Atmospheric Photolysis of Methyl Ethyl, Diethyl, and Propyl Ethyl Ketones: Temperature‐Dependent UV Absorption Cross Sections

International audience ; Ketone photolysis is a potentially important source of HO x radicals in the upper troposphere. To represent this photolysis, models need to include actinic flux, quantum yield, and absorption cross sections over a range of atmospherically relevant conditions. This work seeks to improve the representation of ketone ultraviolet (UV) absorption by quantifying it as a function of temperature. We present observations of 1-nm resolution absorption cross sections from 200 to 335 nm of methyl ethyl ketone (MEK) and diethyl ketone (DEK) at temperatures between 242 and 320 K, as well as propyl ethyl ketone (PEK) cross sections at 296 K. Our measured room temperature absorption cross sections agree to within 2%, 2%, and 5% with previous studies for MEK, DEK, and PEK spectra, respectively. We parameterize the temperature dependence of the cross sections of MEK and DEK using a two-state model, which reproduces our experimental results well. With additional assumptions, this model can be applied to the temperature dependence of PEK in the absence of experimental data. This model is appropriate for atmospherically relevant temperatures both inside and outside the temperatures used in this study and is suitable for incorporation into model atmospheric photolysis schemes. R programs to facilitate usage of these data are included in the supporting information. Inclusion of temperature-dependent absorption cross sections in atmospheric photolysis calculations decreased the rate coefficients of MEK, DEK, and PEK photolysis in the upper troposphere when compared to those calculated using only the room temperature cross sections; the decrease can be as large as 20-25%.