Testing approaches for calculating stomatal ozone fluxes from passive samplers

Current ozone (O-3) 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 O-3 fluxes requires hourly O-3 and meteorological data, which are not always available. Large datasets of O-3 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 O-3 flux calculations with 3-year time series of O-3 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 O-3 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 1 week to 1 month yielded similar errors, which is important in the cost-efficiency terms of the chosen passive sampler exposure periodicity. Onemajor 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 O-3 passive sampler measurements may be used to estimate O-3 fluxes at remote forest sites as a valuable risk assessment tool. (C) 2016 Elsevier B.V. All rights reserved.