Long-term measurements and model calculations of formaldehyde at rural European monitoring sites

Several years of formaldehyde measurements at six rural European sites are compared with EMEP oxidant model calculations. The model results agree well with the measured values both with regard to average seasonal cycles as well as on an episodic day-to-day basis at all sites except for Ispra in Northern Italy. For several of the sites the agreement is better during the summer months whereas the model underestimates the concentrations in winter. The model results show that formaldehyde in summer is mainly controlled by photochemical processes such as reaction with OH, photolysis and formation through NO+peroxy radicals. Furthermore, in spite of the short chemical lifetime of formaldehyde in summer, emission pulses of volatile organic compounds (VOC) and isoprene influence formaldehyde concentrations even 48 h after the emission. These results indicate that formaldehyde is well suited for validating photochemical transport models on a long-range (European) scale. Furthermore, the reasonable agreement between model calculations and measurements for carbonyls presented here, combined with previous findings for non-methane hydrocarbons (NMHC) and evaluation of the condensed EMEP chemical mechanism against a detailed Master chemical mechanism gives encouraging support that the EMEP model describes the main ozone-forming photochemical processes in a reasonable way.