Formaldehyde over the eastern Mediterranean during MINOS: Comparison of airborne in-situ measurements with 3D-model results

[摘要] Formaldehyde (HCHO) is an important intermediate product in the photochemicaldegradation of methane and non-methane volatile organic compounds. In August 2001,airborne formaldehyde measurements based on the Hantzsch reaction technique wereperformed during the Mediterranean INtensive Oxidant Study, MINOS. The detection limit ofthe instrument was 42 pptv (1s) at a time resolution of180 s (10-90%). The overall uncertainty of the HCHO measurements was 30% at a mixing ratio of300 pptv. In the marine boundary layer over the eastern Mediterranean Sea average HCHO concentrations were of theorder of 1500 pptv, in reasonable agreement with results from a three-dimensional globalchemical transport model of the lower atmosphere including non-methane volatile organiccompound (NMVOC) chemistry. Above the boundary layer HCHO mixing ratios decreasedwith increasing altitude to a minimum level of 250 pptv at about 7 km. At higher altitudes(above 7 km) HCHO levels showed a strong dependency on the airmass origin. In airmassesfrom the North Atlantic/North American area HCHO levels were of the order of300 pptv, a factor of 6 higher than values predicted by the model. Even higher HCHO levels, increasingto values of the order of 600 pptv at 11 km altitude, were observed in easterlies transportingair affected by the Indian monsoon outflow towards the Mediterranean basin. Only a smallpart (~30 pptv) of the large discrepancy between the model results and the measurements ofHCHO in the free troposphere could be explained by a strong underestimation of the uppertropospheric acetone concentration by up to a factor of ten by the 3D-model. Therefore, themeasurement-model difference in the upper troposphere remains unresolved, while theobserved dependency of HCHO on airmass origin might indicate that unknown, relativelylong-lived NMVOCs - or their reaction intermediates - associated with biomass burning areat least partially responsible for the observed discrepancies.

[学科分类] 大气科学