the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Dec 2024
Aircraft Observations of Continental Pollution In the Equatorial Lower Stratosphere over the Tropical Western Pacific During Boreal Winter
Abstract. Recent studies hypothesize that emissions from fires reaching the stratosphere can provide aerosols and aerosol precursors that initiate stratospheric ozone loss and lead to radiative heating of the stratosphere and cooling of the surface. Air from the troposphere enters the stratosphere primarily over the tropical western Pacific (TWP) during boreal winter. We report observations in the TWP of persistent, ubiquitous continental pollution in the tropical tropopause layer (TTL) and lower stratosphere (LS) during the Airborne Tropical TRopopause EXperiment (ATTREX) campaign in February–March 2014. We found concentrations of carbon monoxide (CO) enhanced up to 65 % over background levels in the deep tropics (5° S–15° N, 16 –17 km). Correlations of CO with hydrocarbon and halocarbon species indicated a biomass burning source, with the largest CO enhancements found in warmer, clear air. Satellite observations of CO did not detect the thin pollution layers observed by the aircraft, but did indicate Africa, Indonesia, and the western/central Pacific as geographical hot spots for CO in the TTL. Backward trajectories identified convective encounters in these areas as the dominant sources of polluted air in the TWP. Africa and Indonesia contributed about 60 % of the excess CO, transported to the TWP in two to four weeks. Our study confirms that air in the TTL over the TWP is affected by emissions from distant fires that can rapidly reach the LS in the principal source region for air entering the stratosphere, supporting the view that fires in tropical regions could impact stratospheric ozone and temperatures.
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RC1: 'Comment on egusphere-2024-3832', Anonymous Referee #1, 13 Jan 2025
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The manuscript "Aircraft Observations of Continental Pollution In the Equatorial Lower Stratosphere over the Tropical Western Pacific During Boreal Winter" by Pittmann et al. presents observations from the ATTREX campaign 2014 over the tropical western Pacific (TWP) region obtained with the Global Hawk.
The paper describes the dataset and comes to the conclusion that the UTLS composition in the TWP is dominated during the campaign time by continental pollution.
In my point of view the paper is in general well written, but some major and minor revisions should be conducted for a final publication in ACP.1) The pollution is termed "continental pollution". Even though the analysed pollution is of continental origin, the authors state that this is mostly from biomass burning pollution. Consequently, a re-naming of the manuscript should be considered, as the influence of anthropogenic pollution in the TWP UTLS is minor.
2) The authors make both in the abstract as well as in the conclusions statements about aerosol particles. However, the measurement data does not obtain any data point on aerosol particles. Even though aerosol particles are potentially included in the emission sources, their fate and transformation in the convective ascent as well as microphysical processing and / or sedimentation along the relatively long transit times is uncertain. I would expect substantial scavenging of primary emitted particles in the ascent, either by precipitation scavenging or by ice nucleation such that only a minor fraction of primary aerosol particles will be transported into the UTLS. Whether new particle formation also takes place in the TWP or in the transit to the measurement region is not yet certain. Therefore, the TWP cannot be named an ascent region of aerosols into the LS. And as this is not shown in this manuscript (and it is not its intention), these statements should be either weakened and discussed or should be removed from the manuscript.
Similarly, the role of VSLS are prominently mentioned in the beginning, but the analysis showed that they do not play a significant role in this context, i.e., as lofted species with a high ozone destruction potential.3) The back-trajectories are calculated for 40 days. This is a very long time period for air mass transport, without the consideration of diabatic processes (cloud processing), turbulence and small-scale mixing, such that the longest trajectories only represent a very crude approximation for the air mass history. Especially trajectories who travel over the Indian Ocean to Africa and from there back to the TWP region must be treated carefully. These limitations should be discussed in the manuscript.
The occurrence of convection associated with the trajectories also has to be discussed in a little more detail. As the trajectories are calculated from ERA5 dynamics, there is no guarantee that individual convective cells and elements on the order of <30 km are represented properly in the meteorological re-analysis data. Of course, the satellite data represent reality, but it should be analysed / discussed whether the convection in the re-analysis is co-located with the observed convective activity or whether this activity is sufficiently wide-spread such that air parcels in the convective region have a high probability to be lofted.
The analysis of air parcels in convection also reveals that it is not guaranteed that an air parcel actually has seen the convection, only because it has been in its vicinity: it could also happen that air parcels simply stream around the anvil of the convective towers and are not substantially modified by lofted near surface air.4) The selected / presented compounds often have quite enhanced life times, especially CH4 and CO2, such that convective signals of events more than a few days ago can hardly be distinguished from background conditions. Consequently, the profiles from Fig.2 for methane and carbon dioxide provide a measure for the substantial background variability. For species with shorter lifetime, like CO, the variability is smaller, even though the regimes of convectively and non-convectively influenced air masses cannot be easily distinguished (in Fig.2). In Fig.3 some of the flights show a distinct enhanced CO in the UTLS, which could indicate convective outflow; however, after travel times of more than a week, it is unlikely, that these structures remain that clearly visible, indicating more local or regional convection and further lofting, e.g., by convectively induced gravity waves and their mixing processes.
On the other hand, the shorter lived compounds (C3H8, C2H2) hardly show enhancements in the profiles. Given their shorter chemical lifetime, the air masses must have been lofted several days prior to the observations, if only in the Northern observations any signal can be identified. This rules out transport from Africa, but indicates more regional continental pollution. Even the CO signal in the Southern observations is quite low, and hardly shows UT enhancements. This does not fit well with the analysis from Fig. 14 that the corresponding trajectories could have seen convection only two weeks ago. A separation of continental convection or biomass burning in the vicinity of convection or anthropogenic activity near the convection might elucidate the origin or missing signals a bit better. This is most obvious for the Southern observations, but would also hold true for the rest of the data.5) The MLS data does not really sheds light onto the observations, as the signal cannot be properly distinguished from the background. Therefore, the manuscript could be shortened here, and the figures 12 and 13 could be moved to an appendix or supplement.
6) The pollution is mostly determined in the cloud-free air. As the authors state, that the pollution is mostly transported for several days, and that the trajectories are slowly lofted by radiative cooling, this is not very surprising. On the other hand this is a bit contradictionary to the finding of Fig. 11, which states that the pollution is usually associated with warmer air masses. As the typical temperature profile does not increase quickly in the low latitudes above the thermal tropopause, this needs an explanation.
7) I am missing an explanation, why the pollution is not found in cloudy or moist regions. Is this because the polluted air is at higher altitude than the clouds, i.e., because of the above mentioned lofting?
8) What information can be deduced from Fig. 9, which is not also included in Fig. 10? Can you (re)move Fig. 9 to the supplement, in order to slightly shorten the manuscript and reformulate the statements from the corresponding paragraph? Basically, the same conclusions, i.e., that the highest pollution levels are found in non-saturated regions, can be drawn from Fig. 10, which has better representation via the color coded visibility.
However, the color scale of Fig.10 (using rainbow colours) could be modified to be more perceptually uniform and color-blind / black-white friendly.9) Please discuss the variability in the vertical motion of the individual trajectories: especially trajectories from Africa travel more than 50 hPa upwards and downwards on timescales of a day or two (Fig. 14c, around day 13 to 21). What processes would you associate these relatively rapid air motions including downwelling, before the radiatively driven ascent prior to the measurements?
Citation: https://doi.org/10.5194/egusphere-2024-3832-RC1
Data sets
NASA ATTREX E. J. Jensen et al. https://espoarchive.nasa.gov/archive/browse/attrex/id4
MLS N. Livesey et al. https://disc.gsfc.nasa.gov/datasets?page=1&keywords=AURA%20MLS
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