Abstract
Abstract. We present a source apportionment study of a near-continuous 2-year dataset of volatile organic compounds (VOCs), recorded between October 2017 and November 2019 with a quadrupole-based high-sensitivity proton-transfer-reaction mass-spectrometry (hs-PTR-MS) instrument deployed at the Maïdo observatory (21.1∘ S, 55.4∘ E, 2160 m altitude). The observatory is located on La Réunion island in the southwest Indian Ocean. We discuss seasonal and diel profiles of six key VOC species unequivocally linked to specific sources – acetonitrile (CH3CN), isoprene (C5H8), isoprene oxidation products (Iox), benzene (C6H6), C8-aromatic compounds (C8H10), and dimethyl sulfide (DMS). The data are analyzed using the positive matrix factorization (PMF) method and back-trajectory calculations based on the Lagrangian mesoscale transport model FLEXPART-AROME to identify the impact of different sources on air masses sampled at the observatory. As opposed to the biomass burning tracer CH3CN, which does not exhibit a typical diel pattern consistently throughout the dataset, we identify pronounced diel profiles with a daytime maximum for the biogenic (C5H8 and Iox) and anthropogenic (C6H6, C8H10) tracers. The marine tracer DMS generally displays a daytime maximum except for the austral winter when the difference between daytime and nighttime mixing ratios vanishes. Four factors were identified by the PMF: background/biomass burning, anthropogenic, primary biogenic, and secondary biogenic. Despite human activity being concentrated in a few coastal areas, the PMF results indicate that the anthropogenic source factor is the dominant contributor to the VOC load (38 %), followed by the background/biomass burning source factor originating in the free troposphere (33 %), and by the primary (15 %) and secondary biogenic (14 %) source factors. FLEXPART-AROME simulations showed that the observatory was most sensitive to anthropogenic emissions west of Maïdo while the strongest biogenic contributions coincided with air masses passing over the northeastern part of La Réunion. At night, the observatory is often located in the free troposphere, while during the day, the measurements are influenced by mesoscale sources. Interquartile ranges of nighttime 30 min average mixing ratios of methanol (CH3OH), CH3CN, acetaldehyde (CH3CHO), formic acid (HCOOH), acetone (CH3COCH3), acetic acid (CH3COOH), and methyl ethyl ketone (MEK), representative for the atmospheric composition of the free troposphere, were found to be 525–887, 79–110, 61–101, 172–335, 259–379, 64–164, and 11–21 pptv, respectively.
Highlights
Non-methane volatile organic compounds (NMVOCs) are key players in atmospheric chemistry
We analyzed the 2-year dataset of (O)VOC concentrations obtained at Maïdo observatory between October 2017 and November 2019 in the framework of the OCTAVE project by a combination of (i) diel, seasonal, and inter-annual VOC concentration patterns; (ii) a positive matrix factorization (PMF) algorithm; and (iii) the FLEXPART-AROME mesoscale transport model
The measurements are shown to be useful to characterize the atmospheric background composition, as evidenced by the high average impact of the background– biomass burning source factor identified using the PMF algorithm on the atmospheric (O)VOC load (33 %), which is even larger during the night
Summary
Non-methane volatile organic compounds (NMVOCs) are key players in atmospheric chemistry. In general, the VOCs present in the background factor all have atmospheric lifetimes of at least several days This factor does not contain compounds with short lifetimes (i.e., C5H8, Iox, and C8H10), which is expected as it originates mainly in the free troposphere and represents the impact of emissions from large bodies of land located far away from La Réunion. This location corresponds well with the region of the highest C5H8 mixing ratios modeled with Meso-NH during the 2015 FARCE campaign (Duflot et al, 2019) This biogenic emission hotspot is visible in the field of view corresponding to the strongest secondary biogenic factor contributions where a strong impact of surface emissions is seen at the same location (Fig. 17). Other emission hotspots of human activity did not contribute as strongly to the top 5 % of anthropogenic source factor contributions
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