The relative photolysis rates of HCHO and HCDO have been studied in May 2004 at the European Photoreactor Facility (EUPHORE) in Valencia, Spain. The photolytic loss of HCDO was measured relative to HCHO by long path FT-IR and DOAS detection during the course of the experiment. The isotopic composition of the reaction product H(2) was determined by isotope ratio mass spectrometry (IRMS) on air samples taken during the photolysis experiments. The relative photolysis rate obtained by FTIR is j(HCHO)/j(HCDO) = 1.58 +/- 0.03. The ratios of the photolysis rates for the molecular and the radical channels obtained from the IRMS data, in combination with the quantum yield of the molecular channel in the photolysis of HCHO, Phi(HCHO-->H(2)+CO) (JPL Publication 06-2), are j(HCHO-->H(2)+CO/jHCDO-->HD+CO) = 1.82 +/- 0.07 and j(HCHO-->H+HCO/(jHCDO-->H+DCO + jHCDO-->D+HCO)) = 1.10 +/- 0.06. The atmospheric implications of the large isotope effect in the relative rate of photolysis and quantum yield of the formaldehyde isotopologues are discussed in relation to the global hydrogen budget.

Formaldehyde (HCHO) is a principal intermediate in the photochemical oxidation of hydrocarbons in the troposphere. Isotopic analysis is an important tool for tracing the atmospheric path of gaseous species, and for this purpose, characterization of the isotope effects in the loss processes for formaldehyde is needed. The main loss pathways for formaldehyde in the atmosphere are photolysis and reactions with the radical species of OH, Cl, Br, and NO 3 . In this study, the kinetic isotope effects in the reactions of five different isotopomers of formaldehyde (HCHO) with OH, Cl, Br, and NO 3 radicals are studied in a relative-rate experiment at 298 ( 2 K and 1013 ( 10 mbar. The reaction rates of DCDO, HCDO, H 13 CHO, and HCH 18 O with the four radicals are measured relative to H 2 CO in a smog chamber using long-path FTIR detection. The experimental data are analyzed with a nonlinear least-squares spectral-fitting method using measured high-resolution infrared spectra and cross sections from the HITRAN database. The reaction rates of HCDO and HCH 18 O with OH and Cl are determined relative to HCHO as k OH+HCHO /k OH+HCDO ) 1.28 ( 0.01, k OH+HCHO /k OH+HCH 18 O ) 0.967 ( 0.006, k Cl+HCHO /k Cl+HCDO ) 1.201 ( 0.002, and k Cl+HCHO /k Cl+HCH 18 O ) 1.08 ( 0.01. The reaction rates of HCDO and HCH 18 O with Br and NO 3 are determined relative to HCHO as k Br+HCHO /k Br+HCDO ) 3.27 ( 0.03, k Br+HCHO /k Br+HCH 18 O ) 1.275 ( 0.008, k NO 3 +HCHO /k NO 3 +HCDO ) 1.78 ( 0.01, and k NO 3 +HCHO /k NO 3 +HCH 18 O ) 0.98 ( 0.01. The errors represent 2σ from the statistical analyses, and do not include possible systematic errors.