Tropical tropospheric ozone and carbon monoxide distributions: characteristics, origins and control factors, as seen by IAGOS and IASI
Abstract. The characteristics and seasonal variability of the tropical tropospheric distributions of ozone (O3) and carbon monoxide (CO) were analysed based on in situ measurements provided by the In-service Aircraft for a Global Observing System (IAGOS) program since 1994 and 2002 respectively, combined with observations from the Infrared Atmospheric Sounding (IASI) instrument on board the Met-op A satellite since 2008. The SOFT-IO model, which couples back trajectories with CO emissions inventories, was used to explore the origins and sources of the tropical CO observed by IAGOS. The highest tropical O3 and CO maxima occur over Northern Hemisphere (NH) Africa in the low troposphere (LT) (80 ppb and 850 ppb respectively at 2.5 km over Lagos) during the dry season (January). Despite the active local fires, local anthropogenic (AN) emissions (60 %) are dominant for the CO, and consequently the O3 maxima. The importance of the local AN emissions are highlighted over Central Africa, as they cause a persistent polluted surface layer during the transition seasons (40 % in October and 86 % in April). The second highest O3 and CO maxima are observed over Asia. Local or regional Asian AN emissions cause the CO maximum in the LT (0.5 km) in January, and the O3 maximum in the free troposphere (at 6 km) in the post-monsoon season (April). South China is the only Asian site where O3 peaks in the LT (75 ppb at 2.5 km), due to local fires (30 %) in addition to the local (52 %) and regional (15 %) AN emissions. The highest amount of transported CO in the tropics originates from Africa. The main transport pathway is from the dry-season African regions towards the wet-season ones. Contributions from NH Africa are found over Arabia and Eastern Africa (up to 70 %), and India (40 % in the mid (MT) and 60 % in the upper (UT) troposphere) during the dry season. Transport towards NH South America is found all year long, with significant contributions in the MT and UT (30–40 % over Caracas on an annual basis). In contrast, the impact of the Asian emissions in the LT and MT is limited on a local or regional scale. Export of polluted Asian air masses is important in the UT during the Asian summer monsoon and post-monsoon seasons, when convection is active. The AN Asian contributions are mostly found over Arabia and Eastern Africa (up to 80 %) during the Asian summer monsoon. During the post-monsoon, CO impacted by the Indonesian fires (resp. SouthEast Asian AN emissions) are transported towards Eastern Africa (64 % and 16 %) due to the Tropical Easterly Jet. The lowest O3 and CO levels are observed over South America, due to less strong local emissions in comparison to Asia and Africa. The only important CO and O3 enhancement is observed in the MT during the local fires (October), when O3 and precursors impacted by the local AN and fire emissions are trapped in an anticyclone and transported towards South Africa (5–10 ppb from SH and NH South America respectively).
Maria Tsivlidou et al.
Status: final response (author comments only)
- RC1: 'Comment on acp-2022-686', Anonymous Referee #1, 31 Oct 2022
- RC2: 'Comment on acp-2022-686', Anonymous Referee #2, 15 Nov 2022
- AC1: 'Authors response to the reviewers', Maria Tsivlidou, 15 Feb 2023
Maria Tsivlidou et al.
Maria Tsivlidou et al.
Viewed (geographical distribution)
This paper "Tropical tropospheric ozone and carbon monoxide distributions: characteristics, origins and control factors, as seen by IAGOS and IASI" by M. Tsivlidopu et al. provides a comprenehensive description and assessment of O3 and CO spatial and vertical distributions in the tropical regions. The manuscript is well written, clear and well-organized. By combining multi-annual analysis from IAGOS profiles, IASI retrivials and SOFT-IO outputs, the paper provides a suite of information important to study O3 and CO in the tropical regions like the relative contribution of anthropogenic emissions, biomass burning and transport over different sites/regions. I recommend pubblication after that a few minor points are considered.
1) Paragraph 2.1. The term "accuracy" should not be used in a quantitative way. Do the author refer to the systematic error, instead (since the precision is quantified)?
2) Pag 5, line 127. "The RSE is defined as the *fraction*"...This is unclear. Do you mean *ratio*?
3) Figure 2. The stripes visible for ozone, need more discussion. These discontinuities do not represent geophysical features but uncertainty in the data products. For some regions discontinuities of 5-10 ppb were visible which could be considered a quantification of the uncertainty related to the use of the different a priori profiles. Are these spatial discontinuities visible also in other vertical layers? Moroever a kind of noise that was not discussed in the paper was visible for ozone over the desertic regions of the northern Africa and Arabian peninsula. Since especially the first feature affect the regional average values, I would like to see a discussion about potential impact to the obtained results.
5) Pag 23, line 489: please specify AMA
6) The analysis was mostly based on the IAGOS profiles at selected locations. Along the manuscript, the authors nicely discuss also the intra-regional differences observed at different sites in the same regions and attributed observed differences. Potential limitations for the results upscaling should be also highlighted in the Conclusion section.