Tropospheric formaldehyde retrievals with GOME-2

Biogenic Volatile Organic Compounds (BVOCs) emitted by terrestrial ecosystems impact air quality and climate. The most important BVOC for global tropospheric composition is isoprene, whose annual global emissions (400-600 Tg C/year) account for ~50% of the total global BVOC budget. Tropical ecosystems are generally thought to be responsible for 70-90% of the global isoprene budget. Satellite observations of formaldehyde (CH[subscript 2]O), a high-yield, short lifetime product of isoprene oxidation, provide top-down constraints on surface isoprene emissions. Errors in retrieved satellite slant column densities (SCD - trace gas concentration along the instrument's line of sight) are typically in the region of 40% for scenes with little cloud and aerosol contamination. Error sources arise from instability in the differential optical absorption spectroscopy (DOAS) retrieval procedure, largely due to low signal to noise ratios frequently encountered with space-borne UV spectrometers, coupled to CH[subscript 2]O's faint absorption signal. The essential conversion of SCDs to vertical column densities (VCDs - vertical trace gas concentration extending from the Earth's surface), needed for application of retrieved CH[subscript 2]O values to geochemical modelling schemes, is a further significant contributor to product error (30-60%), being strongly influenced by aerosol, cloud and albedo inhomogeneities at sub-pixel level. A new global 5 year CH[subscript 2]O product (2007-2011) generated with global ozone monitoring experiment 2 (GOME-2) satellite radiance data is presented. SCDs for this are generated with optimised spectroscopic fit settings derived from an extensive sensitivity study of input retrieval parameters for the DOAS portion of the retrieval. SCDs are then converted to vertical column densities VCDs using air mass factors calculated with a significantly upgraded retrieval scheme, accounting for atmospheric radiative transfer due to viewing geometry, surface albedo and atmospheric scattering. New GOME-2 CH[subscript 2]O VCDs are then evaluated against GEOS-Chem modelled CH[subscript 2]O VCDs, and compared to a suite of associated environmental parameters.