Abstract
The Oxygenated Compounds in the Tropical Atmosphere: Variability and Exchanges (OCTAVE) campaign aimed to improve the assessment of the budget and role of oxygenated volatile organic compounds (OVOCs) in tropical regions, and especially over oceans, relying on an integrated approach combining in situ measurements, satellite retrievals, and modeling. As part of OCTAVE, volatile organic compounds (VOCs) were measured using a comprehensive suite of instruments on Reunion Island (21.07° S, 55.38° E) from 7 March to 2 May 2018. VOCs were measured at a receptor site at the Maïdo observatory during the entire campaign and at two source sites: Le Port from 19 to 24 April 2018 (source of anthropogenic emissions) and Bélouve from 25 April to 2 May 2018 (source of biogenic emissions) within a mobile lab. The Maïdo observatory is a remote background site located at an altitude of 2200 m, whereas Bélouve is located in a tropical forest to the east of Maïdo and Le Port is an urban area located northwest of Maïdo. The major objective of this study was to understand the sources and distributions of atmospheric formaldehyde (HCHO) in the Maïdo observatory on Reunion Island. To address this objective, two different approaches were used to quantify and determine the main drivers of HCHO at Maïdo. First, a chemical-kinetics-based (CKB) calculation method was used to determine the sources and sinks (biogenic, anthropogenic/primary, or secondary) of HCHO at the Maïdo site. The CKB method shows that 9% of the formaldehyde formed from biogenic emissions and 89% of HCHO had an unknown source; that is, the sources cannot be explicitly described by this method. Next, a positive matrix factorization (PMF) model was applied to characterize the VOC source contributions at Maïdo. The PMF analysis including VOCs measured at the Maïdo observatory shows that the most robust solution was obtained with five factors: secondary biogenic accounting for 17%, primary anthropogenic/solvents (24%), primary biogenic (14%), primary anthropogenic/combustion (22%), and background (23%). The main contributions to formaldehyde sources as described by the PMF model are secondary biogenic (oxidation of biogenic VOCs with 37%) and background (32%). Some assumptions were necessary concerning the high percentage of unknown HCHO sources of the CKB calculation method such as the biogenic emission factor resulting in large discrepancies between the two methods.
Highlights
Volatile organic compounds (VOCs) are key components of the atmosphere: they are precursors of secondary organic aerosol (SOA) and lead to the photochemical formation of air pollutants such as tropospheric ozone [1,2,3,4,5]
We present oxygenated volatile organic compounds (OVOCs) measurements at the Maïdo observatory, Le Port, and Bélouve sites (Reunion Island) during the OCTAVE campaign from March to May 2018
The range of formaldehyde concentrations measured at the Maïdo observatory (1.76 ± 0.26 ppb) was the same as at Bélouve and Le Port (0.74 ± 0.14 and 1.49 ± 0.24 ppbv on average, respectively)
Summary
Volatile organic compounds (VOCs) are key components of the atmosphere: they are precursors of secondary organic aerosol (SOA) and lead to the photochemical formation of air pollutants such as tropospheric ozone [1,2,3,4,5]. They contributed to the same factor despite the slight difference in the SNR and the percentage of lower values Some of these compounds have a long atmospheric lifetime, e.g., ttoluene = 1.4 days [54]. The background factor showed a slight decrease in concentration during the day, with a minimum at 08:00 UT (0.5 ppb), and high concentrations during the night (0.6 ppb) This dip can be explained by monoterpenes being highly oxidized during the day [75] and their atmospheric residence time is short (nearly one hour). This factor reflected the lifetime of certain species that accumulate in the atmosphere and indicated atmospheric background levels (average concentration about ~0.5 ppb). This factor was named “background” and contributed 23.3% to the sum of data
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