The controlling factors of atmospheric formaldehyde (HCHO) in Amazon as seen from satellite

To understand the relative importance of biogenic emission, biomass burning emission on volatile organic compounds in Amazon, the spatial and temporal correlations between atmospheric column-integrated formaldehyde (HCHO) and fire count, vegetation hydrological states index, surface solar radiation flux, near-surface air temperature are studied using synergized satellite products and reanalysis data. A recently developed microwave-based vegetation index (emissivity difference vegetation index, EDVI) with high temporal resolution is used for the linkages between vegetation and HCHO at daily scale with and without fire contaminations. At large regional scale, EDVI shows highest spatial correlation with HCHO indicating the important controlling effect of biogenic emission. In given subregions with frequent fires, the temporal variations of monthly HCHO show much stronger correlations with fire count. The temporal correlations between monthly HCHO and EDVI are vague and even negative in some subregions. Radiation and temperature show stable positive temporal correlations with HCHO, particularly in areas with few fires. After excluding the samples contaminated by fires, the daily temporal correlation between vegetation (EDVI) and HCHO becomes significant and positive in most areas except the northern rainforest with weak temporal variations of EDVI. We proposed a bilinear model of biogenic-emission-induced HCHO (B-HCHO) using radiation and EDVI as inputs. The bias between modeled long-term mean B-HCHO and satellite observation is less than 20%. And the daily time series of modeled B-HCHO matches observations as well. It is the first time to provide satellite observational evidences of the relative importance of biogenic emission and biomass burning emission on HCHO, the proxy of atmospheric volatile organic compounds concentration. Plain Language Summary Air quality, weather, and climate are deeply affected by aerosol, the small suspending particles in the air. A large part of the aerosol is formed from volatile organic compounds (VOCs), which can be released from vegetation and biomass burning. However, VOCs are hard to measure at large-scale regions. Alternatively, atmospheric formaldehyde (HCHO), the oxidation product of VOCs, is measurable with satellite remote sensing techniques. Meanwhile, fires, vegetation states, and solar radiation also can be observed by satellites. In this study over Amazon, the linkages between HCHO and its potential controlling factors are investigated using satellite remote sensing data sets. Spatially, atmospheric HCHO is high over areas with dense vegetation indicating the importance of vegetation emission of VOCs. In areas with frequent fires, fire becomes more important and can blanket the effects from vegetation emission on VOCs. These two effects can be untangled by using combined satellite observations of fire and vegetation at high temporal resolution. Based on the statistics, we proposed a model to estimate the vegetation-emission-induced HCHO using properties of vegetation and solar radiation as inputs. This study helps us to understand the relative importance of vegetation and fire on determining the organic composition of atmosphere.