Atmospheric Environment 36 (2002) 3127–3135 Factors controlling the diurnal variation of CO above a forested area in southeast Europe Val ! erie Gros a , Kostas Tsigaridis b , Bernard Bonsang c , Maria Kanakidou b, *, Casimiro Pio d a Max Planck Institute for Chemistry, P.O. Box 3060, 50020, Mainz, Germany b Department of Chemistry, Environmental Chemical Processes Laboratory, University of Crete, P.O. Box 1470, Heraklion 71409, Greece c Laboratoire des Sciences du Climat et de l’Environement, Orme des Merisiers, Bat. 709, CE Saclay, 91 191, Gif sur Yvette, France d Departamento de Ambiente, University of Aveiro, 3810-193, Aveiro, Portugal Received 3 September 2001; received in revised form 1 March 2002; accepted 11 March 2002 Abstract Carbon monoxide (CO) measurements have been performed in a forested site in central Greece in the framework of theAEROBIC(AEROsolformationfromBiogenicCarbon)campaigninsummer1997.ThemeanCOobservedduring the whole campaign ranged between 114 and 250ppbv (mean of 170727ppbv), reflecting continental influence. The observedmeandiurnalcycleofCOpresentedaminimumintheearlymorningduetotheefficientdepositionofCOina shallow nocturnal layer sealed from the free tropospheric air during the night (loss rates of about 2ppbvh 1 ). In the earlymorningandinthelateafternoon,asharpandfluctuatingincreaseofCOwasobservedastheconsequenceofCO primary sources, likely by local traffic as suggested by the concomitant enhancements of black carbon (BC) and other combustion tracers. The morning pollution peak (6:30–8:30 local time) preceded slightly the opening of the nocturnal layertothefreetroposphere,whichresultedinCOreductiondowntobackgroundlevelsatabout10:00.Duringtheday (10:00–17:00), a slight but regular increase was observed on CO levels. For lack of simultaneous increase of other anthropogenic tracers, this CO enhancement has been attributed to its photochemical formation initiated by the oxidation of reactive biogenic hydrocarbons. This observed net production of CO averaging 1.2ppbvh 1 is quite well reproduced by a box model containing an explicit chemical scheme of isoprene and a- and b-pinene and taking into accountthemeasuredmixingratiosandthereactivityofallbiogenicorganicreactivecompoundswhenuncertaintiesin measurements and modelling are considered. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Carbon monoxide; Photochemical source; Biogenic hydrocarbons; AEROBIC; Forest site 1. Introduction CO is a widely studied gas due to its importance in atmospheric chemistry. CO is the main reaction partner of hydroxyl radical (OH), which itself is the most powerful oxidant in the troposphere and therefore controls the lifetime of many trace gases. Although in these last two decades much progress has been made in our understanding of the processes controlling CO, still important uncertainties are associated with the magni- tude of its main sources, which are both primary (fossil fuel and biomass combustion) and secondary (oxidation of methane and non-methane hydrocarbons). The secondary sources of CO have a very significant impact on its global budget since they may contribute up to 50% to the total CO in certain regions (Kanakidou and Crutzen, 1999). However, it is especially difficult to constrainthissource,asmanyhydrocarbonssourcesare themselves still associated with large uncertainties. *Corresponding author. Tel.: +30-81-393-633; fax: +30-81- 393-601. E-mail addresses: vgros@mpch-mainz.mpg.de (V. Gros), mariak@chemistry.uch.gr (M. Kanakidou). 1352-2310/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S1352-2310(02)00237-6