Temporal variation of northern midlatitude baseline ozone: 48-year observational record challenges our understanding of tropospheric chemistry

Abstract. A continuous, 48-year measurement record, plus some earlier measurements, of baseline ozone at northern mid-latitudes are analyzed to quantify seasonal cycles and long-term changes of annual mean tropospheric ozone. Long-term changes are similar at all sites, and seasonal cycles are similar in the marine boundary layer (MBL) and in the free troposphere (FT), but with marked differences between those two environments. Over the last half of the 20^th century, ozone concentrations increased by a factor of ~2, the seasonal cycle amplitude increased by nearly 50 %, and its maximum shifted to later in the year by 10 ± 13 days. The long-term increase ended early in the 21^st century, followed by a slow decrease that reversed only a small fraction of the total earlier increase. In contrast, the seasonal cycle returned to near that of the preindustrial period. Simulations by six earth system models agree with the magnitude of the overall ozone increase and the increase ending early this century; however, observations indicate only a post-1950 increase, while models simulate a slower increase beginning in 1850. Consequently, the high bias of model simulations, while modest (~10 %) in recent years, was much larger (~87 %) in the 1950s. Qualitatively similar seasonal cycles and shifts are seen in the measurements and simulations, but simulations do not show the observed strong separation between MBL and FT behavior. We hypothesize that models simulate a background troposphere that is too NOx-rich, implying a lesser role than models simulate for methane in raising background ozone concentrations.