Relative Reaction Rates of HCHO, HCDO, DCDO, H¹³CHO, and HCH¹⁸O with OH, Cl, Br, and NO₃ Radicals

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.