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 observa5 tions of isoprene emission do not exist. Two readily available global-scale long-term observations hold information about surface isoprene activity: gross primary producivity (GPP) and tropospheric formaldehyde column variability (HCHOv). We analyze
multi-year seasonal linear correlations between observed GPP and HCHOv. The observed GPP-HCHOv correlation patterns are used to evaluate a global Earth system
model that embeds three alternative leaf-level isoprene emission algorithms. GPP and
HCHOv are decoupled in the summertime southeast US (r=−0.03). In the Amazon,
GPP-HCHOv are weakly correlated in March-April-May (MAM), correlated in June-July-
August (JJA) and weakly anti-correlated in September-October-November (SON). Iso-
prene 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 anti-correlated
with HCHOv. In an isoprene emission model without soil moisture dependence, iso-
prene emission is anti-correlated with photosynthesis and correlated with HCHOv.
Long-term monitoring of isoprene emission, soil moisture and meteorology is required
in water-limited ecosystems to improve understanding of the factors controlling iso-
prene emission and its representation in global Earth system models.