A regional scale flux-based O3 risk assessment for winter wheat in northern Italy, and effects of different spatio-temporal resolutions*

Tropospheric ozone (O3) is a secondary atmospheric pollutant known to cause negative effects on vegetation in terms of physiological oxidative stress, growth rate reductions and yield losses. In recent years, dose-response relationships based on the O3 stomatal flux and effects on the biomass growth have been defined for several crop species. This study was aimed at developing a dual-sink big-leaf model for winter wheat (Triticum aestivum L.) to map the seasonal Phytotoxic Ozone Dose above a threshold of 6 nmol m-2 s- 1 (POD6) in a domain centered on the Lombardy region (Italy). The model runs on local measured data of air temperature, relative humidity, precipitation, wind speed, global radiation and background O3 concentration provided by regional monitoring networks, and includes parameterizations for the crop's geometry and phenology, the light penetration within the canopy, the stomatal conductance, the atmospheric turbulence, and the soil water availability for the plants. For the 2017 an average POD6 of 2.03 mmol m-2 PLA (Projected Leaf Area) was found for the Lombardy regional domain, corresponding to an average relative yield loss of 7.5 %, using the finest spatio-temporal resolution (1 x 1 km2 and 1-h). An analysis of the model's response to different spatio-temporal resolutions (from 2 x 2 to 50 x 50 km2 and from 1 to 6 h) suggests that coarser resolution maps underestimated the average POD6 regional value from 8 to 16 %, and were unable to detect O3 hotspots. Nevertheless, resolutions of 5 x 5 km2 1-h, and 1 x 1 km2 3-h, can still be considered reliable for the estimation of the O3 risk at the regional level since they presented relatively low root mean squared error. Furthermore, although temperature was the main limiting factor for the wheat stomatal conductance in most of the domain, soil water availability emerged as the key factor for determining the spatial patterns of the POD6.