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The BIO-MAÏDO (Bio-physicochemistry of tropical clouds at Maïdo: processes and impacts on secondary organic aerosols formation) campaign was conducted from 13 March to 4 April 2019 on the tropical island of Réunion. The main objective of the project was to improve understanding of cloud impacts on the formation of secondary organic aerosols (SOA) from biogenic volatile organic compound (BVOC) precursors in a tropical environment. Instruments were deployed at five sites: a receptor site, Maïdo Observatory (MO) at 2165 m a.s.l. and four sites along the slope of the Maïdo mountain. Observations include measurements of volatile organic compounds (VOCs) and characterization of the physical, chemical and biological (bacterial diversity and culture-based approaches) properties of aerosols and cloud water. Turbulent parameters of the boundary layer, radiative fluxes and emissions fluxes of BVOCs from the surrounding vegetation were measured to help interpret observed chemical concentrations in the different phases. Dynamical analyses showed two preferred trajectory routes for air masses arriving at MO during the daytime. Both trajectories correspond to return branches of the trade winds associated with upslope thermal breezes, where air masses likely encountered cloud processing. The highest mixing ratios of oxygenated VOCs (OVOCs) were measured above the site located in the endemic forest and the highest contribution of OVOCs to total VOCs at MO. Chemical compositions of particles during daytime showed higher concentrations of oxalic acid, a tracer of cloud processing and photochemical aging, and a more oxidized organic aerosol at MO than at other sites. Approximately 20 % of the dissolved organic compounds were analyzed. Additional analyses by ultra-high-resolution mass spectrometry will explore the complexity of the missing cloud organic matter.

This study underlines the important role of the transported black carbon (BC) mass concentration in the West African monsoon (WAM) area. BC was measured with a micro-aethalometer integrated in the payload bay of the unmanned research aircraft ALADINA (Application of Light-weight Aircraft for Detecting IN situ Aerosol). As part of the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) project, 53 measurement flights were carried out at Savè, Benin, on 2–16 July 2016. A high variability of BC (1.79 to 2.42±0.31 µg m−3) was calculated along 155 vertical profiles that were performed below cloud base in the atmospheric boundary layer (ABL). In contrast to initial expectations of primary emissions, the vertical distribution of BC was mainly influenced by the stratification of the ABL during the WAM season. The article focuses on an event (14 and 15 July 2016) which showed distinct layers of BC in the lowermost 900 m above ground level (a.g.l.). Low concentrations of NOx and CO were sampled at the Savè supersite near the aircraft measurements and suggested a marginal impact of local sources during the case study. The lack of primary BC emissions was verified by a comparison of the measured BC with the model COSMO-ART (Consortium for Small-scale Modelling–Aerosols and Reactive Trace gases) that was applied for the field campaign period. The modelled vertical profiles of BC led to the assumption that the measured BC was already altered, as the size was mainly dominated by the accumulation mode. Further, calculated vertical transects of wind speed and BC presume that the observed BC layer was transported from the south with maritime inflow but was mixed vertically after the onset of a nocturnal low-level jet at the measurement site. This report contributes to the scope of DACCIWA by linking airborne BC data with ground observations and a model, and it illustrates the importance of a more profound understanding of the interaction between BC and the ABL in the WAM region.

Biogenic fluxes from soil at a local and regional scale are crucial to study air pollution and climate. Here we present field measurements of soil fluxes of nitric oxide (NO) and ammonia (NH3) observed over four different land cover types, i.e. bare soil, grassland, maize field, and forest, at an inland rural site in Benin, West Africa, during the DACCIWA field campaign in June and July 2016. At the regional scale, urbanization and a massive growth in population in West Africa have been causing a strong increase in anthropogenic emissions. Anthropogenic pollutants are transported inland and northward from the megacities located on the coast, where the reaction with biogenic emissions may lead to enhanced ozone production outside urban areas, as well as secondary organic aerosol formation, with detrimental effects on humans, animals, natural vegetation, and crops. We observe NO fluxes up to 48.05 ngN m−2 s−1. NO fluxes averaged over all land cover types are 4.79±5.59 ngN m−2 s−1, and maximum soil emissions of NO are recorded over bare soil. NH3 is dominated by deposition for all land cover types. NH3 fluxes range between −6.59 and 4.96 ngN m−2 s−1. NH3 fluxes averaged over all land cover types are -0.91±1.27 ngN m−2 s−1, and maximum NH3 deposition is measured over bare soil. The observations show high spatial variability even for the same soil type, same day, and same meteorological conditions. We compare point daytime average measurements of NO emissions recorded during the field campaign with those simulated by GEOS-Chem (Goddard Earth Observing System Chemistry Model) for the same site and find good agreement. In an attempt to quantify NO emissions at the regional and national scale, we also provide a tentative estimate of total NO emissions for the entire country of Benin for the month of July using two distinct methods: upscaling point measurements and using the GEOS-Chem model. The two methods give similar results: 1.17±0.6 and 1.44 GgN month−1, respectively. Total NH3 deposition estimated by upscaling point measurements for the month of July is 0.21 GgN month−1.

During the monsoon season, pollutants emitted by large coastal cities and biomass burning plumes originating from central Africa have complex transport pathways over southern West Africa (SWA). The Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) field campaign has provided numerous dynamical and chemical measurements in and around the super-site of Savè in Benin ( ≈ 185 km away from the coast), which allows quantification of the relative contribution of advected pollutants. Through the combination of in situ ground measurements with aircraft, radio-sounding, satellite, and high-resolution chemistry-transport modeling with the CHIMERE model, the source attribution and transport pathways of pollutants inland (here, NOx and CO) are carefully analyzed for the 1–7 July 2016 period. The relative contributions of different sources (i.e., emissions from several large coastal cities) to the air quality in Savè are characterized. It is shown that a systematic diurnal cycle exists with high surface concentrations of pollutants from 18:00 to 22:00 UTC. This evening peak is attributed to pollution transport from the coastal city of Cotonou (Benin). Numerical model experiments indicate that the anthropogenic pollutants are accumulated during the day close to the coast and transported northward as soon as the daytime convection in the atmospheric boundary layer ceases after 16:00 UTC, reaching 8° N at 21:00 UTC. When significant biomass burning pollutants are transported into continental SWA, they are mixed with anthropogenic pollutants along the coast during the day, and this mixture is then transported northward. At night, most of the coastal anthropogenic plumes are transported within the planetary boundary layer (below about 500 m above ground level), whereas the biomass burning pollutants are mostly transported above it, thus generally not impacting ground level air quality.

As part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx), a series of aerosol and gas-phase measurements were deployed aboard the SAFIRE ATR42 research aircraft in summer 2014. The present study focuses on the four flights performed in late June early July over two forested regions in the south of France. We combine in situ observations and model simulations to aid in the understanding of secondary organic aerosol (SOA) formation over these forested areas in the Mediterranean and to highlight the role of different gas-phase precursors. The non-refractory particulate species measured by a compact aerosol time-of-flight mass spectrometer (cToF-AMS) were dominated by organics (60 to 72 %) followed by a combined contribution of 25 % by ammonia and sulfate aerosols. The contribution from nitrate and black carbon (BC) particles was less than 5 % of the total PM1 mass concentration. Measurements of non-refractory species from off-line transmission electron microscopy (TEM) showed that particles have different mixing states and that large fractions (35 %) of the measured particles were organic aerosol containing C, O, and S but without inclusions of crystalline sulfate particles. The organic aerosol measured using the cToF-AMS contained only evidence of oxidized organic aerosol (OOA), without a contribution of fresh primary organic aerosol. Positive matrix factorization (PMF) on the combined organic–inorganic matrices separated the oxidized organic aerosol into a more-oxidized organic aerosol (MOOA), and a less-oxidized organic aerosol (LOOA). The MOOA component is associated with inorganic species and had higher contributions of m∕z 44 than the LOOA factor. The LOOA factor is not associated with inorganic species and correlates well with biogenic volatile organic species measured with a proton-transfer-reaction mass spectrometer, such as isoprene and its oxidation products (methyl vinyl ketone, MVK; methacroleine, MACR; and isoprene hydroxyhydroperoxides, ISOPOOH). Despite a significantly high mixing ratio of isoprene (0.4 to 1.2 ppbV) and its oxidation products (0.2 and 0.8 ppbV), the contribution of specific signatures for isoprene epoxydiols SOA (IEPOX-SOA) within the aerosol organic mass spectrum (m∕z 53 and m∕z 82) were very weak, suggesting that the presence of isoprene-derived SOA was either too low to be detected by the cToF-AMS, or that SOA was not formed through IEPOX. This was corroborated through simulations performed with the Polyphemus model showing that although 60 to 80 % of SOA originated from biogenic precursors, only about 15 to 32 % was related to isoprene (non-IEPOX) SOA; the remainder was 10 % sesquiterpene SOA and 35 to 40 % monoterpene SOA. The model results show that despite the zone of sampling being far from industrial or urban sources, a total contribution of 20 to 34 % of the SOA was attributed to purely anthropogenic precursors (aromatics and intermediate or semi-volatile compounds). The measurements obtained during this study allow us to evaluate how biogenic emissions contribute to increasing SOA concentrations over Mediterranean forested areas. Directly comparing these measurements with the Polyphemus model provides insight into the SOA formation pathways that are prevailing in these forested areas as well as processes that need to be implemented in future simulations.

Total OH reactivity, defined as the total loss frequency of the hydroxyl radical in the atmosphere, has proved to be an excellent tool to identify the total loading of reactive species in ambient air. High levels of unknown reactivity were found in several forests worldwide and were often higher than at urban sites.Our study presents atmospheric mixing ratios of biogenic compounds and total OH reactivity measured during late spring 2014 at the forest of downy oak trees of the Observatoire de Haute Provence (OHP), France. Air masses were sampled at two heights: 2 m, i.e., inside the canopy, and 10 m, i.e., above the canopy, where the mean canopy height is 5 m.We found that the OH reactivity at the site mainly depended on the main primary biogenic species emitted by the forest, which was isoprene and to a lesser extent by its degradation products and long-lived atmospheric compounds (up to 26 % during daytime). During daytime, no significant missing OH reactivity was reported at the site, either inside or above the canopy. However, during two nights we determined a missing fraction of OH reactivity up to 50 %, possibly due to unmeasured oxidation products. We confirmed that no significant oxidation of the primary species occurred within the canopy; primary compounds emitted by the forest were fast transported to the atmosphere. Finally, the OH reactivity at this site was maximum 69 s−1, which is a high value for a forest characterized by a temperate climate. Observations in various and diverse forests in the Mediterranean region are therefore needed to better constrain the impact of reactive gases over this area.

A large dataset of reactive trace gases was collected for the first time over West Africa during the African Monsoon Multidisciplinary Analysis (AMMA) field experiment in August 2006. Volatile Organic Compounds (VOC from C5–C9) were measured onboard the two French aircrafts the ATR-42 and the Falcon-20 by a new instrument AMOVOC (Airborne Measurement Of Volatile Organic Compounds). The goal of this study is (i) to characterize VOC distribution in the tropical region of West Africa (ii) to determine the impact of deep convection on VOC distribution and chemistry in the tropical upper troposphere (UT) and (iii) to characterize its spatial and temporal extensions. Experimental strategy consisted in sampling at altitudes between 0 and 12 km downwind of Mesoscale Convective Systems (MCS) and at cloud base. Biogenic and anthropogenic VOC distribution in West Africa is clearly affected by North to South emission gradient. Isoprene, the most abundant VOC, is at maximum level over the forest (1.26 ppb) while benzene reaches its maximum over the urban areas (0.11 ppb). First, a multiple physical and chemical tracers approach using CO, O3 and relative humidity was implemented to distinguish between convective and non-convective air masses. Then, additional tools based on VOC observations (tracer ratios, proxy of emissions and photochemical clocks) were adapted to characterize deep convection on a chemical, spatial and temporal basis. VOC vertical profiles show a "C-shaped" trend indicating that VOC-rich air masses are transported from the surface to the UT by deep convective systems. VOC mixing ratios in convective outflow are up to two times higher than background levels even for reactive and short-lived VOC (e.g. isoprene up to 0.19 ppb at 12 km-altitude) and are dependent on surface emission type. As a consequence, UT air mass reactivity increases from 0.52 s−1 in non-convective conditions to 0.95 s−1 in convective conditions. Fractions of boundary layer air contained in convective outflow are estimated to be 40 ± 15%. Vertical transport timescale is calculated to be 25 ± 10 min between 0 to 12 km altitude. These results characterize deep convection occurring over West Africa and provide relevant information for tropical convection parameterization in regional/global models.

During June, July and August 2006 five aircraft took part in a campaign over West Africa to observe the aerosol content and chemical composition of the troposphere and lower stratosphere as part of the African Monsoon Multidisciplinary Analysis (AMMA) project. These are the first such measurements in this region during the monsoon period. In addition to providing an overview of the tropospheric composition, this paper provides a description of the measurement strategy (flights performed, instrumental payloads, wing-tip to wing-tip comparisons) and points to some of the important findings discussed in more detail in other papers in this special issue. The ozone data exhibits an "S" shaped vertical profile which appears to result from significant losses in the lower troposphere due to rapid deposition to forested areas and photochemical destruction in the moist monsoon air, and convective uplift of ozone-poor air to the upper troposphere. This profile is disturbed, particularly in the south of the region, by the intrusions in the lower and middle troposphere of air from the southern hemisphere impacted by biomass burning. Comparisons with longer term data sets suggest the impact of these intrusions on West Africa in 2006 was greater than in other recent wet seasons. There is evidence for net photochemical production of ozone in these biomass burning plumes as well as in urban plumes, in particular that from Lagos, convective outflow in the upper troposphere and in boundary layer air affected by nitrogen oxide emissions from recently wetted soils. This latter effect, along with enhanced deposition to the forested areas, contributes to a latitudinal gradient of ozone in the lower troposphere. Biogenic volatile organic compounds are also important in defining the composition both for the boundary layer and upper tropospheric convective outflow. Mineral dust was found to be the most abundant and ubiquitous aerosol type in the atmosphere over Western Africa. Data collected within AMMA indicate that injection of dust to altitudes favourable for long-range transport (i.e. in the upper Sahelian planetary boundary layer) can occur behind the leading edge of mesoscale convective system (MCS) cold-pools. Research within AMMA also provides the first estimates of secondary organic aerosols across the West African Sahel and have shown that organic mass loadings vary between 0 and 2 μg m−3 with a median concentration of 1.07 μg m−3. The vertical distribution of nucleation mode particle concentrations reveals that significant and fairly strong particle formation events did occur for a considerable fraction of measurement time above 8 km (and only there). Very low concentrations were observed in general in the fresh outflow of active MCSs, likely as the result of efficient wet removal of aerosol particles due to heavy precipitation inside the convective cells of the MCSs. This wet removal initially affects all particle size ranges as clearly shown by all measurements in the vicinity of MCSs.