Changes in anthropogenic precursor emissions drive shifts in the ozone seasonal cycle throughout the northern midlatitude troposphere

Abstract

Abstract. Simulations by six Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth system models indicate that the seasonal cycle of baseline tropospheric ozone at northern midlatitudes has been shifting since the mid-20th century. Beginning in ∼ 1940, the magnitude of the seasonal cycle increased by ∼10 ppb (measured from seasonal minimum to maximum), and the seasonal maximum shifted to later in the year by about 3 weeks. This shift maximized in the mid-1980s, followed by a reversal – the seasonal cycle decreased in amplitude and the maximum shifted back to earlier in the year. Similar changes are seen in measurements collected from the 1970s to the present. The timing of the seasonal cycle changes is generally concurrent with the rise and fall of anthropogenic emissions that followed industrialization and the subsequent implementation of air quality emission controls. A quantitative comparison of the temporal changes in the ozone seasonal cycle at sites in both Europe and North America with the temporal changes in ozone precursor emissions across the northern midlatitudes found a high degree of similarity between these two temporal patterns. We hypothesize that changing precursor emissions are responsible for the shift in the ozone seasonal cycle; this is supported by the absence of such seasonal shifts in southern midlatitudes where anthropogenic emissions are much smaller. We also suggest a mechanism by which changing emissions drive the changing seasonal cycle: increasing emissions of NOx allow summertime photochemical production of ozone to become more important than ozone transported from the stratosphere, and increasing volatile organic compounds (VOCs) lead to progressively greater photochemical ozone production in the summer months, thereby increasing the amplitude of the seasonal ozone cycle. Decreasing emissions of both precursor classes then reverse these changes. The quantitative parameter values that characterize the seasonal shifts provide useful benchmarks for evaluating model simulations, both against observations and between models.

Arctic Tropospheric Ozone Trends

“…Springtime negative trends at Summit may also be due to emission reductions over North America. Our results do not suggest a shift in the O 3 seasonal cycle toward higher concentrations in the spring (i.e., moving back toward pre‐industrial O 3 seasonality) as reported at NH mid‐latitudes (Bowman et al., 2022). Another explanation for decreasing springtime O 3 at the surface could be that reductions in snow cover due to climate warming (Mudryk et al., 2020) are leading to more O 3 dry deposition to Scandinavian forests.…”

“…While such increases were reported previously at Utqiaġvik (A. Christiansen et al., 2022; Cooper et al., 2020) and Alert (Sharma et al., 2019), we confirm this tendency over the wider Arctic. Emission reductions of NO x in Europe and North America, and more recently over eastern Asia, have led to increasing wintertime O 3 at mid‐latitudes due to less nitrogen oxide titration of O 3 (Bowman et al., 2022; Jhun et al., 2015; T. Wang et al., 2022). This can explain observed increases in wintertime surface Arctic O 3 , influenced primarily by transport of air masses from Europe (Hirdman et al., 2010).…”

Multidecadal increases in global tropospheric ozone derived from ozonesonde and surface site observations: can models reproduce ozone trends?

“…Much of the evidence around tropospheric ozone changes has come from the analysis of surface ozone trends, especially over the United States and Europe (Yan et al, 2018a, b;Lefohn et al, 2010;Simon et al, 2015). Changes over these regions since the 1990s are often characterized by shifts in the magnitude of the seasonal cycle (Bowman et al, 2022) and decreasing peak and summertime ozone values, in contrast to the increasing annual mean ozone driven by increasing low-percentile (e.g., 5th and 10th percentile) ozone. However, changes occurring at the surface may differ from changes above the boundary layer due to the increased importance of transport processes over emissions in the FT. Trends throughout the troposphere can be investigated via satellites measuring total ozone throughout the entire atmospheric column after accounting for the stratospheric contribution.…”

Transboundary transport of air pollution in eastern Canada

“…This also suggests that anthropogenic emissions in this region have the effect of shifting the annual O 3 maximum later in the year, as has been discussed in other studies. 52,53…”

“…This also suggests that anthropogenic emissions in this region have the effect of shiing the annual O 3 maximum later in the year, as has been discussed in other studies. 52,53 NO X and SO 2 are reactive gases with a relatively short lifetime in the atmosphere. Therefore, the absolute differences in NO X and SO 2 concentrations between the simulations are largely constrained to the source regions (Fig.…”