. Twenty years of observed tropospheric ozone increases across the Northern Hemisphere

Abstract
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-Situ Aircraft measurement for 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 mid-latitudes. Annual median ozone increased in nearly all levels of the troposphere, above all regions, including the eastern US 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 US and western Europe.