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

Terrestrial volcanism is known to emit mercury (Hg) into the atmosphere. However, despite many years of investigation, its net impact on the atmospheric Hg budget remains insufficiently constrained, in part because the transformations of Hg in volcanic plumes as they age and mix with background air are poorly understood. Here we report the observation of complete gaseous elemental mercury (GEM) depletion events in dilute and moderately aged (& SIM;3-7 hours) volcanic plumes from Piton de la Fournaise on Reunion Island. While it has been suggested that co-emitted bromine could, once photochemically activated, deplete GEM in a volcanic plume, we measured low bromine concentrations in both the gas- and particle-phase and observed complete GEM depletion even before sunrise, ruling out a leading role of bromine chemistry here. Instead, we hypothesize that the GEM depletions were mainly caused by gas-particle interactions with sulfate-rich volcanic particles (mostly of submicron size), abundantly present in the dilute plume. We consider heterogeneous GEM oxidation and GEM uptake by particles as plausible manifestations of such a process and derive empirical rate constants. By extrapolation, we estimate that volcanic aerosols may scavenge 210 Mg y(-1) (67-480 Mg y(-1)) of Hg from the atmosphere globally, acting effectively as atmospheric mercury sink. While this estimate is subject to large uncertainties, it highlights that Hg transformations in aging volcanic plumes must be better understood to determine the net impact of volcanism on the atmospheric Hg budget and Hg deposition pathways

Bronbepaling van zuurstofhoudende vluchtige organische stoffen in de afgelegen, tropische atmosfeer

Volatile organic compounds (VOCs) have a significant impact on the oxidative capacity of the atmosphere and thus on air quality and climate. Among them, the oxygenated VOCs (OVOCs) represent the largest sink of OH in the remote marine atmosphere. Large uncertainties in the global budget of OVOCs still exist due to the incomplete representation of their chemistry in atmospheric models, and poor characterisation of the terrestrial emissions as well as the ocean-atmosphere exchanges for these compounds and their precursors. This is partly due to a paucity in data, especially in remote tropical regions. Analysis of a 2-year, near-continuous data set of (O)VOC concentrations, registered at the Maïdo observatory (21°S, 54°E, 2160 m altitude), located on La Réunion, is presented. The work presented consists of i) the development of a mesoscale Lagrangian particle dispersion model, FLEXPART-AROME, ii) the study of biomass burning (BB) signals recorded at the observatory, and iii) a source attribution study of (O)VOCs at La Réunion relying on a combination of a multivariate statistical model, and back-trajectory calculations generated with FLEXPART-AROME.Les composés organiques volatils (COV) ont un impact significatif sur la capacité oxydante de l’atmosphère et donc sur la qualité de l’air et le climat. Parmi eux, les COV oxygénés (COVO) représentent le plus grand puits des radicaux OH dans l’atmosphère marine isolée. De larges incertitudes existent toujours dans le bilan global des COVO, en raison d’une représentation incomplète de leur chimie, d’une mauvaise caractérisation de leurs émissions terrestres ainsi que des échanges océan-atmosphère associés à ces espèces chimiques et de leurs précurseurs. Ceci est en partie la conséquence d’un manque de données dans les régions tropicales isolées. L’objet de cette thèse porte sur la présentation et l’analyse d’une base de données quasi continue de 2 ans de mesures in situ de COVO obtenue à l’observatoire du Maïdo (21°S, 54°E, 2160 m d’altitude), situé sur l’ile de la Réunion. Le travail présenté consiste en i) le développement d’un modèle dispersif Lagrangien mésoéchelle FLEXPART-AROME, ii) l’étude de cas de feux de biomasses mesurés à l’observatoire, et iii) l’attribution des sources de COVO à la Réunion en s’appuyant sur un modèle statistique multivariable et des calculs de rétro-trajectoires obtenus à l’aide de FLEXPART-AROME.Vluchtige organische stoffen (VOS) hebben een grote impact op de oxide rende capaciteit van de atmosfeer en dus ook op de luchtkwaliteit en het klimaat. Van de vluchtige organische stoffen zijn de zuurstofhoudende VOS (ZH-VOS) de oorzaak van het grootste verlies van OH in de afgelegen marine atmosfeer. Grote onzekerheden op de globale abundantie van deze ZH-VOS bestaan door de onvolledige representatie van de relevante chemie in at mosfeermodellen, en beperkte karakterisatie van directe emissies van het oppervlak en de interactie tussen atmosfeer en oceaan. Dit is gedeeltelijk te wijten aan een gebrek aan data in dit type regio’s. We presenteren hier de analyse van een 2-jaar durende dataset aan (ZH-)VOS concentraties geregistreerd aan het Maïdo observatorium (21°ZB, 54°OL, 2160 m hoogte), gelegen op La Réunion, een tropisch eiland in de zuidwestelijke Indische Oceaan. Het werk omvat i) de ontwikkeling van een atmosferisch transport model dat transport op mesoschaal in rekening brengt (FLEXPART AROME), ii) de studie van pluimen afkomstig van brandhaarden in Afrika en Madagascar die geobserveerd werden op Maïdo, en iii) een analyse van de volledige 2-jaar lange dataset door een combinatie van een multivariaat statistisch model en de berekening van luchtmassatransport achterwaards in de tijd bekomen met het FLEXPART-AROME model

Development of turbulent scheme in the FLEXPART-AROME v1.2.1 Lagrangian particle dispersion model

International audienceThe FLEXible PARTicle dispersion model FLEX-PART, first released in 1998, is a Lagrangian particle dispersion model developed to simulate atmospheric transport over large and mesoscale distances. Due to FLEXPART's success and its open source nature, different limited area model versions of FLEXPART were released making it possible to run FLEXPART simulations by ingesting WRF (Weather Research Forecasting model), COSMO (Consor-tium for Small-scale Modeling) or MM5 (mesoscale community model maintained by Penn State university) meteorological fields on top of the ECMWF (European Centre for Medium-Range Weather Forecasts) and GFS (Global Forecast System) meteorological fields. Here, we present a new FLEXPART limited area model that is compatible with the AROME mesoscale meteorological forecast model (the Applications of Research to Operations at Mesoscale model). 1 FLEXPART-AROME was originally developed to study mesoscale transport around La Réunion, a small volcanic island in the southwest Indian Ocean with a complex orographic structure, which is not well represented in current global operational models. We present new turbulent modes in FLEXPART-AROME. They differ from each other by dimensionality, mixing length parameterization, turbulent transport constraint interpretation and time step configuration. A novel time step was introduced in FLEXPART-AROME. Performances of new turbulent modes are compared to the ones in FLEXPART-WRF by testing the conservation of well-mixedness by turbulence, the dispersion of a point release at the surface and the marine boundary layer evolution around Réunion. The novel time step configura-1 Applications de la Recherche à l'Opérationnel à Méso-Echelle. tion proved necessary to conserve the well-mixedness in the new turbulent modes. An adaptive vertical turbulence time step was implemented, allowing the model to adapt on a finer timescale when significant changes in the local turbulent state of the atmosphere occur

Study of biogenic volatile organic compound emissions and depositions over a mixed temperate forest by PTR-TOF-MS and eddy covariance

Volatile organic compounds (VOCs) play a key role in atmospheric chemistry. These gases impact air quality by participating in the formation of ozone and secondary organic aerosols and extend the lifetime of methane in the atmosphere. Approximately 90% of global VOC emissions are biogenic (BVOCs), and since forests are the main emitters of BVOCs, these ecosystems deserve special attention in order to better characterize BVOCs exchanges with the atmosphere. Traditionally, flux measurements were mainly limited to a few dominant BVOC species, such as isoprene, terpenes and methanol, due to technical measurement limitations. Most of the measured species were found to be emitted by vegetation, but some studies detected significant net depositions of compounds such as methanol, driven by environmental factors favouring the formation of surface wetness. This observation supports the need for a more detailed and complete picture of BVOC bidirectional exchanges at forest sites as well as the mechanisms controlling these fluxes, which are essential to better characterize the in-canopy atmospheric chemistry. To address this gap, BVOC fluxes were measured in spring-summer 2022 using a PTR-TOF-MS instrument (PTR-TOF-4000, Ionicon Analytik GmbH) over a mixed temperate forest in the Belgian Ardenne (Vielsalm), which is part of the ICOS network. The use of a PTR-TOF-MS instrument, deployed at Vielsalm in the framework of ACTRIS, allows for the simultaneous detection of a very wide range of VOC-related ion masses with increased sensitivity (especially for ions at high m/z ratios) and higher mass resolving power compared to conventional PTR-Quad-MS instruments. O3 fluxes were also simultaneously acquired using fast and slow ozone analysers in order to complete the BVOC fluxes dataset and BVOC+O3 concentration profiles were frequently measured at seven levels along the flux tower from ground level up to 51 m. (Un)calibrated BVOC mixing ratios were first derived from results of the Ionicon Data Analyzer software (IDA, Ionicon Analytik GmbH) obtained on a near-daily basis, and these concentrations were then used in a computational tool based on InnFLUX (Atmospheric Physics and Chemistry Group, University of Innsbruck) to compute fluxes by eddy covariance. Along with these fluxes, uncertainties and limits of detection (LODs) were estimated, and quality control statistical tests were performed. During the whole measurement campaign, about 570 m/z peaks were detected by the IDA software. Based on this extensive dataset, our goals are to: (1) perform spectral analysis (which often turns out to be trickier for low signal-to-noise ratios), (2) determine the significant fluxes based on LODs, (3) analyse their response to meteorological variables, phenology and O3 concentrations/fluxes, (4) attempt compound attribution based on scientific literature and site-related information, and (5) establish the budget of BVOC emissions and depositions for the forest site. The obtained results will be compared to BVOC studies previously conducted at the Vielsalm ICOS forest site with a PTR-Quad-MS instrument and should improve our previous BVOC budget estimates at the site.BERTRAC13. Climate actio

Improved Spatial Resolution in Modeling of Nitrogen Oxide Concentrations in the Los Angeles Basin.

The extent to which emission control technologies and policies have reduced anthropogenic NOx emissions from motor vehicles is large but uncertain. We evaluate a fuel-based emission inventory for southern California during the June 2021 period, coinciding with the Re-Evaluating the Chemistry of Air Pollutants in CAlifornia (RECAP-CA) field campaign. A modified version of the Fuel-based Inventory of Vehicle Emissions (FIVE) is presented, incorporating 1.3 km resolution gridding and a new light-/medium-duty diesel vehicle category. NOx concentrations and weekday-weekend differences were predicted using the WRF-Chem model and evaluated using satellite and aircraft observations. Model performance was similar on weekdays and weekends, indicating appropriate day-of-week scaling of NOx emissions and a reasonable distribution of emissions by sector. Large observed weekend decreases in NOx are mainly due to changes in on-road vehicle emissions. The inventory presented in this study suggests that on-road vehicles were responsible for 55-72% of the NOx emissions in the South Coast Air Basin, compared to the corresponding fraction (43%) in the planning inventory from the South Coast Air Quality Management District. This fuel-based inventory suggests on-road NOx emissions that are 1.5 ± 0.4, 2.8 ± 0.6, and 1.3 ± 0.7 times the reference EMFAC model estimates for on-road gasoline, light- and medium-duty diesel, and heavy-duty diesel, respectively

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

International audienceWe 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

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

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 Maido observatory (21.1 degrees S, 55.4 degrees E; 2160ma.s.l.), located on the remote island of La Reunion 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 (similar to 45 +/- 17 %) during the wet season (15 March-23 April). On the other hand, sulfate dominated (similar to 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 (similar to 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

Mercury in the free troposphere and bidirectional atmosphere–vegetation exchanges – insights from Maïdo mountain observatory in the Southern Hemisphere tropics

International audienceAtmospheric mercury (Hg) observations in the lower free troposphere (LFT) can give important insights into Hg redox chemistry and can help constrain Hg background concentrations on a regional level. Relatively continuous sampling of LFT air, inaccessible to most ground-based stations, can be achieved at highaltitude observatories. However, such high-altitude observatories are rare, especially in the Southern Hemisphere (SH), and atmospheric Hg in the SH LFT is unconstrained. To fill this gap, we continuously measured gaseous elemental mercury (GEM; hourly) and reactive mercury (RM; integrated over ∼ 6-14 d) for 9 months at Maïdo mountain observatory (2160 m a.s.l.) on remote Réunion Island (21.1 • S, 55.5 • E) in the tropical Indian Ocean. GEM exhibits a marked diurnal variation characterized by a midday peak (mean: 0.95 ng m −3 ; SD: 0.08 ng m −3) and a nighttime low (mean: 0.78 ng m −3 ; SD: 0.11 ng m −3). We find that this diurnal variation is likely driven by the interplay of important GEM photo-reemission from the islands' vegetated surfaces (i.e. vegetation + soil) during daylight hours (8-22 ng m −2 h −1), boundary layer influences during the day, and predominant LFT influences at night. We estimate GEM in the LFT based on nighttime observations in particularly dry air masses and find a notable seasonal variation, with LFT GEM being lowest from December to March (mean 0.66 ng m −3 ; SD: 0.07 ng m −3) and highest from September to November (mean: 0.79 ng m −3 ; SD: 0.09 ng m −3). Such a clear GEM seasonality contrasts with the weak seasonal variation reported for the SH marine boundary layer but is in line with modeling results, highlighting the added value of continuous Hg observations in the LFT. Maïdo RM is 10.6 pg m −3 (SD: 5.9 pg m −3) on average, but RM in the cloud-free LFT might be about twice as high

Mercury in the free troposphere and bidirectional atmosphere–vegetation exchanges – insights from Maïdo mountain observatory in the Southern Hemisphere tropics

International audienceAtmospheric mercury (Hg) observations in the lower free troposphere (LFT) can give important insights into Hg redox chemistry and can help constrain Hg background concentrations on a regional level. Relatively continuous sampling of LFT air, inaccessible to most ground-based stations, can be achieved at highaltitude observatories. However, such high-altitude observatories are rare, especially in the Southern Hemisphere (SH), and atmospheric Hg in the SH LFT is unconstrained. To fill this gap, we continuously measured gaseous elemental mercury (GEM; hourly) and reactive mercury (RM; integrated over ∼ 6-14 d) for 9 months at Maïdo mountain observatory (2160 m a.s.l.) on remote Réunion Island (21.1 • S, 55.5 • E) in the tropical Indian Ocean. GEM exhibits a marked diurnal variation characterized by a midday peak (mean: 0.95 ng m −3 ; SD: 0.08 ng m −3) and a nighttime low (mean: 0.78 ng m −3 ; SD: 0.11 ng m −3). We find that this diurnal variation is likely driven by the interplay of important GEM photo-reemission from the islands' vegetated surfaces (i.e. vegetation + soil) during daylight hours (8-22 ng m −2 h −1), boundary layer influences during the day, and predominant LFT influences at night. We estimate GEM in the LFT based on nighttime observations in particularly dry air masses and find a notable seasonal variation, with LFT GEM being lowest from December to March (mean 0.66 ng m −3 ; SD: 0.07 ng m −3) and highest from September to November (mean: 0.79 ng m −3 ; SD: 0.09 ng m −3). Such a clear GEM seasonality contrasts with the weak seasonal variation reported for the SH marine boundary layer but is in line with modeling results, highlighting the added value of continuous Hg observations in the LFT. Maïdo RM is 10.6 pg m −3 (SD: 5.9 pg m −3) on average, but RM in the cloud-free LFT might be about twice as high