Downscaled Solar-Induced Chlorophyll Fluorescence from TROPOMI over the Greater Toronto Area

Abstract:

Solar-Induced chlorophyll Fluorescence (SIF) has been shown to be a valuable proxy for photosynthetic activity on regional scales. Measurements of SIF have been available from space for the past few decades however, they have been limited in both spatial and temporal resolution due to large satellite footprints or long return times of the satellite instruments. The TROPOspheric Monitoring Instrument (TROPOMI) launched in 2017 has allowed for an unprecedented combination of high temporal resolution and moderate spatial resolution of 3.5 x 7 km at nadir however, this resolution is still too coarse to resolve details on the local scale. Using a method introduced by Turner et al. (2019) we have used overlapping TROPOMI footprints over a two-week period in conjunction with the MODerate-resolution Imaging Spectrometer’s (MODIS) vegetation indices, to downscale SIF data to a resolution of 500 x 500 m over the Greater Toronto Area (GTA), the most populous urban area in Canada. This analysis has allowed us to identify locations with large photosynthetic activity on local scales. We noted particularly strong signals from both croplands and forested regions. The TROPOMI SIF results are compared to SIF retrieved from the Orbiting Carbon Observatory-2 and -3, which both take periodic high-resolution measurements over the GTA. The results of this analysis allow us to monitor the seasonality of SIF in both urban centers and more rural areas. We investigate the impact of the urban heat island effect in Toronto on the seasonality of SIF. In addition, we compare measurements of SIF to ground- and satellite-based measurements of CO₂ concentrations as well as Net Ecosystem Exchange (NEE) from the Vegetation Photosynthesis and Respiration Model, to investigate the relationship between NEE and SIF across different regions of the GTA. This analysis will eventually allow us to estimate the role of vegetation in CO₂ removal via photosynthesis and its seasonality in the GTA.