Abstract
Abstract. Oxygenated volatile organic compounds (OVOCs) are important products of the photo-oxidation of hydrocarbons. They influence the oxidizing capacity and the ozone-forming potential of the atmosphere. In the summer of 2008, 2 months of emission restrictions were enforced in Beijing to improve air quality during the Olympic Games. Observational evidence reported in related studies that these control measures were efficient in reducing the concentrations of primary anthropogenic pollutants (CO, NOx and non-methane hydrocarbons, i.e., NMHCs) by 30–40%. In this study, the influence of the emission restrictions on ambient levels of OVOCs was explored using a neural network analysis with consideration of meteorological conditions. Statistically significant reductions in formaldehyde (HCHO), acetaldehyde (CH3CHO), methyl ethyl ketone (MEK) and methanol were found to be 12.9, 15.8, 17.1 and 19.6%, respectively, when the restrictions were in place. The effect of emission controls on acetone was not detected in neural network simulations, probably due to pollution transport from surrounding areas outside Beijing. Although the ambient levels of most NMHCs were reduced by ~35% during the full control period, the emission ratios of reactive alkenes and aromatics closely related to automobile sources did not present much difference (< 30%). A zero-dimensional box model based on the Master Chemical Mechanism version 3.2 (MCM3.2) was applied to evaluate how OVOC production responds to the reduced precursors during the emissions control period. On average, secondary HCHO was produced from the oxidation of anthropogenic alkenes (54%), isoprene (30%) and aromatics (15%). The importance of biogenic sources for the total HCHO formation was almost on par with that of anthropogenic alkenes during the daytime. Anthropogenic alkenes and alkanes dominated the photochemical production of other OVOCs such as acetaldehyde, acetone and MEK. The relative changes of modeled HCHO, CH3CHO, methyl vinyl ketone and methacrolein (MVK + MACR) before and during the pollution controlled period were comparable to the estimated reductions in the neural network, reflecting that current mechanisms can largely explain secondary production of those species under urban conditions. However, it is worth noting that the box model overestimated the measured concentrations of aldehydes by a factor of 1.4–1.7 without consideration of loss of aldehydes on aerosols, and simulated MEK was in good agreement with the measurements when primary sources were taken into consideration. These results suggest that the understanding of the OVOCs budget in the box model remains incomplete, and that there is still considerable uncertainty in particular missing sinks (unknown chemical and physical processes) for aldehydes and absence of direct emissions for ketones.
Highlights
Oxygenated volatile organic compounds (OVOCs), such as aldehydes, ketones and alcohols, are mostly produced in the atmosphere by the oxidation of biogenic, anthropogenic hydrocarbons and other organic species (Finlayson-Pitts and Pitts, 2000), and directly emitted by vegetation (Park et al, 2013), biomass burning (Yokelson et al, 2009; Mason et al, 2001; Andreae and Merlet, 2001), fossil fuel combustion (Schauer et al, 1999, 2002) and industries (Singh et al, 1994)
Using a neural network analysis, we concluded that the emission controls taken in Beijing had a notable effect on reducing the ambient concentrations of formaldehyde, acetaldehyde, methyl ethyl ketone (MEK) and methanol
The influence of emission restrictions on OVOCs was found to be statistically significant when the variations due to meteorological conditions were excluded by the multilayer perceptron (MLP) network analysis
Summary
Oxygenated volatile organic compounds (OVOCs), such as aldehydes, ketones and alcohols, are mostly produced in the atmosphere by the oxidation of biogenic, anthropogenic hydrocarbons and other organic species (Finlayson-Pitts and Pitts, 2000), and directly emitted by vegetation (Park et al, 2013), biomass burning (Yokelson et al, 2009; Mason et al, 2001; Andreae and Merlet, 2001), fossil fuel combustion (Schauer et al, 1999, 2002) and industries (Singh et al, 1994). It is essential to understand the sources, sinks and chemistry of OVOCs quantitatively, while the role of OVOCs in heavily polluted regions remains largely unexplored partly due to the lack of reliable observations of them
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have