Tropospheric Ozone Is Still Increasing across the Northern Hemisphere (Invited Presentation)

Tropospheric ozone is an important greenhouse gas, is detrimental to human health and crop and ecosystem productivity, and controls the oxidizing capacity of the troposphere. Due to its high spatial and temporal variability, the current in situ monitoring network has been insufficient for quantifying net tropospheric ozone changes across the entire Northern Hemisphere on time scales of two decades or less. This presentation will show, using frequent commercial aircraft profiles since the mid-1990s, that there has been a consistent increase of free tropospheric ozone above eleven polluted regions, widely distributed across the tropics and mid-latitudes of the Northern Hemisphere. Boundary layer (BL) ozone has decreased above some mid-latitude regions where ozone precursor emissions have diminished, mainly in the warm season. However, BL ozone reductions have been offset by free tropospheric increases, leading to overall increases in the tropospheric column ozone (TCO) values. The largest TCO increases occurred above Malaysia/Indonesia, Southeast Asia and India, while the largest increases of BL ozone occurred above India, Malaysia/Indonesia and Northeast China/Korea (NCK). This analysis is the first to report BL increases above the Persian Gulf, which are comparable to those above NCK. Strikingly, extremely low ozone values once commonly observed above Malaysia/Indonesia, Southeast Asia and India (1994-2004) have almost completely disappeared in recent years (2011-2016). On the hemispheric scale, the largest increases in the lower and mid-troposphere stretched from the Persian Gulf and NCK, southward to the equatorial regions of Africa (Gulf of Guinea) and Malaysia/Indonesia. Model analysis indicates that the ozone increases reported here are driven by increases of anthropogenic and biomass burning emissions. The net result of shifting anthropogenic ozone precursor emissions has led to an increase of ozone across the Northern Hemisphere, despite emissions reductions at mid-latitudes.