A framework is presented for defining the present atmospheric chemistry of West Antarctica and the photochemistry of the atmosphere during the early- to mid-Holocene transition (9,000-6,000 yr BP). Little is known on the contributions of local and regional sources to the photochemical composition of the current atmosphere of West Antarctica and the early- to mid-Holocene atmosphere. Measurements over the West Antarctic continent were scarce in regards of exploring its present photochemical composition, while the contribution of the atmospheric oxidative capacity, namely OH radicals, to the methane decline over the early- to mid-Holocene has been assumed negligible. In this study, investigation of the present atmospheric chemistry was run during Antarctic summer at the WAIS Divide site, through multi-week continuous measurements of photochemical active species: atmospheric NO, O₃, H₂O₂, MHP, and snow NO₃^-, NO₂^- and H₂O₂. Then, estimation of the mixing ratios of past atmospheric oxidants from ice-core records was made possible from the latest findings on air-ice partition equilibrium of reversibly deposited species, HCHO and H₂O₂. Results from the West Antarctic campaign show that the levels of short-lived species above this region, such as NO_X, are driven by local sources, mainly snowpack emissions, whereas regional air mass transport controls the atmospheric concentrations of species with longer lifetime, such as O₃. Atmospheric estimates from the early- to mid-Holocene reveal that both methane sources and sink (low-latitude emissions and oxidation with OH) contributed to the CH₄ budget between 12,000-9,000 yr BP, whereas only the sources drove the decline of CH₄ between 9,000-6,000 yr BP.

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