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

Abstract

Abstract. In situ measurements in the upper troposphere–lower stratosphere (UTLS) have been performed in the framework of the European research infrastructure IAGOS (In-service Aircraft for a Global Observing System) for ozone since 1994 and for carbon monoxide (CO) since 2002. The flight tracks cover a wide range of longitudes in the northern extratropics, extending from the North American western coast (125° W) to the eastern Asian coast (135° E) and more recently over the northern Pacific Ocean. Several tropical regions are also sampled frequently, such as the Brazilian coast, central and southern Africa, southeastern Asia, and the western half of the Maritime Continent. As a result, a new set of climatologies for O3 (August 1994–December 2013) and CO (December 2001–December 2013) in the upper troposphere (UT), tropopause layer, and lower stratosphere (LS) are made available, including gridded horizontal distributions on a semi-global scale and seasonal cycles over eight well-sampled regions of interest in the northern extratropics. The seasonal cycles generally show a summertime maximum in O3 and a springtime maximum in CO in the UT, in contrast to the systematic springtime maximum in O3 and the quasi-absence of a seasonal cycle of CO in the LS. This study highlights some regional variabilities in the UT, notably (i) a west–east difference of O3 in boreal summer with up to 15 ppb more O3 over central Russia compared with northeast America, (ii) a systematic west–east gradient of CO from 60 to 140° E, especially noticeable in spring and summer with about 5 ppb by 10 degrees longitude, (iii) a broad spring/summer maximum of CO over northeast Asia, and (iv) a spring maximum of O3 over western North America. Thanks to almost 20 years of O3 and 12 years of CO measurements, the IAGOS database is a unique data set to derive trends in the UTLS at northern midlatitudes. Trends in O3 in the UT are positive and statistically significant in most regions, ranging from +0.25 to +0.45 ppb yr−1, characterized by the significant increase in the lowest values of the distribution. No significant trends of O3 are detected in the LS. Trends of CO in the UT, tropopause, and LS are almost all negative and statistically significant. The estimated slopes range from −1.37 to −0.59 ppb yr−1, with a nearly homogeneous decrease in the lowest values of the monthly distribution (5th percentile) contrasting with the high interregional variability in the decrease in the highest values (95th percentile).

Tropospheric Ozone Assessment Report: Present-day distribution and trends of tropospheric ozone relevant to climate and global atmospheric chemistry model evaluation

“…Ozone is particularly low (20-40 nmol mol -1 ) above Southeast Asia during boreal summer and autumn, likely due to deep convective uplift of low-ozone air masses (Ziemke et al, 2010;Cooper et al, 2013;Strode et al, 2017). While this analysis lacks observations above SE Asia during winter and spring, a recent IAGOS analysis of this region including observations through 2013 shows that upper tropospheric ozone peaks during the spring biomass burning season (40-50 nmol mol -1 ) (Cohen et al, 2018). In the tropics, relatively high ozone is found above regions known to be impacted by biomass burning, particularly over South America in SON (Yamasoe et al, 2015), and West Africa in DJF (Sauvage et al, 2005).…”

Global-scale distribution of ozone in the remote troposphere from ATom and HIPPO airborne field missions

“…3) and can be linked to the occurrence -or absence -of stratospheric air sampling during ATom and HIPPO. However, it is straightforward to remove stratospheric airmasses from airborne data using filters based on meteorology (potential vorticity) or composition (H2O/O3) (e.g., Cohen et al, 2018). In the absence of stratospheric air mixing (< 8 km in Fig.…”

“…IAGOS O3 and CO observations in the northern Atlantic UTLS provide a measurementbased climatology at commercial aircraft cruise altitudes for comparison to ATom. Simultaneous measurements of O3 and CO are of particular interest because CO provides a long-lived tracer of continental emissions, which helps to differentiate O3 sources (Cohen et al, 2018). We note that while IAGOS measurements encompass hundreds of seasonal flights (depending on the region), ATom sampled within each latitude band and season on one or two flights only (Fig.…”

Ice-supersaturated air masses in the northern mid-latitudes from regular in-situ observations by passenger aircraft: vertical distribution, seasonality and tropospheric fingerprint

“…Another source of data, but for the occurrence frequency of cirrus clouds originates from long-term analyses of satellite observations (Stubenrauch et al, 2010;Stubenrauch et al, 2013). In their 6-year climatology Stubenrauch et al (2010) report cirrus cloud coverage fractions for northern mid-latitudes of 35% in January and 27% in July from AIRS-LMD (2003, and respective fractions of 34% and 21% from TOVS -Path B 540 (1987to 1995, and 42% and 40% from CALIPSO (2006CALIPSO ( to 2007. The compilation of our annual cycle of ISSR occurrence and the respective observations from space-borne sensors is shown in Figure 12b.…”