The gas phase tropospheric removal of fluoroaldehydes (CxF2x+1CHO, x = 3, 4, 6)

Abstract

The rate coefficient of the OH reaction with the perfluoroaldehydes C₃F₇CHO and C₄F₉CHO have been determined in the temperature range 252–373 K using the pulsed laser photolysis–laser induced fluorescence (PLP-LIF) method: k_C3F7CHO+OH = (2.0 ± 0.6) × 10⁻¹² exp[−(369 ± 90)/T] and k_C4F9CHO+OH = (2.0 ± 0.5) × 10⁻¹² exp[−(356 ± 70)/T] cm³ molecule⁻¹ s⁻¹, corresponding to (5.8 ± 0.6) × 10⁻¹³ and (6.1 ± 0.5) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, respectively, at 298 K. The UV absorption cross sections of these two aldehydes and CF₃(CF₂)₅CH₂CHO have been measured over the range 230–390 nm at 298 K and also at 328 K for CF₃(CF₂)₅CH₂CHO. The obtained results for C₃F₇CHO and C₄F₉CHO are in good agreement with two recent determinations but the maximum value of the absorption cross section for CF₃(CF₂)₅CH₂CHO is over a factor of two lower than the single one recently published. The photolysis rates of C₃F₇CHO, C₄F₉CHO and CF₃(CF₂)₅CHO have been measured under sunlight conditions in the EUPHORE simulation chamber in Valencia (Spain) at the beginning of June. The photolysis rates were, respectively, J_obs = (1.3 ± 0.6) × 10⁻⁵, (1.9 ± 0.8) × 10⁻⁵ and (0.6 ± 0.3) × 10⁻⁵ s⁻¹. From the J_obs measurements and calculated photolysis rate J_calc, assuming a quantum yield of unity across the atmospheric range of absorption of the aldehydes, quantum yields J_obs/J_calc = (0.023 ± 0.012), (0.029 ± 0.015) and (0.046 ± 0.028) were derived for the photodissociation of C₃F₇CHO, C₄F₉CHO and CF₃(CF₂)₅CHO, respectively. The atmospheric implication of the data obtained in this work is discussed. The main conclusion is that the major atmospheric removal pathway for fluoroaldehydes will be photolysis, which under low NO_x conditions, may be a source of fluorinated carboxylic acids in the troposphere.