Atmos. Chem. Phys., 18, 5415–5453, 2018 https://doi.org/10.5194/acp-18-5415-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Climatology and long-term evolution of ozone and carbon monoxide in the upper troposphere–lower stratosphere (UTLS) at northern midlatitudes, as seen by IAGOS from 1995 to 2013 Yann Cohen1,2, Hervé Petetin1, Valérie Thouret1, Virginie Marécal2, Béatrice Josse2, Hannah Clark1, Bastien Sauvage1, Alain Fontaine1, Gilles Athier1, Romain Blot1, Damien Boulanger3, Jean-Marc Cousin1, and Philippe Nédélec1 1Laboratoire d’Aérologie, Université de Toulouse, CNRS, UPS, France 2Centre National de Recherches Météorologiques, CNRS-Météo-France, UMR 3589, Toulouse, France 3Observatoire Midi-Pyrénées, Toulouse, France Correspondence: Yann Cohen ([email protected]) Received: 21 August 2017 – Discussion started: 19 September 2017 Revised: 2 February 2018 – Accepted: 16 March 2018 – Published: 20 April 2018 Abstract. In situ measurements in the upper troposphere– broad spring/summer maximum of CO over northeast Asia, lower stratosphere (UTLS) have been performed in the and (iv) a spring maximum of O3 over western North Amer- framework of the European research infrastructure IAGOS ica. Thanks to almost 20 years of O3 and 12 years of CO (In-service Aircraft for a Global Observing System) for measurements, the IAGOS database is a unique data set to ozone since 1994 and for carbon monoxide (CO) since 2002. derive trends in the UTLS at northern midlatitudes. Trends The flight tracks cover a wide range of longitudes in the in O3 in the UT are positive and statistically significant in northern extratropics, extending from the North American most regions, ranging from C0.25 to C0.45 ppb yr−1, char- western coast (125◦ W) to the eastern Asian coast (135◦ E) acterized by the significant increase in the lowest values of and more recently over the northern Pacific Ocean. Sev- the distribution. No significant trends of O3 are detected in eral tropical regions are also sampled frequently, such as the LS. Trends of CO in the UT, tropopause, and LS are al- the Brazilian coast, central and southern Africa, southeast- most all negative and statistically significant. The estimated ern Asia, and the western half of the Maritime Continent. As slopes range from −1.37 to −0.59 ppb yr−1, with a nearly a result, a new set of climatologies for O3 (August 1994– homogeneous decrease in the lowest values of the monthly December 2013) and CO (December 2001–December 2013) distribution (5th percentile) contrasting with the high interre- in the upper troposphere (UT), tropopause layer, and lower gional variability in the decrease in the highest values (95th stratosphere (LS) are made available, including gridded hor- percentile). izontal distributions on a semi-global scale and seasonal cy- cles over eight well-sampled regions of interest in the north- ern extratropics. The seasonal cycles generally show a sum- mertime maximum in O3 and a springtime maximum in CO 1 Introduction in the UT, in contrast to the systematic springtime maximum in O3 and the quasi-absence of a seasonal cycle of CO in the Ozone plays an important role in the thermal structure of the LS. This study highlights some regional variabilities in the stratosphere and in the oxidizing capacity of the troposphere UT, notably (i) a west–east difference of O3 in boreal sum- as a major source of hydroxyl radicals (OH). Tropospheric mer with up to 15 ppb more O3 over central Russia compared ozone is also a strong greenhouse gas (IPCC, 2013, AR5). with northeast America, (ii) a systematic west–east gradi- In contrast with carbon dioxide (CO2) and methane (CH4), ent of CO from 60 to 140◦ E, especially noticeable in spring the scientific understanding concerning its radiative forcing and summer with about 5 ppb by 10 degrees longitude, (iii) a is still at a medium level. Nevertheless, its impact on sur- Published by Copernicus Publications on behalf of the European Geosciences Union. 5416 Y. Cohen et al.: IAGOS from 1995 to 2013 face temperature has been shown to reach its maximum when as an interannually variable signal. Also, with a dense ge- changes in O3 mixing ratios occur in the upper troposphere– ographical coverage spreading over the midlatitudinal zonal lower stratosphere (UTLS) region (e.g. Riese et al., 2012). band (except the Pacific Ocean in so far as regular measure- Carbon monoxide (CO) is a precursor for CO2 and tropo- ments only started in 2012), spatial distributions seen by IA- spheric O3. It is also a major sink for OH radicals in an GOS are a source of complementary information for local unpolluted atmosphere (Logan et al., 1981; Lelieveld et al., observations, quantifying the spatial variability at synoptic 2016), thus increasing the lifetime of CH4, such that CO and intercontinental scales. The monitoring period is now emissions are considered “virtually certain” to cause posi- sufficient to derive representative climatologies and long- tive radiative forcing (IPCC, 2013, AR5). Because of its ∼ 2- term evolution of both species over different regions of in- month lifetime (Edwards et al., 2004), it can be used as a terest. tracer of combustion processes, mainly anthropogenic emis- Since the 1990s, several observation systems have been sions and biomass burning (Granier et al., 2011), although monitoring these two species, aiming at a better understand- it is also a product of the oxidation of CH4 and isoprene ing of the processes controlling their spatial distribution and (C6H8). Identifying the contributions from different factors temporal evolution, (multi-)decadal trends and interannual to O3 concentrations in the UTLS is more difficult than for variability in the UTLS. Satellite-based instruments provide CO. First, the O3 mixing ratio is influenced by transport from global coverage but with limited vertical resolution while both the stratosphere (e.g. Hsu and Prather, 2009; Stevenson sondes, lidar (light detection and ranging), and aircraft in et al., 2013) and the troposphere (e.g. Barret et al., 2016; situ measurements during ascent and descent phases offer a Zhang et al., 2016). Second, tropospheric O3 is produced precise view of the vertical profile over a limited area. Neu from a wide variety of precursors (non-methane volatile or- et al.(2014b) have recently presented a comprehensive inter- ganic compounds, CO, HO2) emitted from numerous surface comparison of currently available satellite O3 climatologies sources, both natural and anthropogenic. Third, nitrogen ox- (gathering limb and nadir viewing) in the 300–70 hPa range. ides (NOx) implicated in the production of O3 are not only Although suffering from a coarse vertical resolution, such an emitted at the surface by combustion processes but are also analysis offers a basis for a comparison of the large-scale produced in the free troposphere by lightning. In parallel, ni- spatio-temporal characteristics of the O3 distribution (e.g. trogen oxides can be released by the decomposition of reser- zonal mean cross sections, seasonal variability, interannual voir species such as peroxyacetyl nitrate (PAN), a long-lived variability). The authors point out the large differences in species that can be transported at intercontinental scales in these satellite-based climatologies in the UTLS. Observing the free troposphere (e.g. Hudman et al., 2004). this layer often requires a more detailed picture, with a higher Establishing trends in O3 and CO with observations is im- spatio-temporal resolution and multiple species, which can portant for evaluating the impact of the reduction in anthro- all be addressed with aircraft platforms. Tilmes et al.(2010) pogenic emissions in the northern midlatitudes on the at- proposed an integrated picture of O3, CO, and H2O clima- mospheric chemical composition. Trends in both gases are tologies over the Northern Hemisphere, mostly representa- also important for assessing changes in atmospheric radia- tive of North America and Europe, based on research aircraft tive forcing. Furthermore, both observed trends and clima- campaigns. They provide a valuable data set representing the tologies (i.e. time-averaged data) are used as reference val- vertical gradients of H2O, O3, and CO in three regimes: trop- ues in order to evaluate the ability of models to reproduce ics, subtropics, and the polar region in the Northern Hemi- the past atmospheric composition and thus to forecast future sphere. Compared to commercial aircraft, research aircraft changes. Consequently, assessing the mean distribution and can fly at higher altitudes. Such a study has taken the most ad- the long-term evolution of O3 and CO is within the focus of vantage of this, which has made tropopause-referenced tracer several research programs dealing with the understanding of profiles and tracer–tracer correlations possible and has high- atmospheric composition regarding air quality and climate lighted further the transport and mixing processes in a wider issues. vertical range. However, the IAGOS program using commer- The present study is based on the measurements of these cial aircraft as a multi-species measurement platform enables two trace gases, available in one single data set: the IA- a better characterization of interregional differences, given its GOS database (http://www.iagos.org). In the framework of higher spatio-temporal resolution. One of the objectives of the IAGOS program (In-service Aircraft for a Global Ob- the present study is to emphasize the use of IAGOS as