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Twenty Years of Observed Tropospheric Ozone Increases Across the Northern Hemisphere

A. Gaudel1,2, O.R. Cooper1,2, K. Chang3,2, V. Thouret4, J. Ziemke5, S. Strode6,7, P. Nédélec4 and R. Blot4

1Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309; 303-497-6563, E-mail: audrey.gaudel@noaa.gov
2NOAA Earth System Research Laboratory, Chemical Sciences Division (CSD), Boulder, CO 80305
3National Research Council Post-Doc, Boulder, CO 80305
4Laboratoire dAérologie, The National Center for Scientific Research (CNRS), and Universite Paul Sabatier Toulouse III, Toulouse, France
5University of Maryland, College Park, MD 20742
6Universities Space Research Association (USRA), Colombia, MD 21046
7NASA Goddard Space Flight Center (GSFC), Greenbelt, MD 20771

Tropospheric ozone is the third most important greenhouse gas, is detrimental to human health and crop and ecosystem productivity, and controls the oxidizing capacity of the troposphere. Due to tropospheric ozone’s high spatial and temporal variability, the current in situ monitoring network has been insufficient for quantifying ozone’s net global change on time scales less than two decades. The In-service Aircraft for a Global Observing System (IAGOS) has used commercial aircraft to monitor ozone worldwide since 1994. Here we show IAGOS observations that demonstrate 20 years of ozone increases above seven polluted regions distributed across the northern tropics and midlatitudes. Annual median ozone increased in nearly all levels of the troposphere, above all regions, including the eastern U.S. and western Europe, where ozone precursor emissions have decreased. Ozone reductions were limited to extreme ozone pollution events in the lower troposphere of the eastern U.S. and western Europe.

Figure 1

Figure 1. Annual differences in the 50th percentile of ozone (nmol mol-1). The differences are calculated at each vertical pressure level between 1994–2004 and 2011–2016 above Europe (blue), Northeast U.S. (green), Northeast China / Korea (red), Southeast Asia (cyan), India (orange), and between 1998–2005 and 2011–2016 above Persian Gulf (black). The statistically significant differences between both periods at each pressure level are indicated with a square.