Understanding the effect of international ship NOx emissions on surface O3 concentration using three different attribution approaches

We use global atmospheric chemistry models (GACMs) to understand the extent to which NOx emissions from ships affect the surface ozone concentration and its seasonal cycle over continental regions. Here we use three different attribution approaches: NOx-tagged, combined-tagged and perturbation; implemented in three different GACMs, for understanding the qualitative and quantitative differences in employing these approaches for attributing surface ozone to NOx emitted from ships sailing over various marine regions. Using the NOx-tagged method, we attribute ~10 % contribution of ship NOx emissions to the simulated surface ozone over the receptor regions considered in our study. The summer peak in this contribution (eg. Over NW Europe) was mainly from NOx emissions over nearby marine regions (such as Baltic and North Seas). During the non-summer months, we simulate large contributions to surface ozone from remote marine regions (such as North Pacific). The combined-tagged approach attributes only ~7.7 % relative contribution from ship-tagged component, which is a smaller relative contribution compared to the NOx-tagged approach, due to the attribution considering an equally weighted small contribution of reactive carbon emitted from ships. Perturbation approach simulates the impact of changing NOx emissions on surface ozone concentration, which in our study is ~2.3 % of the ozone in the unperturbed simulation. This impact simulated using the perturbation approach is smaller compared to the contributions simulated using the tagging approaches discussed above. We explore the reason for the smaller impact simulated using the perturbation approach compared to the tagged contributions, by simulating NOx-tagged and combined-tagged perturbations in ship NOx emissions and quantify the role of both perturbed and unperturbed tagged emission sources in contributing to the response to perturbation. The non-tagged surface ozone response to ship-NOx perturbation is ~2.5-5 times smaller than the response from the ship-tagged ozone component, because of compensating negative responses due to reduction in ozone production from other contributing tagged-components such as land-based natural and anthropogenic ozone precursor NOx sources.