Abstract. Isoprene is important in atmospheric chemistry, but its seasonal emission pattern – especially in the tropics, where most isoprene is emitted – is incompletely understood. We set out to discover general, biome-independent relationships between large-scale isoprene emission and a series of potential predictor variables, including both observed and model-estimated variables related to gross primary production (GPP) and canopy temperature. To this end we used remotely sensed atmospheric concentrations of formaldehyde, an intermediate oxidation product of isoprene, as a proxy for isoprene emission in 22 regions selected to span high to low latitudes, to sample major biomes, and to minimize interference from pyrogenic sources of volatile organic compounds that could interfere with the isoprene signal. Formaldehyde concentrations showed the highest average seasonal correlations with remotely sensed (r = 0.85) and model-estimated (r = 0.80) canopy temperatures. Both variables predicted formaldehyde concentrations better than air temperature (r = 0.56) and a "reference" isoprene model that includes both temperature and GPP (r = 0.49), and far better than either remotely sensed green vegetation cover (r = 0.25) or model-estimated GPP (r = 0.14). GPP in tropical regions was anti-correlated with formaldehyde concentration (r = –0.30), which peaks during the dry season. We conjecture that the positive correlations of isoprene emission with primary production, and with air temperature, found in temperate forest regions arise simply because all three peak during the relatively short growing season. In most tropical regions, where the seasonal cycles of GPP and canopy temperature are very different, isoprene emission is revealed to depend on canopy temperature but not at all on GPP. The lack of a general correlation between GPP and formaldehyde concentration is consistent with experimental evidence that isoprene emission is decoupled from photosynthesis, and with the likely adaptive significance of isoprene emission in protecting leaves against heat damage and oxidative stress. In contrast, the high correlation between canopy temperature and formaldehyde concentration indicates the importance of including canopy temperature explicitly in large-scale models.
Isoprene emissions track the seasonal cycle of canopy temperature, not primary production: evidence from remote sensing