Strongly reduced transport emissions lead to decrease of ozone in the planetary boundary layer
May 18, 2021
Strongly reduced transport emissions lead to decrease of ozone in the planetary boundary layer
The Covid-19 pandemic resulted in greatly reduced anthropogenic emissions, especially during spring and early summer 2020. In particular, the emissions by transport were significantly lower. This gave the unique opportunity to study the effect of emission reductions on various trace gases. For example, emissions of reactive species comprising nitrogen oxide (NOx), non-methane hydrocarbons and carbon monoxide (CO), lead to the formation of ozone (O3), a pollutant and a greenhouse gas. In their recent study, Mertens et al. (2021) improved the understanding of key processes concerning the role of anthropogenic emissions in the atmosphere by means of numerical simulations of the global atmosphere.
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Figure 1: Ground-level mixing ratios of ozone (in nmol/mol) for the BAU simulation (left) and the difference (right, in nmol/mol) between BAU and COVID19 (BAU minus COVID19) for the period 15. – 30. March 2020. (Figure: CC-BY 3.0, Mertens et al.)
The idealized study shows a decrease of lower tropospheric ozone of 8% over Europe in May 2020 due to the (idealized) emission reductions. The calculated reductions, by using the coupled global-to-regional chemistry-climate model MECO(n), are in line with observed changes in ground level ozone. This O3 reduction is mainly due to the decreased ozone precursors, namely nitrogen oxides and non-methane hydrocarbons, from anthropogenic origin. Over Europe, atmospheric concentrations of reactive species are considerably lower, with e.g. a reduction in NOx in the order of 20%. Interestingly, the results also show that the ozone reduction is comparably lower than the reduction of the total NOx emissions (around 20% compared to about 8%). This is a result of a compensating mechanism: Although anthropogenic emissions of reactive species are reduced, more ozone is produced from natural emissions of those reactive species. Eventually, this results in an increase of the ozone productivities of the remaining anthropogenic emissions. Additionally, the atmospheric lifetime of ozone is increasing.
The study by Mertens et al. (2021) shows that the simple conclusion that a reduction of anthropogenic emissions leads to a reduction of ozone of comparable size is not generally valid. Additionally, the study highlights that due to the rather long lifetime of ozone, the emissions in other parts of the world strongly influence European ozone levels. Therefore, reducing emissions only in Europe will most likely not lead to envisaged ozone decreases in Europe. With respect to potential mitigation options, the result demonstrates that detailed assessments are needed to judge, whether planned emissions reductions are sufficient to decrease tropospheric ozone burdens substantially.
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Figure 2: Contributions to lower tropospheric ozone columns (in percent) in the BAU (blue; reference period), COVID19 (orange) simulation and their differences (red, in percentage-points) during May 2020. “Avia” is aviation and “ship” is shipping. Other natural sources include emissions from lightning, soils, biomass burning and methane depletion. Categories marked with two asterisks refer to the vertical axis on the right, all others to the vertical axis on the left. (Figure: CC-BY 3.0, Mertens et al.)
