isoAOT40: An improved ozone exposure index based on the Annual Ozone Spectrum Profile (AO3SP)

Abstract

In recognition of the rising threats of ground-level ozone (O₃) pollution to forests, agricultural crops, and other types of vegetation, accurate and realistic risk assessment is urgently needed. The accumulated O₃ exposure over a concentration threshold of 40 nmol mol⁻¹ (AOT40) is the most commonly used metric to investigate O₃ exposure and its effects on vegetation and to conduct vegetation risk assessment. It is also used by international regulatory authorities for deriving critical levels and setting standards to protect vegetation against surface O₃. However, fixed periods of the growing season are used universally, yet growing seasons vary with latitudes and elevations, and the periods of plant lifespan also differ among annual species. Here, we propose the concept of the Annual O₃ Spectrum Profile (AO₃SP) and apply it to calculate the profile of AOT40 throughout the year (AAOT40SP, Annual AOT40 Spectrum Profile) using the International Organization for Standardization (ISO) weeks as a shorter window ISO-based accumulated exposure. Using moving time periods of three (for crops) or six (for forests) months, the _isoAOT40 behavior throughout the year can be examined as a diagnostic tool for O₃ risks in the short- or long-term during the lifecycle of local vegetation. From this analysis, AOT40 (_isoAOT40) that is most representative for the local conditions and specific situations can be identified, depending on the exact growing season and lifecycle of the target vegetation. We applied this novel approach to data from five background monitoring stations located at different elevations in Cyprus. Our results show that the AAOT40SP approach can be used for improved and more realistic assessment of O₃ risks to vegetation. The AO₃SP approach can also be applied using metrics other than AOT40 (exposure- or flux-based), adding a new dimension to the way O₃ risk to vegetation is assessed.

Keywords

Air pollution / AOT40 index / Ozone risk assessment / Critical levels / Vegetation exposure metric