Relationships between photosynthesis and formaldehyde as a probe of isoprene emission

Atmospheric oxidation of isoprene emission from land plants affects radiative forcing of global climate change. There is an urgent need to understand the factors that control isoprene emission variability on large spatiotemporal scales but such direct observations of isoprene emission do not exist. In this study we apply three readily available global-scale long-term observationally based data sets to explore surface isoprene activity: gross primary productivity (GPP) derived from FLUXNET measurements, satellite-based solar-induced chlorophyll fluorescence (SiF) and satellite-based tropospheric formaldehyde column variability (HCHOv). The observed correlations are used to evaluate a global Earth system model that embeds three alternative leaf-level isoprene emission algorithms. On daily time scales, observed SiF-HCHOv correlations are not statistically significant across the globe. On seasonal to interannual time scales, GPP and HCHOv are decoupled in the summertime in the southeast US (r=-0.03). The SiF-HCHOv correlations show similar patterns as the GPP-HCHOv. Isoprene emission algorithms that include soil moisture dependence demonstrate greater skill in reproducing the observed interannual seasonal GPP-HCHOv correlations in the southeast US and the Amazon. In isoprene emission models that include soil moisture dependence, isoprene emission is correlated with photosynthesis and anticorrelated with HCHOv. In an isoprene emission model without soil moisture dependence, isoprene emission is anticorrelated with photosynthesis and correlated with HCHOv. The relationships between photosynthesis, isoprene emission and HCHOv are investigated at 9 flux tower sites for which time-varying isoprene emission was measured using eddy covariance. At the Missouri Ozarks site, which experienced the severe drought in central US in July 2012, substantial reductions in isoprene flux and SiF were observed, but no concomitant reduction in HCHO column was found. Long-term monitoring of isoprene emission, soil moisture and meteorology is required in water-limited ecosystems to improve understanding of the factors controlling isoprene emission and its representation in global Earth system models.