Measurements of volatile organic compounds over West Africa

Abstract

Abstract. In this paper we describe measurements of volatile organic compounds (VOC) made using a Proton Transfer Reaction Mass Spectrometer (PTR-MS) aboard the UK Facility for Atmospheric Airborne Measurements during the African Monsoon Multidisciplinary Analyses (AMMA) campaign. Observations were made during approximately 85 h of flying time between 17 July and 17 August 2006, above an area between 4 • N and 18 • N and 3 • W and 4 • E, encompassing ocean, mosaic forest, and the Sahel desert. High time resolution observations of counts at mass to charge (m/z) ratios of 42, 59, 69, 71, and 79 were used to calculate mixing ratios of acetonitrile, acetone, isoprene, the sum of methyl vinyl ketone and methacrolein, and benzene respectively using laboratory-derived humidity-dependent calibration factors. Strong spatial associations between vegetation and isoprene and its oxidation products were observed in the boundary layer, consistent with biogenic emissions followed by rapid atmospheric oxidation. Acetonitrile, benzene, and acetone were all enhanced in airmasses which had been heavily influenced by biomass burning. Benzene and acetone were also elevated in airmasses with urban influence from cities such as Lagos, Cotonou, and Niamey. The observations provide evidence that both deep convection and mixing associated with fair-weather cumulus were responsible for vertical redistribution of VOC emitted from the surface. Profiles over the ocean showed a depletion of acetone in the marine boundary layer, but no significant decrease for acetonitrile.

HO<sub>x</sub> observations over West Africa during AMMA: impact of isoprene and NO<sub>x</sub>

“…The two instruments were in good agreement with each other for the species in common (acetone, isoprene and MVK/MACR) Murphy et al, 2010). NO x measurements were made by a chemiluminescence TECO 42C analyser with a heated molybdenum filament to enable conversion of NO 2 to NO.…”

Airborne observations of IEPOX-derived isoprene SOA in the Amazon during SAMBBA

“…The instrument measured a range of hydrocarbons and oxygenated hydrocarbons with a typical cycle time of around 3-5 s. Isoprene was calibrated against gas standards provided by Apel-Reimer Environmental (Broomfield, CO, USA). The instrument was the same as used during OP3 and full instrumental, operational and calibration details are described in Murphy et al (2010). Accuracy for isoprene is estimated at ± 15 % and data were validated against offline gas chromatography analysis of samples taken using the aircraft's Whole Air Sampling (WAS) system, using the methods described by Hopkins et al (2011).…”

Ozone photochemistry in boreal biomass burning plumes

“…Measurements used for identification of biomass burning plume air masses included acetonitrile (CH 3 CN) and CO. A proton transfer reaction mass spectrometer (PTR-MS) (Murphy et al, 2010) provided measurements of CH 3 CN with an average time of 1 s and mean precision of 37 pptv over all BORTAS flights. CO was measured using a vacuum-UV resonance fluorescence instrument (Gerbig et al, 1999) with an average time of 1 s, precision of 1 ppbv, and accuracy of 3 %.…”