Analysis of Volatile Organic Compounds during the OCTAVE Campaign: Sources and Distributions of Formaldehyde on Reunion Island

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.

Solar UV Radiation in the Tropics: Human Exposure at Reunion Island (21° S, 55° E) during Summer Outdoor Activities

“…UVI depends on other parameters such as ozone, cloud, albedo, and aerosols. The atmospheric chemical composition is very different on each side of the boundary layer [44][45][46]. The low aerosol load above the boundary layer induces higher UVR.…”

Measurement report: Source apportionment of volatile organic compounds at the remote high-altitude Maïdo observatory

“…Calibration factors for compounds that were not present in the calibration mixture -i.e., HCOOH and CH 3 COOH -were determined indirectly, as mentioned in Verreyken et al (2020). Specifically, for CH 3 COOH, the calibration factor was inferred from that of CH 3 COCH 3 by considering the calculated collision rate constants of H 3 O + with CH 3 COOH and CH 3 COCH 3 (Su, 1994;Zhao and Zhang, 2004), by considering the contribution of the protonated molecules to the respective product ion distributions (Schwarz et al, 2009;Inomata and Tanimoto, 2010), and by assuming the same hs-PTR-MS transmission efficiency for ions with a mass difference of 2 Da. The same principle was applied to calculate the calibration factor of HCOOH from that of CH 3 CHO.…”

Tropospheric and stratospheric NO retrieved from ground-based Fourier-transform infrared (FTIR) measurements

“…The SFIT4 v0.9.4.4 retrieval code, updated from SFIT2 (Pougatchev et al, 1995), based on the optimal estimation method (Rodgers, 2000) is applied to retrieve the NO profile from the infrared spectra observed at Xianghe and Maïdo.…”

“…Table 2 lists the systematic and random retrieval uncertainties in the NO total column at Xianghe and Maïdo, including the contributions from the major uncertainty sources, which are estimated based on the optimal estimation method (Rodgers, 2000). It is assumed that the systematic uncertainty in a priori profile is 10 %, and the random uncertainty in a priori profile is calculated from the covariance matrix of the monthly a priori profiles in 2018.…”