Origin of water-soluble organic aerosols at the Maïdo high-altitude observatory, Réunion Island in the tropical Indian Ocean

Abstract

Abstract. The tropical and subtropical Indian Ocean (IO) is expected to be a significant source of water-soluble organic aerosols (WSOAs), which are important factors relevant to cloud condensation nuclei and ice nuclei of aerosol particles. Current atmospheric numerical models significantly underestimate the budget of organic aerosols and their precursors, especially over tropical oceans. This is primarily due to poor knowledge of sources and the paucity of observations of these parameters considering spatial and temporal variation over the tropical open ocean. To evaluate the contribution of sources to WSOA as well as their formation processes, submicrometer aerosol sampling was conducted at the high-altitude Maïdo observatory (21.1° S, 55.4° E, 2,160 m a.s.l), located on the remote island of La Réunion in the southwest IO. The aerosol samples were continuously collected during local daytime and nighttime, which corresponded to the ambient conditions of the marine boundary layer (MBL) and free troposphere (FT), respectively, from March 15 to May 24, 2018. Chemical analysis showed that organic matter was the dominant component of submicrometer water-soluble aerosol (~45 ± 17 %) during the wet season (March 15–April 23), whereas sulfate dominated (~77 ± 17 %) during the dry season (April 24–May 24). Measurements of the stable carbon isotope ratio of water-soluble organic carbon (WSOC) suggested that marine sources contributed significantly to the observed WSOC mass in both the MBL and the FT in the wet season, whereas a mixture of marine and terrestrial sources contributed to WSOC in the dry season. The distinct seasonal changes in the dominant source of WSOC were also supported by Lagrangian trajectory analysis. Positive matrix factorization analysis suggested that marine secondary OA dominantly contributed to the observed WSOC mass (~70 %) during the wet season, whereas mixtures of marine and terrestrial sources contributed during the dry season in both MBL and FT. Overall, this study demonstrates that the effect of marine secondary sources is likely important up to the FT in the wet season, which may be responsible for cloud formation as well as direct radiative forcing over oceanic regions.

Observed in-plume gaseous elemental mercury depletion suggests significant mercury scavenging by volcanic aerosols

“…Filter-based observations of inorganic ions and organic carbon. Submicron and supermicron aerosols were sampled at Maïdo from the 15th of March 2018 to the 24th of May 2018 in the framework of the OCTAVE (Oxygenated Compounds in the Tropical Atmosphere: Variability and Exchanges) project 38 during its intensive observation period, as already described in detail in Simu et al (2021). 39 Briey, ambient aerosols were collected using two high-volume air samplers at a ow rate of 1130 L min −1 (at local temperature and pressure) for each.…”

“…Submicron and supermicron aerosols were sampled at Maïdo from the 15th of March 2018 to the 24th of May 2018 in the framework of the OCTAVE (Oxygenated Compounds in the Tropical Atmosphere: Variability and Exchanges) project 38 during its intensive observation period, as already described in detail in Simu et al (2021). 39 Briey, ambient aerosols were collected using two high-volume air samplers at a ow rate of 1130 L min −1 (at local temperature and pressure) for each. One of the samplers performed daytime sampling (running between 07:00 and 18:00 local time), while the other performed nighttime sampling (running between 22:00 and 05:00 local time).…”

“…It has already been reported that sulfate dominated the mass of inorganic ions at Maïdo during the April-June 2018 volcanic episode. 39 To estimate variation of PM0.95 sulfate on the 29th of April we construct a PM0.95 sulfate proxy (PM0.95 sulfate proxy ). We assume that volcanic PM0.95 sulfate can be expressed as a function of observed SO 2 , a clear volcanic plume tracer and precursor for secondary sulfate, and the DMPS-derived particle volume (PV0.7 DMPS ), which is proportional to PM0.7.…”

Observed in-plume gaseous elemental mercury depletion suggests significant mercury scavenging by volcanic aerosols

“…Submicron and supermicron aerosols were sampled at Maïdo from 15 March 2018 to 24 May 2018 in the framework of the OCTAVE (Oxygenated Compounds in the Tropical Atmosphere: Variability and Exchanges) project 33 during its intensive observation period, as already described in detail in Simu et al (2021) 34 . Briefly, ambient aerosols were collected using two high-volume air samplers at a flow rate of 1130 L/min (at local temperature and pressure) for each.…”