Seasonal variation in atmospheric pollutants transport in central Chile: dynamics and consequences

Central Chile faces atmospheric pollution issues all year long as a result of elevated concentrations of fine particulate matter during the cold months and tropospheric ozone during the warm season. In addition to public health issues, environmental problems regarding vegetation growth and water supply, as well as meteorological feedback, are at stake. Sharp spatial gradients in regional emissions, along with a complex geographical situation, make for variable and heterogeneous dynamics in the localization and long-range transport of pollutants, with seasonal differences. Based on chemistry–transport modeling with Weather Research Forecasting (WRF)–CHIMERE, this work studies the following for one winter period and one summer period: (i) the contribution of emissions from the city of Santiago to air pollution in central Chile, and (ii) the reciprocal contribution of regional pollutants transported into the Santiago basin. The underlying 3-dimensional advection patterns are investigated. We find that, on average for the winter period, 5 to 10 μgm−3 of fine particulate matter in Santiago come from regional transport, corresponding to between 13 % and 15 % of average concentrations. In turn, emissions from Santiago contribute between 5 % and 10 % of fine particulate matter pollution as far as 500 km to the north and 500 km to the south. Wintertime transport occurs mostly close to the surface. In summertime, exported precursors from Santiago, in combination with mountain–valley circulation dynamics, are found to account for most of the ozone formation in the adjacent Andes cordillera and to create a persistent plume of ozone of more than 50 ppb (parts per billion), extending along 80 km horizontally and 1.5 km vertically, and located slightly north of Santiago, several hundred meters above the ground. This work constitutes the first description of the mechanism underlying the latter phenomenon. Emissions of precursors from the capital city also affect daily maxima of surface ozone hundreds of kilometers away. In parallel, cutting emissions of precursors in the Santiago basin results in an increase in surface ozone mixing ratios in its western area.

Tropospheric O 3 is a secondary pollutant formed by the photochemical oxidation of volatile organic compounds (VOC) in the presence of nitrogen oxides (NO x ). The essential role of photolysis in its production explains that harmful levels are mostly observed in summertime (e.g. Walcek and Yuan, 1995;Seinfeld and Pandis, 2006). O 3 is noxious for human health, causing 40 respiratory disorders such as asthma (Lippmann, 1991). Furthermore, its deposition on plant leaves affects their photosynthesis and evaporation ability, hence damaging crop yields (Hill and Littlefield, 1969). Tropospheric O 3 is also a powerful greenhouse gas (GHG) as well as a photochemical oxidant hence playing a key role in the atmosphere :::::::::::::::: (Ehhalt et al., 2001).
Chile is a narrow band of land bordered by the Pacific ocean on the western side and the Andes cordillera on the eastern side. Air motions are thus influenced by sea-land atmospheric interactions and mountain-valley circulation, in addition to more synoptic patterns. The intensity of these atmospheric regimes, which are partly governed by radiative processes, are modulated seasonally. So do emissions of primary pollutants and photochemical reactions involved in the creation of secondary pollutants 60 (e.g. Gramsch et al., 2006;Barraza et al., 2017). Moreover, despite a well developed network of air quality monitoring stations across the country, the spatial and temporal density of the data does not allow for a detailed observation-based study of atmospheric pollutants transport. As a result, chemistry-transport modeling offers a solution to cope with this issue and provide insights regarding the magnitude and mechanisms of advection of pollutants at the regional scale.
The present work studies, for one summer month and one winter month in 2015 in central Chile, through chemistry-65 transport simulations with WRF-CHIMERE, (i) the contribution of pollutants emitted in Santiago to the regional atmospheric composition, (ii) the reciprocal contribution of regional emissions to air pollution in the capital city basin, (iii) the corresponding 3-dimensional advection patterns of particulate matter and ozone. The methodology and data are described in Section 2, the relative contributions of transport and the underlying advection processes are presented in Section 3. These results are discussed in Section 4 and conclusions are gathered in Section 5.

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2 Data and methods

Modeling setup
The chemistry-transport simulations are performed with the Weather Research and Forecasting (WRF) mesoscale numerical weather model from the US National Center for Atmospheric Research (Skamarock et al., 2008) to simulate the meteorological fields, and CHIMERE to compute chemistry and transport (Mailler et al., 2017). Anthropogenic emissions are based on the 75 EDGAR HTAP V2 inventory (Janssens-Maenhout et al., 2015). The simulation domain has a 5 km spatial resolution, extending over 200 latitudinal and 100 longitudinal grid points, and is centered on Santiago (white domain CHILE_5K in Figure 1a). The parameterizations and model configuration used for WRF are presented in Table A1. WRF is applied to 60 vertical levels up to the highest elevation of 50 hPa. Initial and boundary conditions rely on the NCEP FNL analysis data sets, with a 1 • by 1 • spatial resolution and 6-hour temporal resolution, from the Global Forecast System (NCEP, 2000). Land-use and orography 80 are extracted from the modified IGBP MODIS 20-category database with 30 sec resolution (Friedl et al., 2010). CHIMERE is a Eulerian 3-dimensional regional Chemistry-Transport Model, able to reproduce gas-phase chemistry, aerosols formation, transport and deposition. In this work, the 2017 off-line version of CHIMERE is used (Mailler et al., 2017). The model configuration is described in Table A1, with the same horizontal domain as for WRF, applied on 30 vertical levels up to 150 hPa.
Emissions downscaled from the HTAP V2 inventory and input into CHIMERE are shown in Figures 1b through 1e. The seasonality of BC emissions is clear, given the major role played by residential heating which takes place mostly in wintertime.

Simulation validation
Surface meteorology and pollutants concentrations are validated using data from the automated air quality and meteorology monitoring network of Chile, known as Sistema de Información Nacional de Calidad del Aire (https://sinca.mma.gob.cl/index. bias : on surface relative humidity :::: with ::::::: average ::::: mean ::::: biases : of -15% to -20% but shows fair correlations around 0.8 to 0.9. Surface winds are fairly reproduced, with limited biases and wind gusts well captured, although the correlations can be low for some locations. Figures   is also well reproduced with hourly correlations (not shown here) of 0.67 for Viña del Mar and 0.8 to 0.9 for the three other 145 sites. In parallel, summertime NO x mixing ratios within the Santiago area ::::::: Santiago :::: city (not shown here) are well captured by the model with mean biases between 0.05 and 1.23 ppb for three stations in Santiago (Las Condes, Puente Alto -southeastern Santiago -and Independencia), associated with decent hourly correlations between 0.43 and 0.59.
In conclusion, PM 2.5 in wintertime and O 3 and its precursors in summertime, the key pollutants for their respective seasons, as well as meteorological conditions, are fairly reproduced by the model for a selection of sites throughout central Chile, which gives confidence in the model outputs that are described and analyzed in the following sections.
From these mean wind fields, the dominant advection pathways of pollutants can be inferred. Polluted air masses are on average blown towards the cordillera ::::: Andes : and the north during daytime in summertime, and have more complex dynamics in wintertime but also transport northward in general. Deeper and more turbulent planetary boundary layer heights during 185 daytime, as observed in summertime ( Fig. 3a) also enable the vertical export of pollutants up into the free troposphere (FT) where they can be advected farther, while wintertime shallower boundary layers ( Fig. 3b) imply more stagnation of air masses. 3.1 Impact of emissions from Santiago on regional atmospheric composition 3.1.1 Wintertime PM 2.5 Consistently with the mean wind fields and emission rates of pollutants in central Chile discussed previously, anthropogenic 190 emissions in the Metropolitan Area ::: city : of Santiago significantly influence surface atmospheric composition over a large region, over land, the Andes and the Pacific ocean. The following results are based on the analysis of sensitivity to emissions from Santiago described in Section 2.1: the difference between the simulation with (baseline case) and without (contribution case) emissions from Santiago yields their contribution to atmospheric composition over the domain. Figures 4a and 4b show the average wintertime PM 2.5 plume (absolute and relative, respectively) attributable to emissions from the capital ::: city : area.

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The direct western vicinity of the Santiago basin receives, on average 5 µg m −3 to 15 µg m −3 coming from the capital ::: city, a few tens of kilometers from the source, corresponding to more than 30% of the signal simulated in the baseline scenario for this area. At the scale of hundreds of kilometers, the export drops to a few µg m −3 , corresponding to 5% to 20%. It is worth noting that the relative contribution of emissions from Santiago remains greater than 5% on a area as large as more than 8 • meridionally and 3 • zonally, hence stressing the significant impact of the capital ::: city on atmospheric composition for the whole 200 region (Fig. 4b). In particular, the southern part of the plume is transported over the Andes down to Argentina with a large spread of several degrees of longitude, whereas the northern part extends mostly along the coast in a narrower manner and transports as far as the boundary of the simulation domain.
More specifically, urban areas along the north-south axis of Santiago (Curicó, San Fernando, Rancagua and Los Andes in Fig. 4a) receive 1 µg m −3 to 2 µg m −3 from Santiago on a hourly basis on average, corresponding to 4% to 8% of the baseline 205 concentrations (Fig. 4c). Sporadically, up to more than 20 µg m −3 in Rancagua and 9 µg m −3 in San Fernando, Curicó and Los Andes can be attributed to emissions from Santiago. These significant contributions likely lead to alert thresholds crossing for some hours in these cities.
On the eastern side of the Santiago basin is the Andes cordillera. We examine the contribution of Santiago emissions in a village :::::::: mountain :::::: locality : (San Gabriel : , :::::: 1250 m :::: a.s.l.) and a summit (Maipo volcano, ::::::: 5264 m :::: a.s.l.) along the Maipo canyon, 210 southeast of Santiago. We find that for the village ::: San ::::::: Gabriel 34% (1 µg m −3 ) of PM 2.5 , on average, is transported from the metropolitan area ::::: urban ::::: basin. This is consistent with the mountain-valley circulation patterns aforementioned, leading to the intrusion of urban air masses deep into the canyon (Lapere et al., 2021b). We acknowledge that the estimate of 34% is probably larger than reality since the HTAP inventory does not capture properly local emissions in the village of San Gabriel, which likely dominate the signal, especially with wood burning for residential heating being largely used in such villages 215 in wintertime. In the Maipo volcano area(summit at 5264 m a.s.l.), a small contribution in absolute value is found (less than 0.5 µg m −3 ) although it can reach up to more than 2 µg m −3 occasionally, but this corresponds to 20% of the signal there on average. This area is covered in snow during wintertime, so that despite the small magnitude of the import of PM 2.5 it can lead to significant radiative effects when deposited ::::::::::::::: (Rowe et al., 2019), especially given the large fraction of black carbon ::: BC in PM 2.5 emitted in Santiago(, : around 15% at Independencia -not shown here) :::::::: according :: to ::: the :::::: HTAP :::::::: emissions :::::::: inventory.

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The Viña del Mar-Valparaíso area is the second largest populated region of Chile, located on the ocean coast :::: coast :: of :::::: central ::::: Chile, :::::::::::: approximately :::::: 100 km : west of Santiago. In wintertime, it is downwind of Santiago, which leads to an average import of particulate matter from the capital city of 3 µg m −3 (18%) and sporadically up to 18 µg m −3 . Again, air quality in this urban area is worsened by export from Santiago, by a significant share. Further north, at the location of La Serena, which also suffers from bad air quality in wintertime, the contribution of Santiago emissions is more moderate but still remains significant in 225 absolute value although its contribution is only 1%.

Summertime O 3
In summertime, except for PM 2.5 emitted by biomass burning events (not considered here), O 3 is the pollutant raising concern.
Combined significant emissions of NO x and VOC are required, in the presence of sunlight, to generate high mixing ratios of O 3 . However, a lot of non-linearities are involved in the tropospheric O 3 cycle, so that the sensitivity to its precursors emissions  important variable to determine whether O 3 pollution is high is its daily maximum mixing ratio, on which we will mostly focus hereafter. Figure 5a shows the average of maximum hourly O 3 mixing ratio at ground-level observed each day in the baseline case 235 (SB). In the baseline scenario, O 3 is mostly found at harmful levels near the area :: in ::: the :::::: vicinity : of Santiago, and mainly on its eastern side. Again, mountain-valley circulation accounts for this observation: afternoon westerlies blow O 3 precursors, present in large amounts in urban air masses, towards the Andes. NO x lifetime is a few hours at most, depending on the reactivity of VOC and NO 2 density (e.g. Laughner and Cohen, 2019), while most VOC have an atmospheric lifetime of several days (e.g. Monod et al., 2001), so that on the way, NO x are more consumed than VOC. Consequently, while urban air is mostly a NO x -rich 240 environment, precursors ratios become more balanced along with the export so as to create more favorable conditions for O 3 formation once reaching less urbanized areas. Such a mechanism is observed for Paris and its suburbs for instance (e.g. Menut et al., 2000). Export by easterlies occur less frequently and mostly at night when O 3 cannot be created due to lack of sunlight, which is why the rural area west of Santiago shows smaller O 3 maxima. This mechanism explains the large concentrations of O 3 mostly found east of Santiago. Except for the center part of the domain where average daily maxima reach more than 245 100 ppb, O 3 pollution is less concerning elsewhere in central Chile where they range between 10 ppb and 35 ppb. maximum, dots show the average. "ns" indicates that the distribution in the SB and SC scenarios cannot be distinguished at the 90% level based on a t-test. "*" indicates that they are different at the 99% level based on that same t-test. Figure 5b shows the decrease in O 3 daily maxima induced by eliminating emissions of the Santiago basin. The spatial pattern is again consistent with the mechanism introduced previously: emissions of precursors from Santiago are the main origin for O 3 formation in the cordillera :::::: Andes and north of the city, with a reduction of more than 50 ppb of the daily maxima over this area when the capital city no longer emits pollutants.
emissions in the vicinity of the Maipo summit is small but remains significant with a decrease by 4 ppb of the daily maxima and average mixing ratios. Given the nature of the locations Valle Nevado and Maipo, the narrow distribution of O 3 hourly mixing ratio, averaging at 33 ppb and ranging between 25 ppb and 40 ppb, recovered in scenario SC for those sites is indicative of the background concentration for the region, i.e. the distribution of O 3 mixing ratio that would be observed at a site not 260 influenced by anthropogenic emissions.
On the other hand, the western and southern areas adjacent to Santiago are barely sensitive to its emissions. In Figure 5c, the distribution of hourly O 3 mixing ratio at Viña del Mar and Rancagua is nearly the same in both scenarios, except for the maximum at Rancagua which is a few ppb lesser in scenario SC than SB. For those two locations, the difference in O 3 distribution with or without Santiago emissions is not significant at the 90% level, while for all other locations, distributions 265 are significantly different at the 99% level.
Regarding the regional transport of PM 2.5 in summertime, the plume shows a similar extent to O 3 in summertime, i.e. an area of influence lesser than in wintertime near the surface (not shown here). O 3 is barely produced in wintertime due to a zenith angle of the sun closer to the horizon and shorter duration of days, resulting in less radiative power and barely active photolysis, so that the question of the export of its precursors at that season is less relevant.

Contribution of regional emissions to atmospheric composition in Santiago
Similarly to the previous approach, we can, in a symmetric manner, deduce the contribution of transport from remote sources to atmospheric composition in the Santiago Metropolitan Area ::::::: Santiago :::: city. This is achieved by looking at concentrations in the contribution case, that exclusively originate from non-local sources. Hereafter the analysis focuses again on PM 2.5 in wintertime and O 3 in summertime.  Figure 4b indicated that local emissions largely dominate the wintertime PM 2.5 signal for Santiago, with 50% to 100% of the mean surface concentration of PM 2.5 originating locally within the white rectangle. Nevertheless, the contribution of transport is also significant although heterogeneous in Santiago. Figure 6a shows that in the baseline scenario, the northern and western parts of Santiago feature higher levels of PM 2.5 , with average concentrations ranging between 30 µg m −3 and 100 µg m −3 for 280 the whole metropolis. In the contribution scenario (Fig. 6b), this pattern is partly recovered, with a smoother gradient from west to east. The underlying conclusion is twofold. First, the transport of pollutants in Santiago mostly comes from the west, which is consistent with the presence of the cordillera ::::::: mountain ::::: range : in the east not featuring many sources of pollution, and the dominant westerly daytime wind direction in wintertime. Second, the districts of Santiago facing the worst air quality are also the ones where the transport of pollutants is larger.

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However, the differing patterns between Fig. 6a and 6b also shows that local sources in these districts are also stronger. If emissions were similar, the observed gradient in WB would be closer to that in WC. Consistently with this observed westward gradient, we define 5 zones of interest, comprising 6 grid points each along a meridional axis (rectangles and dots in Fig. 6b).
This arrangement ensures that most of the city is covered while maintaining the west-east variability. For each zone we look at the distribution of PM 2.5 , averaged over the 6 grid points, in scenarios WB and WB-WC (Fig. 6c) and WC (Fig. 6d).

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When averaged meridionally, the westward gradient is also obtained in scenario WB, and conserved when the contribution of transport is substracted (WB versus (WB-WC) in Fig. 6c). On average, the contribution of transport to PM 2.5 concentration ranges between 10 µg m −3 for the westernmost area and 5 µg m −3 for the easternmost part, with a monotonic spatial variation (Fig. 6d). However, this amount always corresponds to 13% to 15% of the WB concentrations. This number is well in line with Barraza et al. (2017) that found 9% of PM 2.5 in Santiago coming from coastal sources, for the period 2011-2012. It is 295 worth noting that given the observed westward gradient, we also recover that coastal sources are likely the main contributor to imported PM 2.5 . Again, transport is larger in western Santiago, and can sporadically reach up to 30 µg m −3 , but does not constitute a greater share than in the east. This averaged picture provides a first clue as to the main origin of PM 2.5 transport of Santiago but the picture can be refined by looking at the joint distribution of hourly wind direction and PM 2.5 concentrations as shown in Fig. 7. At the selected 300 southeastern location, mostly clean air comes from the east, i.e. from the Andes where pollutants sources are scarce (less than 5 µg m −3 for almost every hour), while winds blowing from the southwest can transport concentrations as high as more than 20 µg m −3 for some hours, pointing to the southern cities of Rancagua ::::::: (34.2 • S, ::::::: 70.7 • W) : or San Fernando ::::::: (34.6 • S, :::::: 71 • W) mentioned previously, or the southwestern urban location of Melipilla (33.6 • S, 71.2 • W). At the northeastern site, winds mainly come from the north where only a handful of urban areas are found, hence leading to a transport seldom exceeding 10 µg m −3 .

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In the center of the metropolitan area :::::::: metropolis, winds are either southwesterlies (from the Rancagua and Melipilla areas) or northwesterlies (from the Viña del Mar-Valparaíso area), with both cases leading to similar amounts of imported PM 2.5 mostly above 10 µg m −3 , i.e. above average. The picture is similar for the northwestern and southwestern sites although wind directions are shifted.
In the northwest, center and northeast, the dominant wind directions also coincide with higher maximum relative contributions 310 of transport over the period along these directions (black diamonds in Fig. 7). Sporadically, significant transport events can also come from less frequently observed directions. The maximum relative contribution obtained for the northwest point when winds are from NNE is 75% for example, while such winds occur less than 1% of the time. For southwest and southeast locations, these maximum relative transport episodes are observed when winds blow from the south, in particular in the southeast where it can reach up to 100%.

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In summary, wintertime PM 2.5 concentrations in Santiago are significantly (5 µg m −3 to 10 µg m −3 on average) and always (at least 1 µg m −3 for every hour) affected by transport, with identifiable origins, and although the different districts are not equally affected in absolute value, the relative burden of imported particulate matter is equivalent.

Summertime O 3
Based on the same approach, we find that the summertime transport of NO x within the Santiago basin never exceeds 0.5 ppb, 320 while average values in the SB scenario are between 5 ppb and 40 ppb, with a similar westward gradient as observed for PM 2.5 (not shown here). At the 90% level, NO x transport is thus not significant. Similarly, the transport of VOC is homogeneous over the whole basin at 2 ppb on average, while between 20 ppb and 60 ppb in the SB scenario, which is again not significant at the 90% level. Santiago is thus not affected by the transport of O 3 precursors.
However, the picture within the Metropolitan Area ::::::: Santiago :::: city in SB and SC scenarios is complex. In the baseline scenario, 325 the eastern area of Santiago is more affected by O 3 pollution compared to the western area ( Fig. 8a and 8c), consistently with observations and the literature (e.g. Menares et al., 2020), due to a more balanced VOC/NO x ratio than in the western area.
At Independencia (center :: of Santiago), the VOC/NO x ratio at emission is between 1:1 and 2:1 on average. Contrarily, at Las Condes (eastern Santiago) the VOC/NO x ratio at emission is around 6:1 on average, in the baseline case (not shown here). A typical O 3 formation ridge line of the VOC/NO x concentration ratio in urban areas is around 6:1 to 8:1 (e.g. National Research 330 Council, 1991;Sillman, 1999), so that Independencia features a VOC-limited regime, while Las Condes features a balanced regime favorable to O 3 formation, hence the larger amounts found at the latter location.  given that there is no import of precursors as evidenced above, the whole area is set to the background O 3 level of around 30 ppb described in Section 3.1.2, since there is no influence of anthropogenic pollutants anymore. Therefore, this corresponds to an increase (decrease, respectively) of O 3 in western (eastern , respectively) ::::::: Santiago ::: and :: a ::::::: decrease :: of ::: O 3 :: in ::::::: eastern Santiago, thus explaining the dipole obtained in Figure 8b. Such an evolution is also clear in the evolution of the distribution of hourly 340 O 3 mixing ratios across the city between scenario SB and SC (Fig. 8c). While in scenario SB the distribution is shifted towards larger mixing ratios when going eastward, in scenario SC all distributions are equal (significant at the 99% level). The leveling 15 of mixing ratios, with no gradient across the metropolitan area ::: city : in scenario SC constitutes an additional evidence that O 3 in Santiago is not affected by long-range transport, otherwise heterogeneous patterns similar to what is observed in Figure 6b would be obtained. Shades of gray correspond to the zones defined in (b) on which an average is made.

Advection processes
As discussed around Figure 3 and observed in Figure 4 and Figure 5, advection patterns of pollutants differ between wintertime and summertime. So far, the analyses focused on surface fields, but processes along the vertical drive these differences. Figure 9 shows an average latitude/altitude transect, along central Chile, of winds, afternoon mixing layer height and pollutants concentrations for the corresponding season, in the baseline scenario and the Santiago isolated contribution case.

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In wintertime (Fig. 9b and 9c), the boundary layer (solid white line) is shallow, and average winds in the FT are strong, consistently with the observed semi-permanent inversion layer in the region. As a result, the PM 2.5 emitted in large amounts mostly remain trapped within this shallow mixing layer. Injection of polluted air masses into the lower FT can also occur, through mountain venting, as described in Lapere et al. (2021b), explaining why residual concentrations of 1 to 5 µg m −3 are observed higher up in the baseline and contribution scenarios at the latitude of Santiago ( Fig. 9b and 9c). Nevertheless, 355 the long-range export of PM 2.5 from Santiago observed in Figure 4 is mainly driven by advection within the boundary layer, close to the ground, by weak winds, as evidenced by Figure 9c. Except for a shallow residual layer located above the average afternoon boundary layer, pollutants emitted from Santiago remain within it along the transect from Santiago to Viña del Mar.
It is also worth noting that even though export of O 3 close to the surface is limited (Fig. 5b), it is more widespread higher up, with a residual layer originating from Santiago emissions of a few ppb extending 2 km vertically and transporting northward in the FT along 2 • of latitude (Fig. 9e). Given the proximity of the Andes cordillera to the Santiago basin, the formation of the aforementioned O 3 bubble finds its 390 origin in the mountain-valley circulation and the associated mountain venting mechanism. Daytime upslope winds, strong in summertime, lift polluted air masses from the atmospheric boundary layer (ABL) over Santiago into the lower FT, possibly above another location depending on the FT winds direction. McKendry and Lundgren (2000) and Lu and Turco (1996) find that this process is a net sink for boundary layer O 3 in British Columbia and the Los Angeles basin, respectively. Henne et al. (2005) find that the effect of venting in an Alpine environment on FT O 3 concentrations strongly depends on initial mixing 395 ratios within the vented ABL, with either net production if ABL mixing ratios of O 3 are high (urban valleys), or net loss if they are low (remote valleys). However, our study case falls into none of the aforementioned. Such a bubble of O 3 , detached from the ground, with a mixing ratio much higher than at ground level is not found in the literature to our knowledge. More moderate injections, with O 3 mixing ratios lower to similar to surface levels are usually observed. In our case, we find that the venting of precursors from the Santiago polluted ABL leads to the net production of large quantities of O 3 in the FT, larger 400 than at the surface. Schematically, there is a larger export of VOC than NO x in the FT (Fig. ??a and ?? ::: A5a :::: and ::: A5b) which makes for a balanced chemical regime (below 0.5 is NO x -limited, above 0.5 is NO x -rich) at around 0.5 at 2 km altitude, while the regime is close to 1 near the surface hence unfavorable to O 3 production, along the whole transect, due to dominant urban NO x emissions (Fig. ?? ::: A5c). Also, the export of Peroxyacetyl nitrate (PAN), which is a NO x carrier, into the FT, contributes to enhanced O 3 formation (Fig. ?? :: A5d). Figure 10 further sheds light on the dynamics of this mechanism. Early in the day, at 11:00 UTC (Chile is UTC-3 in summertime), both NO x and NMVOC (non-methane VOC) are highly concentrated near the ground (morning peak of emissions from traffic), and start ascending the Andean foothills, but no O 3 is formed in the region given that sunlight is not intense yet (leftmost column in Fig. 10). A few hours later, at 12:00 LT (15:00 UTC), the precursors have a similar distribution as in early morning, but the photolysis starts taking place and O 3 is created at the top of the precursors plume, well above the ground.

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Two factors explain that O 3 is not formed near the source of precursors. First, given the urban environment of the basin, NO x are emitted in large quantities so that the VOC/NO x ratio is adverse to neutral, and O 3 is formed in small amounts (see the related discussion in Sect. 3.2.2). But NO x have a lifetime much shorter than most VOC (e.g. Monod et al., 2001;Laughner and Cohen, 2019), and when the polluted air parcels are lifted up, the ratio becomes more balanced as NO x is consumed closer to the ground. Due to these asymmetric lifetimes and hence export, at some point on the vertical the ratio becomes favorable 415 ( Fig. ?? ::: A5c) and O 3 is formed in large quantities. The second factor comes from the increase of photolysis rates with altitude.
Several hundred meters above the ground, near or above the mixing layer, photolysis rates of NO 2 are much faster than at ground level (e.g. Pfister et al., 2000) hence favoring the formation of O 3 , all other things being equal. In our simulation, on average, the photolysis of NO 2 is 20% faster 1000 m above ground than at the surface. Also, Figure 9d shows that at the point where the O 3 plume is denser, winds are weak to null on average, so that precursors stagnate, again allowing for more O 3 420 creation.
At 14:00 LT (17:00 UTC), the vertical export of precursors is even more important due to the maximum development of the deep mixing layer and the full intensity of upslope winds, with two main consequences. First, NO x levels in Santiago decrease compared to VOC due to vertical export earlier in the day, leading to a more favorable ratio  In addition to being a pollutant, tropospheric O 3 is a strong greenhouse gas, estimated to contribute between 0.2 to 0.6 W m −2 to present climate global radiative forcing (Myhre et al., 2013). The process described above may then imply radiative effects, either directly through greenhouse effect, indirectly through its impact on moisture, clouds and atmospheric circulation, or through its interaction with the cycles of other greenhouse gases (Mohnen et al., 1993). Estimating this impact is however not in the scope of the present work. Nevertheless, it shows that although the venting of pollutants can be beneficial from the urban 435 air pollution perspective, longer-term effects on climate are a corollary worth investigating.
Despite our good confidence in the model, it is not possible to strengthen conclusiveness with observations on the newly 490 evidenced O 3 bubble we find as there are no local measurements of O 3 profiles for our study period. Besides, tropospheric ozone column products from satellite data are usually not fit for analysis in mountainous regions (e.g. Kar et al., 2010).
However, Seguel et al. (2013) conducted ozone profile measurements in the Santiago region :::::::::: Metropolitan ::::::: Region in summer 2011 that match our results quite well. First, they find a similar 35 ppb free troposphere background O 3 mixing ratio. Second, they evidence several occurrences of deep residual layers as intense as 100 ppb of O 3 slightly north of Santiago (at the location 495 of La Colina, between Santiago and Los Andes) in early afternoon, measured between 1.5 km and 2.5 km above ground. Such secondary layers higher up are consistent with our findings. If a vertical profile is taken north of Santiago in Fig 9d, a bell shaped profile is recovered, of maximum intensity near 2 km above ground, much like Figure 9 in Seguel et al. (2013). Thus, our simulation results agree very well with the measurements conducted in Seguel et al. (2013), despite being for a different year, hence strengthening our conclusions on the existence of the newly evidenced O 3 bubble and its seasonal persistence.

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Similarly to our findings, Seguel et al. (2013) also concluded that the residual layer is coming from pollutants venting from Santiago. However their measurement-based approach did not allow to evidence (i) the exact formation mechanism of this bubble, (ii) its persistent character even through nighttime (measurements presented are only for daytime), (iii) its horizontal extent (measurements are discrete in space). Our modeling approach based on sensitivity analysis confirms the primary role of Santiago emissions and gives a clearer and continuous 3-dimensional picture of the phenomenon, while agreeing with the 505 findings of these previous measurements.

Conclusions
Based on chemistry-transport modeling with WRF-CHIMERE, the present work investigates the transport of atmospheric pollutants in central Chile for one winter month and one summer month. Our findings show that emissions of the Santiago Metropolitan Area :::: from ::::::: Santiago :::: city greatly affect atmospheric composition in its vicinity and farther, with a contribution of a few µg m −3 to PM 2.5 in wintertime corresponding to 5% to 10% of surface concentrations as far as 4 • north and 4 • 525 :::::: 500 km :: to ::: the ::::: north :::: and :::::: 500 km :: to : south. This transport is mostly driven by surface winds within the boundary layer above land, and takes place in the free troposphere over the ocean hence explaining its long northward range. The spatial extent of the effect on surface concentrations of O 3 precursors emitted in Santiago in summertime is lesser. Nevertheless, daily peaks of O 3 in the direct vicinity of Santiago are reduced by up to 50 ppb on average when emissions from the metropolitan area ::: city are eliminated. Conversely, the contribution of long-range transport of PM 2.5 in wintertime is responsible for 5 µg m −3 to 530 10 µg m −3 on average in downtown Santiago, corresponding to around 14% of the baseline concentration. While the transport of O 3 precursors to Santiago in summertime is not significant, if emissions of precursors were to be decreased in the city, its western districts would see their O 3 mixing ratios increase on average, despite daily peaks largely dropping, while the eastern area would improve for every quantile. This phenomenon is linked to heterogeneous emissions within the city, which make for currently higher (lower, respectively) than background levels in the east(the west, respectively) : , ::: and :::::: lower :::: than 535 ::::::::: background ::: in :: the ::::: west. When all emissions are cut, the whole area is brought to background, hence the respective variations.
The vertical export of precursors above Santiago in summertime, in relation with unperturbed mountain-valley circulation and venting, creates a persistent O 3 bubble of more than 50 ppb on average, around 1000 m above the ground, slightly north of the city. Daytime upslope winds, the heterogeneous lifetimes of precursors, and increasing vertical profiles of photolysis rate account for this formation, which impact should be looked at in greater detail in terms of surface air quality improvement (or 540 worsening), and photo-chemical and greenhouse effects.  Table A2. Simulation scores for daily average low-level meteorology for wintertime and summertime 2015. T2 is the 2 m air temperature ( • C), RH the surface relative humidity, U10 10 m zonal wind and V10 10 m meridional wind speed (m s −1 ). MB is the mean bias, NRMSE the normalized root mean square error and R the Pearson correlation coefficient.  Table A3. Simulation scores for meteorological vertical profiles for four days in July 2015 at noon local time at Quinta Normal station in Santiago. The same notations as in Table A2 apply. Figure A1. ::::::