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 formation 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; 2160 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 15 March to 24 May 2018. Chemical analysis showed that organic matter was the dominant component of submicrometer water-soluble aerosol (∼ 45 ± 17 %) during the wet season (15 March–23 April). On the other hand, sulfate dominated (∼ 77 ± 17 %) during the dry season (24 April–24 May), most of which was attributable to the effect of volcanic eruption. 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 organic aerosol (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 affect cloud formation as well as direct radiative forcing over oceanic regions.

Evaluation of the Sources, Precursors, and Processing of Aerosols at a High-Altitude Tropical Site

“…The MSA/ nss -SO 4 2– ratio previously observed in subtropical and tropical oceanic areas varies between 0.005 and 0.070. − Our result shows that the MSA/ nss -SO 4 2– ratio measured at the MO (0.0007) is 6 to 40 times lower than that observed in other regions. The MSA average concentrations observed in our study (0.78 ng m –3 ) are much lower than those reported in a recent study at Réunion by factors of 10 to 20 (11.7–8.3 ng m –3 ) . These differences could be related to the several days under the detection limit concentrations during our field campaign.…”

“…Offline PM 10 filter measurements show that organic matter (OM) was the dominant fraction (39%, on average) of the aerosol composition followed by SO 4 2– (31%), Na + (8.4%), NO 3 – (6.9%), NH 4 + (5.1%), Cl – (4.2%), K + (0.44%), Mg 2+ (0.32%), and Ca 2+ (0.37%) (Figure ). This composition is similar to long-term observations of PM 10 obtained at the MO . In order to assess the impact of the marine sources, it was first assumed that NaCl interacts with acidic species (such as HNO 3 or H 2 SO 4 ), resulting in the formation of gas-phase NH 4 Cl or HCl, leaving the aerosols enriched in nitrate and nonsea salt ( nss ) sulfate and depleted in Cl – . , The deposition of HCl gas may cause another possible loss mechanism of chlorine to the sea surface .…”

“…This composition is similar to long-term observations of PM 10 obtained at the MO. 83 In order to assess the impact of the marine sources, it was first assumed that NaCl interacts with acidic species (such as HNO 3 or H 2 SO 4 ), resulting in the formation of gas-phase NH 4 Cl or HCl, leaving the aerosols enriched in nitrate and nonsea salt ( nss ) sulfate and depleted in Cl – . 84 , 85 The deposition of HCl gas may cause another possible loss mechanism of chlorine to the sea surface.…”

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

“…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 Briefly, ambient aerosols were collected using two high-volume air samplers at a flow 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…”