ITCZ region (median value 120 pptv). H₂O₂ median value was higher north of 20°S (median value 174 pptv) compare to south of 20°S (median value 148 pptv) in the upper troposphere. According to Mari *et al.* [2002], the mixing ratios of H₂O₂ north of 20°S were attributed to upper tropospheric local photochemistry only, without invoking transport.

[12] Chemical signatures in the SPCZ region are shown in Figure 6. CO in the relatively pristine tropical air sampled during PEM-Tropics B was between 40 and 50 ppbv. Figure 6 shows a well marked latitudinal

gradient with high CO mixing ratios above 10°S, apparently of northern hemispheric origin [Staudt et al., 2001; Avery et al., 2001]. These higher CO mixing ratios are present also in the upper troposphere (12 km) as illustrated in the cross section along the flight track. South of the SPCZ, a maximum in CO is simulated at 12 km. The origin of this upper tropospheric CO plume is related to transport of continental air masses from Australia [Pickering et al., 2001; Mari et al., 2002]. It is interesting to note that no manifest vertical gradient is simulated for CO.

[13] Simulated ozone in the free troposphere has a well marked gradient with relatively low mixing ratios in the SPCZ convective region (20 ppbv at 12 km) compared to ozone mixing ratio south of the SPCZ (40 ppbv at the same altitude). The contrast reflects the convective transport of ozone depleted air from the marine boundary layer to the upper troposphere. It is worth noting that the O₃ minima is not nearly as severe as seen somewhat farther north during Central Equatorial Pacific Experiment (less than 5 ppbv observed) [Kley et al., 1996] and which have also been largely attributed to convection [Lawrence
et al., 1999]. The horizontal variation of CO and O₃ delimits the region south of the SPCZ, in the SPCZ and north of the SPCZ.

[14] Cross sections along the flight track in Figure 6 clearly show the signature of SPCZ-related convection for more soluble species. Mixing ratios of CH₃OOH are significantly enhanced in the upper troposphere up to 14 km, due to transport by convection [Mari et al., 2000]. H₂O₂ is more soluble than CH₃OOH, scavenging by convective precipitation is thus more efficient and low mixing ratios are simulated below the SPCZ column. HCHO is less soluble than H₂O₂ but more soluble than CH₃OOH. Cross section of HCHO does not reveal any particular signature in the convective regions. A minimum of HCHO mixing ratios is simulated in the cloud free region between the SPCZ and ITCZ where no vertical transport is simulated. In this marine convective case, vertical transport of HCHO is limited by convective precipitations as we will see in section 4.3.

## 4.2. Comparison Between Model and Observations

[15] The point by point comparison of observed and simulated mixing ratios is presented in Figure 7.

### 4.2.1. CH₃OOH and H₂O₂

[16] The model captures 41% of the observed CH₃OOH variance but tends to overestimate CH₃OOH mixing ratios: the median simulated-to-observed mixing ratio is 1.2 and the slope of the linear fit is 1.45. Vertical profiles on both sides of the SPCZ reveal that the model overestimation is mainly located in the boundary layer (Figure 8). H₂O₂ is reasonably well represented by the model. The model captures 56% of the observed H₂O₂ variance (Figure 7). The median simulated-to-observed mixing ratio is 0.99 for H₂O₂ and the model reproduces well the vertical variations of H₂O₂ (Figure 8).

### 4.2.2. HCHO

[17] The model captures 60% of the observed variance but tends to underestimate the measured mixing ratios (the median simulated-to-observed mixing ratio is 0.49). In particular, the model fails to reproduce the peak of at 300 hPa north of the SPCZ. Such a maximum of HCHO can be due to advection of HCHO-rich air or enrichment of upper tropospheric air by previous convective events. However, the underestimation of the enhancement of upper tropospheric mixing ratio by convection in the model cannot be excluded because of the underestimation of in the marine boundary layer. Underestimation of HCHO in the marine boundary layer can be due to a missing oceanic source of HCHO precursors (isoprene, acetone) in Meso-NH [Lewis et al., 2001; Zhou and Mopper, 1997; Ayers et al., 1997]. In the model, convection is not an efficient process to increase upper tropospheric and is thus unlikely to serve as a primary source of HOₓ.

### 4.2.3. O₃

[18] In Figures 7 and 8, the median simulated-to-observed mixing ratio for is 1.3 and the model captures only 29% of the observed variance. The model reproduces well the O₃ mixing ratios increase in the upper troposphere above 350 hPa south of the SPCZ but does not simulate the minimum of O₃ at 300 hPa. Instead, the model

simulates minimum mixing ratios in the whole 800-400 hPa region. Pickering et al. [2001] suggested that the plume of high O₃ mixing ratio at 400 hPa may have a nonconvective origin or that the source of the plume was located further back in time, allowing more photochemical processing during transport. Below 400 hPa, the model reproduces well the vertical gradient of ozone but underestimates the observed O₃ in the southern drier region and overestimates the measured ozone in the cloudy region north of 20°S.

### 4.2.4. OH and HO₂

[19] The model captures 79% of the variance of the observed H₂O concentrations. The median simulated-to-observed ratio is 1.0 and the slope of the linear fit is 0.97. The simulation of OH mixing ratio is poorer but still satisfactory with 47% of the variance of the observed OH captured by the model. The median simulated-to-observed ratio is 0.9 and the slope is 0.77. The good correlation of modeled OH H₂O and observed OH and is not surprising although the model tends to overestimate the CH₃OOH mixing ratios and to underestimate the HCHO mixing ratios. According to Mari et al. [2002], the reaction O(¹D) + H₂O is a major HOₓ source in the most of the SPCZ region. Water vapor is well reproduced by the model with the least squares correlation coefficient between model and observations equal to 0.96 and the median ratio of simulated-to-observed mixing ratios equal to 0.64 (see also Figure 4).

## 4.3. Convection, Advection, and Local Chemistry

[20] In order to quantify the relative role of convection, advection, and local chemistry in the SPCZ convective region a series of budget calculations in the model is carried out for HO₂, CH₃OOH, HCHO, H₂O₂, and O₃. These budget terms include sources tendencies of chemical species due to transport, convection (subgrid vertical lifting plus scavenging) and net gas phase chemical production. The net tendency of any chemical species in the model is given by:

$$ \frac{dC}{dt} = \frac{dC}{dt}\bigg|_{\text{chemistry}} + \frac{dC}{dt}\bigg|_{\text{convection}} + \frac{dC}{dt}\bigg|_{\text{advection}} + \frac{dC}{dt}\bigg|_{\text{turbulent mixing}} \quad (1) $$

The budget terms were summed in horizontal and temporal ways by applying the following operator:

$$ \frac{1}{t_2 - t_1} \sum_{t=t_1}^{t=t_2} \sum_{i} \sum_{j} P_{i,j} \Delta x \Delta y \Delta t \quad (2) $$

where P stands for a given process: advection, subgrid-scale convection (including wet deposition), chemistry, and turbulence. Spatial integration is performed over the horizontal subdomain defined by the Cartesian zone in Figure 2. This region corresponds to the convective SPCZ-related area traversed by the aircraft. This region is chosen because of the persistence of convective clouds. Advection process is the summation of horizontal plus vertical advection. It is worth noting that the advection tendencies obtained with this method may depend on the size of the subdomain that was chosen. Budget terms
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Point-by-point comparison between observations and model calculations for (a) CH<sub>3</sub>OOH mixing ratios (in pptv), (b) H<sub>2</sub>O<sub>2</sub> mixing ratios (in ppbv), (c) HCHO mixing ratios (in pptv), (d) O<sub>3</sub> mixing ratios (in pptv), (e) OH mixing ratios (in pptv), and (f) HO<sub>2</sub> mixing ratios (in pptv). One-minute averages corresponding to PEM-Tropics B Flight 10 are shown by crosses. Model results are instantaneous model values at the exact time of the measurements. Model values are linearly interpolated at the location of the measurement. Also listed are the slope (s) of the linear fit, the least squares correlation coefficient (r<sup>2</sup>) between model and observations, and the median ratio (R) of simulated-to-observed mixing ratios. The solid line in each panel is the 1:1 ratio and the dotted lines are the 2:1 and 1:2 ratios.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Vertical profiles south and north of the SPCZ of observations and model calculations for (a) CH₃OOH mixing ratios (in pptv), (b) H₂O₂ mixing ratios (in ppbv), (c) HCHO mixing ratios (in pptv), and (d) O₃ mixing ratios (in pptv). One-minute averages corresponding to PEM-Tropics B Flight 10 are shown by crosses. Model output is given as a solid line.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 9.** Averaged vertical profiles over the SPCZ (see Figure 2) and over the entire duration of the flight of HO₂ source terms (in pptv/d) for (a) total advection, (b) chemistry, (c) convection, and (d) vertical profiles of H₂O mixing ratios (in pptv).</figcaption></figure>

have been integrated temporally over the entire duration of the flight ($t_2 - t_1 = 8$ hours).

### 4.3.1. HO₂

[21] Local chemistry prevails on the HO₂ budget (Figure 9) as expected due to the short lifetime of the radical.

Chemistry source of HO₂ is 4 times greater than convection sources with a strong diurnal variation, ranging from -40 to 40 pptv/d. During the flight over the SPCZ region, scavenging by convective precipitation and convective transport eliminated on average 5 pptv/d of HO₂ at 12

<figure><img src="image_11.png" /><figcaption>**Figure 10.** Same as Figure 9, but for CH₃OOH source terms (in pptv/d) and vertical profiles of CH₃OOH mixing ratios (in pptv).</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 11.** Same as Figure 9, but for H₂O₂ source terms (in ppbv/d) and vertical profiles of H₂O₂ mixing ratios (in ppbv).</figcaption></figure>

km. The loss of HO₂ by convection in the 2–13 km region is partly balanced by the advection source.

### 4.3.2. CH₃OOH

[22] In Figure 10, chemistry is the main source of CH₃OOH in the marine boundary layer (1000 pptv/d). Convection plays an important role in the vertical redistribution of CH₃OOH. Convective upward transport and scavenging by convective liquid precipitation deplete the 1–9 km layer at 100 pptv/d. In the 10–15 km layer over the SPCZ region, transport by the convection brings air in the upper troposphere at 200 pptv/d. CH₃OOH transported by convection can act as a primary source of HOₓ. *Mari et al.* [2002] confirm this hypothesis. The authors found that in the cloud outflow region, CH₃OOH accounted for 22–64% of the total primary source. It is worth noting that in the 2–8 km region, convective loss is mostly balanced by advection. Advected replenishes the midtroposphere in the SPCZ region.

### 4.3.3. H₂O₂

[23] H₂O₂ is a highly soluble gas. It is efficiently scavenged by liquid precipitation in the lower (warm) part of the clouds and to a much lesser extent in glaciated clouds [*Mari et al., 2000; Barth et al., 2001*]. Convection is a sink for H₂O₂ in the 2–13 km region with loss rate up to 0.3 ppbv/d (Figure 11). This effect is partly counteracted by chemical production below 5 km and advection above 3 km.

### 4.3.4. HCHO

[24] The decrease of HCHO mixing ratios is driven by chemical loss within nearly the whole subdomain of budget calculation (Figure 12). Chemistry tendencies become slightly positive after 21 March at 0000 UTC in the boundary layer and 21 March at 0500 UTC above 5 km. HCHO is

more soluble than CH₃OOH. Convection is a net sink for HCHO with loss by convective precipitation and upward transport of 50 pptv/d between 2 and 10 km. However, convection tendency does not exhibit any enhancement in the upper troposphere: vertical transport of HCHO balances the scavenging of HCHO resulting in no net change in the SPCZ region. According to *Mari et al.* [2000], the authors found that HCHO was at the threshold for efficient scavenging allowing convection to increase upper tropospheric mixing ratios. They were studying a continental convective case sampled during the TRACE-A campaign. During this experiment, HCHO in the boundary layer was initially 20 times higher than in the background upper troposphere. In the marine convective case during PEM-Tropics B, HCHO in the background upper troposphere is initially only 3 times smaller than the mixing ratios observed in the boundary layer. In addition, convective updraft were less vigorous over ocean than they were over continent. The vertical gradient is thus significantly lower in the marine case and upward transport is expected to have less impact. The weaker updrafts in marine convection compared to continental convection would give smaller HCHO fluxes to the upper troposphere for the marine case. In addition, smaller vertical HCHO fluxes would be found for the marine case because of the smaller boundary layer concentrations compared to the continental case. In the model, HCHO is thus unlikely to serve as a primary source of HOₓ in this marine upper tropospheric case.

### 4.3.5. O₃

[25] In Figure 13, convection brings O₃-poor air from the boundary layer to higher altitudes inducing negative convection tendency above 5 km [*Folkins et al., 2002*]. As
<figure><img src="image_13.png" /><figcaption>**Figure 12.** Same as Figure 9, but for HCHO source terms (in pptv/d) and vertical profiles of HCHO mixing ratios (in pptv).</figcaption></figure>

a consequence, a minimum in O₃ mixing ratio is obtained between 9 and 12 km. O₃ starts increasing significantly above 12 km whereas convection tendency reaches its largest negative values. Increase of ozone mixing ratio is

due to advection of rich ozone air in the 11–13 km region. Ozone chemical term is low (less than 2 ppbv/d) in the upper troposphere as expected in a NOₓ-limited regime.

<figure><img src="image_14.png" /><figcaption>**Figure 13.** Same as Figure 9, but for O₃ source terms (in ppbv/d) and vertical profiles of O₃ mixing ratios (in ppbv).</figcaption></figure>
## 5. Conclusion

[26] A three-dimensional mesoscale model, Meso-NH, coupled on-line with chemistry and an entraining-detraining plume model is applied to study the factors controlling HOₓ radicals and their precursors during an SPCZ convection event sampled by the PEM-Tropics B aircraft mission.

[27] Near the surface (950 hPa) and at very low pressure (200 hPa), the SPCZ acts as a barrier to mixing of tropical and subtropical air. On the contrary, in the 400–270 hPa region, the convergence zone becomes porous allowing subtropical air masses from the Southern Hemisphere to mix with tropical air from NE of the SPCZ. The present study suggests that air south of the SPCZ can also traverse the SPCZ at midtropospheric levels where it is entrained in the convective clouds.

[28] The point by point comparison of simulated and 1 min average observed mixing ratios reveals that the model is able to reproduce well the HOₓ mixing ratios. The model also captures the contrast between northern and southern air masses. Convection brings O₃-poor air at higher altitude, generating a minimum in O₃ mixing ratio between 9 and 12 km. Ozone local chemistry source is low (<2 ppbv/d) in the upper troposphere as may be expected in a NOₓ-limited regime.

[29] The model captured 56% of the observed H₂O₂ variance and 41% of the observed CH₃OOH variance. Vertical variations of H₂O₂ are remarkably well reproduced. Scavenging by convective precipitation and upward transport eliminate H₂O₂ in the 2–15 km region. The model overestimates the observed CH₃OOH mixing ratio mainly in the boundary layer. Convection plays an important role in the vertical distribution of CH₃OOH with efficient vertical transport from the boundary layer to the 10–15 km region. Therefore, in the upper troposphere over the SPCZ region, CH₃OOH transported by convection can act as a primary source of HOₓ.

[30] The model captures 60% of the observed variance but fails to reproduce a peak of mixing ratio at 300 hPa sampled during the northern spirals. Simulated does not exhibit any increase in the upper troposphere. This result contrasts with continental convective, more vigorous, systems, for which stronger updrafts and steeper vertical gradients of mixing ratios lead to strong enhancement of HCHO in the upper troposphere. Thus, in this marine case, HCHO is unlikely to serve as a primary source of HOₓ in the upper troposphere. Underestimation of HCHO in the marine boundary layer suggests a missing oceanic source of HCHO precursors (isoprene and acetone).

[31] **Acknowledgments.** Computational resources were provided by the Institute for Development and Resources in Intensive Scientific Computing. J. Duron, S. Prieur, D. Gazen, and J. Escobar are acknowledged for their precious assistance. This work was supported by the French Programme National de Chimie Atmospherique. The authors want to thank H. Fuelberg, M. Lawrence, and an anonymous reviewer for their useful comments.

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