Testing approaches for calculating stomatal ozone fluxes from passive samplers

Current ozone (O₃) levels are high enough to negatively affect vegetation and may become worse in the future. Ozone risk assessments have recently shifted from exposure-based to flux-based metrics. Modeling stomatal O₃ fluxes requires hourly O₃ and meteorological data, which are not always available. Large datasets of O₃ concentrations measured with passive samplers exist worldwide, and usually provide weekly to monthly means. We tested the suitability of using aggregated data instead of hourly data for O₃ flux calculations with 3-year time series of O₃ data from 24 Spanish air quality stations. Five different approaches and three different parameterizations were tested. Ozone-averaged values in combination with hourly meteorological data provided the most robust estimates of accumulated O₃ fluxes (Phytotoxic Ozone Dose with no threshold, POD0), and the median of the absolute percent error (MAPE) due to aggregation came close to 5%. Aggregations from 1week to 1month yielded similar errors, which is important in the cost-efficiency terms of the chosen passive sampler exposure periodicity. One major limitation of these approaches is that they are not suitable for high POD thresholds, and that accuracy of the measurements with passive samplers has to be strictly assured in order to finally obtain acceptable errors. A combination of meteorological data and O₃ passive sampler measurements may be used to estimate O₃ fluxes at remote forest sites as a valuable risk assessment tool.