Long-term changes in northern mid-latitude tropospheric ozone concentrations: Synthesis of two recent analyses

Abstract

The HTAP (Hemispheric Transport of Air Pollution) and TOAR (Tropospheric Ozone Assessment Report) programs synthesized historical ozone measurements and compared them with recent measurements to quantify the long-term increase of tropospheric ozone concentrations that occurred at northern mid-latitudes in the 20th century due to the increase and subsequent stabilization of anthropogenic ozone precursor emissions. The two studies disagree regarding the magnitude of the increase; the HTAP study found an approximate doubling of baseline ozone concentrations, while the TOAR study suggested a change of about half that magnitude in the well-mixed planetary boundary layer. Here we explore the reasons for this difference, and reconcile the results. The two efforts agree regarding measurements of historical ozone concentrations, but disagree on the comparison of the historical to the modern data. HTAP analyzed seasonal mean historical and modern data from the same or closely equivalent, baseline sites. TOAR selected pre-1975 measurement data sets based upon four data selection criteria; averages of these selected data sets from all seasons, site locations and site environments were compared with annual means of modern data collected at rural European surface sites. The TOAR comparison approach introduced several biases that led to underestimates of the overall change of baseline ozone. We conclude that northern mid-latitude tropospheric baseline ozone concentrations, which are relevant for radiative forcing, increased by a factor of 2.1 ± 0.2 over the 1950–2000 period.