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FTIR time-series of biomass burning products (HCN, C2H6, C2H2, CH3OH, and HCOOH) at Reunion Island (21° S, 55° E) and comparisons with model data

by C. Vigouroux, T. Stavrakou, C. Whaley, B. Dils, V. Duflot, C. Hermans, N. Kumps, J. M. Metzger, F. Scolas, G. Vanhaelewyn, J. F. Müller, D. B A Jones, Q. Li, M. De Mazière show all authors
Atmospheric Chemistry and Physics ()

Abstract

Reunion Island (21 degrees S, 55 degrees E), situated in the Indian\nOcean at about 800 km east of Madagascar, is appropriately located to\nmonitor the outflow of biomass burning pollution from Southern Africa\nand Madagascar, in the case of short-lived compounds, and from other\nSouthern Hemispheric landmasses such as South America, in the case of\nlonger-lived species. Ground-based Fourier transform infrared (FTIR)\nsolar absorption observations are sensitive to a large number of biomass\nburning products. We present in this work the FTIR retrieval strategies,\nsuitable for very humid sites such as Reunion Island, for hydrogen\ncyanide (HCN), ethane (C2H6), acetylene (C2H2), methanol (CH3OH), and\nformic acid (HCOOH). We provide their total columns time-series obtained\nfrom the measurements during August-October 2004, May-October 2007, and\nMay 2009-December 2010. We show that biomass burning explains a large\npart of the observed seasonal and interannual variability of the\nchemical species. The correlations between the daily mean total columns\nof each of the species and those of CO, also measured with our FTIR\nspectrometer at Reunion Island, are very good from August to November (R\n>= 0.86). This allows us to derive, for that period, the following\nenhancement ratios with respect to CO: 0.0047, 0.0078, 0.0020, 0.012,\nand 0.0046 for HCN, C2H6, C2H2, CH3OH, and HCOOH, respectively. The HCN\nground-based data are compared to the chemical transport model\nGEOS-Chem, while the data for the other species are compared to the\nIMAGESv2 model. We show that using the HCN/CO ratio derived from our\nmeasurements (0.0047) in GEOS-Chem reduces the underestimation of the\nmodeled HCN columns compared with the FTIR measurements. The comparisons\nbetween IMAGESv2 and the long-lived species C2H6 and C2H2 indicate that\nthe biomass burning emissions used in the model (from the GFED3\ninventory) are probably underestimated in the late September-October\nperiod for all years of measurements, and especially in 2004. The\ncomparisons with the short-lived species, CH3OH and HCOOH, with\nlifetimes of around 5 days, suggest that the emission underestimation in\nlate September-October 2004, occurs more specifically in the\nSoutheastern Africa-Madagascar region. The very good correlation of\nCH3OH and HCOOH with CO suggests that, despite the dominance of the\nbiogenic source of these compounds on the global scale, biomass burning\nis their major source at Reunion Island between August and November.

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