Elucidation of the photochemical reactions involved in the production and destruction of formaldehyde, hydrogen peroxide and methyl hydroperoxide

A study has been made of three gases in the atmosphere, formaldehyde, hydrogen peroxide and methyl hydroperoxide involving observations, chemical transport modelling and basic physical chemistry. Observations of these atmospheric gases were made in Melbourne, Australia from September 1999 until February 2001. During the sampling period meteorological measurements and mixing ratios of other gases were acquired and subsequently used to investigate the seasonal and diurnal changes in formaldehyde, hydrogen peroxide and methyl hydroperoxide mixing ratios. Over the observation period, the average mixing ratios of formaldehyde, hydrogen peroxide and methyl hydroperoxide were 747 ± 14 ppt, 828 ± 15 ppt and 171 ± 4 ppt respectively. Seasonal cycles for these gases are evident; the atmospheric mixing ratios in the first spring/summer period compared to the winter period were 642 ± 42/469 ± 24 ppt, 802 ± 76/325 ± 74 ppt and 124 ± 6/76 ± 2 ppt respectively for formaldehyde, hydrogen peroxide and methyl hydroperoxide. The diurnal cycles for formaldehyde, hydrogen peroxide and methyl hydroperoxide were obvious, peaking at about 11:00, 16:00 and 20:00 respectively during summer, and a little later during winter. A regional chemical transport model, TAPM/CTM, was used to simulate the 18 months of observations. Also, as part of the study, major production and destruction rates were determined and used in a simple kinetic framework to calculate the mixing ratios of formaldehyde, hydrogen peroxide and methyl hydroperoxide for midday summer baseline periods. While TAPM/CTM was able to reproduce the observed mixing ratios of all three gases, the simple kinetic approach is satisfactory for formaldehyde and hydrogen peroxide but unsatisfactory for methyl hydroperoxide. The observed and modelled mixing ratios of methyl hydroperoxide were lower than those calculated by the simple kinetic approach. This could be due to lower loss rates used to calculate the values. If the photochemical loss was too low, the photolysis rate coefficient would have to increase significantly from 6 × 10–6 s–1 to about 1.2 × 10–5 s–1 to decrease the calculated mixing ratios to near the observed values. This implies that there would be an increase in the absorption cross sections of methyl hydroperoxide and/or an increase in actinic flux, over those used in the calculations. Although higher absorption cross sections of methyl hydroperoxide were measured in two recent studies, they only result in an increase of ~20% in the total UV photolysis, rate resulting a small decrease of 5% in the methyl hydroperoxide mixing ratios. The other major loss of methyl hydroperoxide is reaction with hydroxyl radicals, and a recent measurement of this rate coefficient is substantially higher than used previously. Using this new rate coefficient substantially improves the comparison of the calculated mixing ratios with the observed and modelled mixing ratios. The branching nature of the methyl peroxy radical was investigated. When nitric oxide was high, in polluted urban air, formaldehyde production was preferred compared to the tendency to produce more methyl hydroperoxide when air masses were of marine origin, and nitric oxide was low. While this is expected from current knowledge, where either formaldehyde or methyl hydroperoxide is preferentially produced during the reaction sequence of methane oxidation, this branching has not been well demonstrated previously from atmospheric observations.