ALBERT

Description: The European Union (EU)-funded project Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) investigates the relationship between weather, climate, and air pollution in southern West Africa—an area with rapid population growth, urbanization, and an increase in anthropogenic aerosol emissions. The air over this region contains a unique mixture of natural and anthropogenic gases, liquid droplets, and particles, emitted in an environment in which multilayer clouds frequently form. These exert a large influence on the local weather and climate, mainly owing to their impact on radiation, the surface energy balance, and thus the diurnal cycle of the atmospheric boundary layer. In June and July 2016, DACCIWA organized a major international field campaign in Ivory Coast, Ghana, Togo, Benin, and Nigeria. Three supersites in Kumasi, Savè, and Ile-Ife conducted permanent measurements and 15 intensive observation periods. Three European aircraft together flew 50 research flights between 27 June and 16 July 2016, for a total of 155 h. DACCIWA scientists launched weather balloons several times a day across the region (772 in total), measured urban emissions, and evaluated health data. The main objective was to build robust statistics of atmospheric composition, dynamics, and low-level cloud properties in various chemical landscapes to investigate their mutual interactions. This article presents an overview of the DACCIWA field campaign activities as well as some first research highlights. The rich data obtained during the campaign will be made available to the scientific community and help to advance scientific understanding, modeling, and monitoring of the atmosphere over southern West Africa.

Description: Airborne measurements of the halogenated trace gases methyl chloride, methyl bromide and chloroform were conducted over the Atlantic Ocean and 1000 km of pristine tropical rainforest in Suriname and French Guyana (3–6° N, 51–59° W) in October 2005. In the boundary layer (0–1.4 km), maritime air masses initially low in forest hydrocarbons, advected over the forest by southeasterly trade winds, were measured at various distances from the coast. Since the organohalogens presented here have relatively long atmospheric lifetimes (0.4–1.0 years) in comparison to the transport times (1–2 days), emissions will accumulate in air traversing the rainforest. The distributions of methyl chloride, methyl bromide and chloroform were analyzed as a function of forest contact time and the respective relationship used to determine fluxes from the rainforest during the long dry season. Emission fluxes have been calculated for methyl chloride and chloroform as 9.4 (±4.0 2σ) and 0.34 (0.14± 2σ) μg m−2 h−1, respectively. No significant flux from the rainforest was observed for methyl bromide within the limits of these measurements. The flux of methyl chloride was in general agreement with the flux measured over the same region in March 1998 during the LBA Claire project using a different analytical system. This confirms that the rainforest is a strong source for methyl chloride and suggests that this emission is relatively uniform throughout the year. In contrast the chloroform flux derived here is a factor of three less than previous measurements made in March 1998 suggesting a pronounced ecosystem variation. The differences in chloroform fluxes could not be attributed to either temperature or rainfall changes. The global extrapolation of the derived fluxes led to 1.5 (±0.6 2σ) Tg yr−1 for methyl chloride, which is in the range of the missing source postulated by previous model studies and 55 (±22 2σ) Gg yr−1 for chloroform.

Description: Cloud condensation nuclei (CCN) size distributions and numbers were measured for the first time at Puy-de-Dôme high altitude (1465 m a.s.l) site in Central France. Majority of the measurements were done at constant supersaturation (SS) of 0.24%, which was also deduced to be representative of the typical in-cloud SS at the site. CCN numbers during summer ranged from about 200 up to 2000 cm−3 and during winter from 50 up to 3000 cm−3. Variability of CCN number was explained by both particle chemistry and size distribution variability. The higher CCN concentrations were measured in continental, in contrast to marine, air masses. Aerosol CCN activity was described with a single hygroscopicity parameter κ. Range of this parameter was 0.29 ± 0.13 in summer and 0.43 ± 0.19 in winter. When calculated using SS of 0.51% during summer, κ of 0.22 ± 0.07 was obtained. The decrease with increasing SS is likely explained by the particle size dependent chemistry with smaller particles containing higher amounts of freshly emitted organic species. Higher κ values during winter were for the most part explained by the observed aged organics (analysed from organic m/z 44 ratio) rather than from aerosol organic to inorganic volume fraction. The obtained κ values also fit well within the range of previously proposed global continental κ of 0.27 ± 0.21. During winter, the smallest κ values and the highest organic fractions were measured in marine air masses. CCN closure using bulk AMS chemistry led to positive bias of 5% and 2% in winter and summer, respectively. This is suspected to stem from size dependent aerosol organic fraction, which is underestimated by using AMS bulk mass composition. Finally, the results were combined with size distributions measured from interstitial and whole air inlets to obtain activated droplet size distributions. Cloud droplet number concentrations were shown to increase with accumulation mode particle number, while the real in-cloud SS correspondingly decreased. These results provide evidence on the effects of aerosol particles on maximum cloud supersaturations. Further work with detailed characterisation of cloud properties is proposed in order to provide more quantitative estimates on aerosol effects on clouds.

Description: Measurements of the mole fraction of the CO2 and its isotopes were performed in Paris during the MEGAPOLI winter campaign (January–February 2010). Radiocarbon (14CO2) measurements were used to identify the relative contributions of 77% CO2 from fossil fuel consumption (CO2ff from liquid and gas combustion) and 23% from biospheric CO2 (CO2 from the use of biofuels and from human and plant respiration: CO2bio). These percentages correspond to average mole fractions of 26.4 ppm and 8.2 ppm for CO2ff and CO2bio, respectively. The 13CO2 analysis indicated that gas and liquid fuel contributed 70% and 30%, respectively, of the CO2 emission from fossil fuel use. Continuous measurements of CO and NOx and the ratios CO/CO2ff and NOx/CO2ff derived from radiocarbon measurements during four days make it possible to estimate the fossil fuel CO2 contribution over the entire campaign. The ratios CO/CO2ff and NOx/CO2ff are functions of air mass origin and exhibited daily ranges of 7.9 to 14.5 ppb ppm−1 and 1.1 to 4.3 ppb ppm−1, respectively. These ratios are consistent with different emission inventories given the uncertainties of the different approaches. By using both tracers to derive the fossil fuel CO2, we observed similar diurnal cycles with two maxima during rush hour traffic.

Description: Three years of greenhouse gases measurements, obtained using a gas chromatograph (GC) system located at the Puy de Dôme station at 1465 m a.s.l. in Central France are presented. The GC system was installed in 2010 at Puy de Dôme and was designed for automatic and accurate semi-continuous measurements of atmospheric carbon dioxide, methane, nitrous oxide and sulfur hexafluoride mole fractions. We present in detail the instrumental set up and the calibration strategy, which together allow the GC to reach repeatabilities of 0.1 μmol mol−1, 1.2, 0.3 nmol mol−1 and 0.06 pmol mol−1 for CO2, CH4, N2O and SF6, respectively. Comparisons of the atmospheric time series with those obtained using other instruments shown that the GC system meets the World Meteorological Organization recommendations. The analysis of the three-year atmospheric time series revealed how the planetary boundary layer height drives the mole fractions observed at a mountain site such as Puy de Dôme where air masses alternate between the planetary boundary layer and the free troposphere. Accurate long-lived greenhouse gases measurements collocated with 222Rn measurements as an atmospheric tracer, allowed us to determine the CO2, CH4 and N2O emissions in the catchment area of the station. The derived CO2 surface flux revealed a clear seasonal cycle with net uptake by plant assimilation in the spring and net emission caused by the biosphere and burning of fossil fuel during the remainder of the year. We calculated a mean annual CO2 flux of 1150 t(CO2) km−2. The derived CH4 and N2O emissions in the station catchment area were 5.6 t(CH4) km−2 yr−1 and 1.5 t(N2O) km−2 yr−1, respectively. Our derived annual CH4 flux is in agreement with the national French inventory, whereas our derived N2O flux is five times larger than the same inventory.

Description: A Saharan dust event was observed in a rural area in the Maurienne Valley (French Alps) in summer 2000. Detailed data on PM10, particle numbers, and aerosol chemistry (ionic species and Elemental Carbon (EC) and Organic Carbon (OC)) are presented. The comparative evolutions of particle numbers and chemistry (calcium, sodium, and sulfate) show that the overall period included two episodes of dust particles with very distinct chemistry, followed by an episode with a large increase of the concentrations of species with an anthropogenic origin. The overall data set does not indicate large interactions between the dust particles and compounds from anthropogenic origin (sulfate, nitrate) or with organic carbon, all of these species showing very low concentrations. Simplistic calculations indicate that these concentrations are consistent with our current knowledge of adsorption processes of gases on mineral dust in a clean air mass.

Description: Detailed investigations of the chemical and microphysical properties of atmospheric aerosol particles were performed at the puy-de-Dôme (pdD) research station (1465 m) in autumn (September and October 2008), winter (February and March 2009), and summer (June 2010) using a compact Time-of-Flight Aerosol Mass Spectrometer (cToF-AMS). Over the three campaigns, the average mass concentrations of the non-refractory submicron particles ranged from 10 μg m−3 up to 27 μg m−3. Highest nitrate and ammonium mass concentrations were measured during the winter and during periods when marine modified airmasses were arriving at the site, whereas highest concentrations of organic particles were measured during the summer and during periods when continental airmasses arrived at the site. The measurements reported in this paper show that atmospheric particle composition is strongly influenced by both the season and the origin of the airmass. The total organic mass spectra were analysed using positive matrix factorisation to separate individual organic components contributing to the overall organic particle mass concentrations. These organic components include a low volatility oxygenated organic aerosol particle (LV-OOA) and a semi-volatile organic aerosol particle (SV-OOA). Correlations of the LV-OOA components with fragments of m/z 60 and m/z 73 (mass spectral markers of wood burning) during the winter campaign suggest that wintertime LV-OOA are related to aged biomass burning emissions, whereas organic aerosol particles measured during the summer are likely linked to biogenic sources. Equivalent potential temperature calculations, gas-phase, and LIDAR measurements define whether the research site is in the planetary boundary layer (PBL) or in the free troposphere (FT)/residual layer (RL). We observe that SV-OOA and nitrate particles are associated with air masses arriving from the PBL where as particle composition measured from RL/FT airmasses contain high mass fractions of sulphate and LV-OOA. This study provides unique insights into the effects of season and airmass variability on regional aerosol particles measured at an elevated site.

Description: Airborne measurements of the halogenated trace gases methyl chloride, methyl bromide and chloroform were conducted over the Atlantic Ocean and about 1000 km of pristine tropical rainforest in Suriname and French Guyana (3–6° N, 51–59° W) in October 2005. In the boundary layer (0–1.4 km), maritime air masses, advected over the forest by southeasterly trade winds, were measured at various distances from the coast. Since the organohalogens presented here have relatively long atmospheric lifetimes (0.4–1.0 years) in comparison to the advection times from the coast (1–2 days), emissions will accumulate in air traversing the rainforest. The distributions of methyl chloride, methyl bromide and chloroform were analyzed as a function of time the air spent over land and the respective relationship used to determine net fluxes from the rainforest for one week within the long dry season. Net fluxes from the rainforest ecosystem have been calculated for methyl chloride and chloroform as 9.5 (±3.8 2σ) and 0.35 (±0.15 2σ)μg m-2 h−1, respectively. No significant flux was observed for methyl bromide within the limits of these measurements. The global budget of methyl chloride contains large uncertainties, in particular with regard to a possible source from tropical vegetation. Our measurements are used in a large-scale approach to determine the net flux from a tropical ecosystem to the planetary boundary layer. The obtained global net flux of 1.5 (±0.6 2σ) Tg yr-1 for methyl chloride is at the lower end of current estimates for tropical vegetation sources, which helps to constrain the range of tropical sources and sinks (0.82 to 8.2 Tg yr-1 from tropical plants, 0.03 to 2.5 Tg yr-1 from senescent/dead leaves and a sink of 0.1 to 1.6 Tg yr-1 by soil uptake). Nevertheless, these results show that the contribution of the rainforest ecosystem is the major source in the global budget of methyl chloride. For chloroform, the extrapolated global net flux from tropical ecosystems is 56 (±23 2σ) Gg yr−1, which is of minor importance compared to the total global sources and might be already contained in the soil emission term.

Description: Chemical Ionisation Mass Spectrometer measurements of hydroxyl radical (OH) and the sum of hydroperoxy and organic peroxy (HO2+RO2) radicals were conducted during the MEGAPOLI summer field campaign at the SIRTA observatory near Paris, France, in July 2009. OH and (HO2+RO2) showed a typical diurnal variation with averaged daytime maxima values around 5×106 and 1.2×108 molecule cm−3, respectively. Simultaneously, a large number of ancillary measurements, such as NOx, O3, HONO, HCHO and other VOCs were also conducted. These data provide an opportunity to assess our understanding of the radical chemistry in a suburban environment by comparing the radical observations to calculations. First, OH mixing ratios were estimated by a simple Photo Stationary State (PSS) calculation. PSS calculations overestimate the OH mixing ratio by 50%, especially at NOx mixing ratios lower than 10 ppb, suggesting that some loss processes were missing in the calculation at low NOx. Then, a photochemical box model simulation based on the Master Chemical Mechanism (MCM) and constrained by ancillary measurements was run to calculate radical concentrations. Three different modelling procedures were tested, varying the way the unconstrained secondary species were estimated, to cope with the unavoidable lack of their measurements. They led to significant differences in simulated radical concentrations. OH and (HO2+RO2) concentrations estimated by two selected model version were compared with measurements. These versions of the model were chosen because they lead, respectively, to the higher and lower simulated radical concentrations and are thus the two extremes versions. The box model showed better results than PSS calculations, with a slight overestimation of 12% and 5%, for OH and (HO2+RO2) respectively, in average for the reference model, and an overestimation of approximately 20% for OH and an underestimation for (HO2+RO2) for the other selected model version. Thus, we can conclude from our study that OH and (HO2+RO2) radical levels agree on average with observations within the uncertainty range. Finally, an analysis of the radical budget, on a daily basis (06:00–18:00 UTC), indicates that HONO photolysis (~35%), O3 photolysis (~23%), and aldehydes and ketones photolysis (~16% for formaldehyde and 18% for others) are the main radical initiation pathways. According to the MCM modelling, the reactions of RO2 with NO2 (~19%), leading mainly to PAN formation, is a significant termination pathway in addition to the main net loss via reaction of OH with NO2 (~50%).

Description: Simulations with the chemistry transport model CHIMERE are compared to measurements performed during the MEGAPOLI (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) summer campaign in the Greater Paris region in July 2009. The volatility-basis-set approach (VBS) is implemented into this model, taking into account the volatility of primary organic aerosol (POA) and the chemical aging of semi-volatile organic species. Organic aerosol is the main focus and is simulated with three different configurations with a modified treatment of POA volatility and modified secondary organic aerosol (SOA) formation schemes. In addition, two types of emission inventories are used as model input in order to test the uncertainty related to the emissions. Predictions of basic meteorological parameters and primary and secondary pollutant concentrations are evaluated, and four pollution regimes are defined according to the air mass origin. Primary pollutants are generally overestimated, while ozone is consistent with observations. Sulfate is generally overestimated, while ammonium and nitrate levels are well simulated with the refined emission data set. As expected, the simulation with non-volatile POA and a single-step SOA formation mechanism largely overestimates POA and underestimates SOA. Simulation of organic aerosol with the VBS approach taking into account the aging of semi-volatile organic compounds (SVOC) shows the best correlation with measurements. High-concentration events observed mostly after long-range transport are well reproduced by the model. Depending on the emission inventory used, simulated POA levels are either reasonable or underestimated, while SOA levels tend to be overestimated. Several uncertainties related to the VBS scheme (POA volatility, SOA yields, the aging parameterization), to emission input data, and to simulated OH levels can be responsible for this behavior. Despite these uncertainties, the implementation of the VBS scheme into the CHIMERE model allowed for much more realistic organic aerosol simulations for Paris during summertime. The advection of SOA from outside Paris is mostly responsible for the highest OA concentration levels. During advection of polluted air masses from northeast (Benelux and Central Europe), simulations indicate high levels of both anthropogenic and biogenic SOA fractions, while biogenic SOA dominates during periods with advection from Southern France and Spain.