Variability of atmospheric dimethylsulphide over the southern Indian Ocean due to changes in ultraviolet radiation

Dimethylsulphide (DMS) is a climatically important component of global biogeochemical cycles, through its role in the sulphur cycle. Changes in ultraviolet radiation (UV) exhibit both positive and negative forcings on the dynamics of production and turnover of DMS and its precursor dimethylsulphoniopropionate (DMSP). In this study we investigate the net forcing of UV on atmospheric DMS. The work is based on a 10-year record of observed DMS at Amsterdam Island in the southern Indian Ocean, and satellite-based retrievals of surface UV and photosynthetically active radiation (PAR). The results show an inverse relationship between UV radiation and atmospheric DMS associated with extreme changes (defined as the greatest 5%) in daily UV, independent of changes in wind speed, sea surface temperature, and PAR

Characterization of carbonaceous aerosols during the MINOS campaign in Crete, July–August 2001: a multi-analytical approach

During the major part of the Mediterranean Intensive Oxidant Study (MINOS) campaign (summer 2001, Crete Isl.), the Marine Boundary Layer (MBL) air was influenced by long range transport of biomass burning from the northern and western part of the Black Sea. During this campaign, carbonaceous aerosols were collected on quartz filters at a Free Tropospheric (FT) site, and at a MBL site together with size-resolved distribution of aerosols. Three Evolution Gas Analysis (EGA) protocols have been tested in order to better characterize the collected aged biomass burning smoke: A 2-step thermal method (Cachier et al., 1989) and a thermo-optical technique using two different temperature programs. The later temperature programs are those used for IMPROVE (Interagency Monitoring of Protected Visual Environments) and NIOSH 5040 (National Institute of Occupational Safety and Health). Artifacts were observed using the NIOSH temperature program and identified as interactions between carbon and dust deposited on the filter matrix at high temperature (T>550ºC) under the pure helium step of the analysis.<br> <br> During the MINOS campaign, Black Carbon (BC) and Organic Carbon (OC) mass concentrations were on average respectively 1.19±0.56 and 3.62±1.08 <font face='Symbol'>m</font>gC/m³ for the IMPROVE temperature program, and 1.09±0.36 and 3.75±1.24 <font face='Symbol'>m</font>gC/m³ for the thermal method. Though these values compare well on average and the agreement between the Total Carbon (TC) measurements sample to sample was excellent (slope=1.00, <i>r</i>²=0.93, n=56), important discrepancies were observed in determining BC concentrations from these two methods (average error of 33±22%). BC from the IMPROVE temperature program compared well with non-sea-salt potassium (nss-K) pointing out an optical sensitivity to biomass burning. On the other hand, BC from the thermal method showed a better agreement with non-sea-salt sulfate (nss-SO₄), considered as a tracer for fossil fuel combustion during the MINOS campaign. The coupling between these two methods for determining BC brings here new insights on the origin of carbonaceous aerosols in a complex mixture of different sources. It brings also to our attention that important deviations in BC levels are observed using three widely used EGA's techniques and most probably none of the EGA tested here are well adapted to fully characterize this aerosol mixture.<br> <br> Spherical, smooth and silico-aluminated fly-ash observed by an Analytical Scanning Electron Microscope (ASEM) confirm the influence of coal combustion on the carbonaceous aerosol load throughout the campaign. A rough calculation based on a BC/nss-SO₄ mass ratio suggests that biomass burning could be responsible for half of the BC concentration recorded during the MINOS campaign.<br> <br> From the plot of BC as a function of TC, two linear correlations were observed corresponding to 2 times series (before and after 12 August). Such good correlations suggest, from a first look, that both BC and OC have similar origin and atmospheric transport. On the other hand, the plot of BC as a function of TC obtained from the 2-step thermal method applied to DEKATI Low Pressure Cascade Impactor samples does not show a similar correlation and points out a non conservative distribution of this ratio with 2 super micron modes enriched in OC, correlated with sea salt aerosols and probably originating from gas-to-particle conversion

Model evaluation of marine primary organic aerosol emission schemes

In this study, several marine primary organic aerosol (POA) emission schemes have been evaluated using the GEOS-Chem chemical transport model in order to provide guidance for their implementation in air quality and climate models. These emission schemes, based on varying dependencies of chlorophyll &lt;i&gt;a&lt;/i&gt; concentration ([chl &lt;i&gt;a&lt;/i&gt;]) and 10 m wind speed (&lt;i&gt;U&lt;/i&gt;&lt;sub&gt;10&lt;/sub&gt;), have large differences in their magnitude, spatial distribution, and seasonality. Model comparison with weekly and monthly mean values of the organic aerosol mass concentration at two coastal sites shows that the source function exclusively related to [chl &lt;i&gt;a&lt;/i&gt;] does a better job replicating surface observations. Sensitivity simulations in which the negative &lt;i&gt;U&lt;/i&gt;&lt;sub&gt;10&lt;/sub&gt; and positive [chl &lt;i&gt;a&lt;/i&gt;] dependence of the organic mass fraction of sea spray aerosol are enhanced show improved prediction of the seasonality of the marine POA concentrations. A top-down estimate of submicron marine POA emissions based on the parameterization that compares best to the observed weekly and monthly mean values of marine organic aerosol surface concentrations has a global average emission rate of 6.3 Tg yr&lt;sup&gt;−1&lt;/sup&gt;. Evaluation of existing marine POA source functions against a case study during which marine POA contributed the major fraction of submicron aerosol mass shows that none of the existing parameterizations are able to reproduce the hourly-averaged observations. Our calculations suggest that in order to capture episodic events and short-term variability in submicron marine POA concentration over the ocean, new source functions need to be developed that are grounded in the physical processes unique to the organic fraction of sea spray aerosol

Characterization of carbonaceous aerosols during the MINOS campaign in Crete, July–August 2001: a multi-analytical approach

International audienceDuring the major part of the Mediterranean Intensive Oxidant Study (MINOS) campaign (summer 2001, Crete Isl.), the Marine Boundary Layer (MBL) air was influenced by long range transport of biomass burning from the northern and western part of the Black Sea. During this campaign, carbonaceous aerosols were collected on quartz filters at a Free Tropospheric (FT) site, and at a MBL site together with size-resolved distribution of aerosols. Three Evolution Gas Analysis (EGA) protocols have been tested in order to better characterize the collected aged biomass burning smoke: A 2-step thermal method (Cachier et al., 1989) and a thermo-optical technique using two different temperature programs. The later temperature programs are those used for IMPROVE (Interagency Monitoring of Protected Visual Environments) and NIOSH 5040 (National Institute of Occupational Safety and Health). Artifacts were observed using the NIOSH temperature program and identified as interactions between carbon and dust deposited on the filter matrix at high temperature (T>550ºC) under the pure helium step of the analysis. During the MINOS campaign, Black Carbon (BC) and Organic Carbon (OC) mass concentrations were on average respectively 1.19±0.56 and 3.62±1.08 mgC/m3 for the IMPROVE temperature program, and 1.09±0.36 and 3.75±1.24 mgC/m3 for the thermal method. Though these values compare well on average and the agreement between the Total Carbon (TC) measurements sample to sample was excellent (slope=1.00, r2=0.93, n=56), important discrepancies were observed in determining BC concentrations from these two methods (average error of 33±22%). BC from the IMPROVE temperature program compared well with non-sea-salt potassium (nss-K) pointing out an optical sensitivity to biomass burning. On the other hand, BC from the thermal method showed a better agreement with non-sea-salt sulfate (nss-SO4), considered as a tracer for fossil fuel combustion during the MINOS campaign. The coupling between these two methods for determining BC brings here new insights on the origin of carbonaceous aerosols in a complex mixture of different sources. It brings also to our attention that important deviations in BC levels are observed using three widely used EGA's techniques and most probably none of the EGA tested here are well adapted to fully characterize this aerosol mixture. Spherical, smooth and silico-aluminated fly-ash observed by an Analytical Scanning Electron Microscope (ASEM) confirm the influence of coal combustion on the carbonaceous aerosol load throughout the campaign. A rough calculation based on a BC/nss-SO4 mass ratio suggests that biomass burning could be responsible for half of the BC concentration recorded during the MINOS campaign. From the plot of BC as a function of TC, two linear correlations were observed corresponding to 2 times series (before and after 12 August). Such good correlations suggest, from a first look, that both BC and OC have similar origin and atmospheric transport. On the other hand, the plot of BC as a function of TC obtained from the 2-step thermal method applied to DEKATI Low Pressure Cascade Impactor samples does not show a similar correlation and points out a non conservative distribution of this ratio with 2 super micron modes enriched in OC, correlated with sea salt aerosols and probably originating from gas-to-particle conversion

Modeling organic aerosol concentrations and properties during winter 2014 in the northwestern Mediterranean region

Organic aerosols are measured at a remote site (Ersa) on the cape of Corsica in the northwestern Mediterranean basin during the winter campaign of 2014 of the CHemistry and AeRosols Mediterranean EXperiment (CharMEx), when high organic concentrations from anthropogenic origins are observed. This work aims to represent the observed organic aerosol concentrations and properties (oxidation state) using the air-quality model Polyphemus with a surrogate approach for secondary organic aerosol (SOA) formation. Because intermediate and semi-volatile organic compounds (I/S-VOCs) are the main precursors of SOAs at Ersa during winter 2014, different parameterizations to represent the emission and aging of I/S-VOCs were implemented in the chemistry-transport model of Polyphemus (different volatility distribution emissions and single-step oxidation vs multi-step oxidation within a volatility basis set – VBS – framework, inclusion of non-traditional volatile organic compounds – NTVOCs). Simulations using the different parameterizations are compared to each other and to the measurements (concentration and oxidation state). The highly observed organic concentrations are well reproduced in all the parameterizations. They are slightly underestimated in most parameterizations. The volatility distribution at emissions influences the concentrations more strongly than the choice of the parameterization that may be used for aging (single-step oxidation vs multi-step oxidation), stressing the importance of an accurate characterization of emissions. Assuming the volatility distribution of sectors other than residential heating to be the same as residential heating may lead to a strong underestimation of organic concentrations. The observed organic oxidation and oxygenation states are strongly underestimated in all simulations, even when multigenerational aging of I/S-VOCs from all sectors is modeled. This suggests that uncertainties in the emissions and aging of I/S-VOC emissions remain to be elucidated, with a potential role of formation of organic nitrate and low-volatility highly oxygenated organic molecules.</p

Absorption properties of Mediterranean aerosols obtained from multi-year ground-based remote sensing observations.

International audienceAerosol absorption properties are of high importance to assess aerosol impact on regional climate. This study presents an analysis of aerosol absorption products obtained over the Mediterranean basin or land stations in the region from multi-year ground-based AERONET observations with a focus on the Absorbing Aerosol Optical Depth (AAOD), Single Scattering Albedo (SSA) and their spectral dependence. The AAOD and Absorption Angström Exponent (AAE) dataset is composed of daily averaged AERONET level 2 data from a total of 22 Mediterranean stations having long time series, mainly under the influence of urban-industrial aerosols and/or soil dust. This dataset covers the 17-yr period 1996-2012 with most data being from 2003-2011 (~89% of level-2 AAOD data). Since AERONET level-2 absorption products require a high aerosol load (AOD at 440 nm > 0.4), which is most often related to the presence of desert dust, we also consider level-1.5 SSA data, despite their higher uncertainty, and filter out data with an Angström exponent < 1.0 in order to study absorption by carbonaceous aerosols. The SSA dataset includes AERONET level-2 products. Sun-photometer observations show that values of AAOD at 440 nm vary between 0.024 ± 0.01 (resp. 0.040 ± 0.01) and 0.050 ± 0.01 (0.055 ± 0.01) for urban (dusty) sites. Analysis shows that the Mediterranean urban-industrial aerosols appear "moderately" absorbing with values of SSA close to ~0.94-0.95 ± 0.04 (at 440 nm) in most cases except over the large cities of Rome and Athens, where aerosol appears more absorbing (SSA ~0.89-0.90 ± 0.04). The aerosol Absorption Angström Exponent (AAE, estimated using 440 and 870 nm) is found to be larger than 1 for most sites over the Mediterranean, a manifestation of mineral dust (iron) and/or brown carbon producing the observed absorption. AERONET level-2 sun-photometer data indicate a possible East-West gradient, with higher values over the eastern basin (AAEEast = 1.39/AAEWest = 1.33). The North-South AAE gradient is more pronounced, especially over the western basin. Our additional analysis of AERONET level-1.5 data also shows that organic absorbing aerosols significantly affect some Mediterranean sites. These results indicate that current climate models treating organics as nonabsorbing over the Mediterranean certainly underestimate the warming effect due to carbonaceous aerosols

Comparison between simulated and observed chemical composition of fine aerosols in Paris (France) during springtime: contribution of regional versus continental emissions

Hourly concentrations of inorganic salts (ions) and carbonaceous material in fine aerosols (aerodynamic diameter, A.D. &lt;2.5 μm) have been determined experimentally from fast measurements performed for a 3-week period in spring 2007 in Paris (France). The sum of these two chemical components (ions and carbonaceous aerosols) has shown to account for most of the fine aerosol mass (PM&lt;sub&gt;2.5&lt;/sub&gt;). This time-resolved dataset allowed investigating the factors controlling the levels of PM&lt;sub&gt;2.5&lt;/sub&gt; in Paris and showed that polluted periods with PM&lt;sub&gt;2.5&lt;/sub&gt; &gt; 15 μg m&lt;sup&gt;−3&lt;/sup&gt; were characterized by air masses of continental (North-Western Europe) origin and chemical composition made by 75% of ions. By contrast, periods with clean marine air masses have shown the lowest PM&lt;sub&gt;2.5&lt;/sub&gt; concentrations (typically of about 10 μg m&lt;sup&gt;−3&lt;/sup&gt;); carbonaceous aerosols contributing for most of this mass (typically 75%). &lt;br&gt;&lt;br&gt; In order to better discriminate between local and continental contributions to the observed chemical composition and concentrations of PM&lt;sub&gt;2.5&lt;/sub&gt; over Paris, a comparative study was performed between this time-resolved dataset and the outputs of a chemistry transport model (CHIMERE), showing a relatively good capability of the model to reproduce the time-limited intense maxima observed in the field for PM&lt;sub&gt;2.5&lt;/sub&gt; and ion species. Different model scenarios were then investigated switching off local and European (North-Western and Central) emissions. Results of these scenarios have clearly shown that most of the ions observed over Paris during polluted periods, were either transported or formed in-situ from gas precursors transported from Northern Europe. On the opposite, long-range transport from Europe appeared to weakly contribute to the levels of carbonaceous aerosols observed over Paris. &lt;br&gt;&lt;br&gt; The model failed to properly account for the concentration levels and variability of secondary organic aerosols (SOA) determined experimentally by the EC-tracer method. The abundance of SOA (relatively to organic aerosol, OA) was as much as 75%, showing a weak dependence on air masses origin. Elevated SOA/OA ratios were also observed for air masses having residence time above ground of less than 10 h, suggesting intense emissions and/or photochemical processes leading to rapid formation of secondary organic aerosols

Clues for a standardised thermal-optical protocol for the assessment of organic and elemental carbon within ambient air particulate matter

Along with some research networking programmes, the European Directive 2008/50/CE requires chemical speciation of fine aerosol (PM_2.5), including elemental (EC) and organic carbon (OC), at a few rural sites in European countries. Meanwhile, the thermal-optical technique is considered by the European and US networking agencies and normalisation bodies as a reference method to quantify EC–OC collected on filters. Although commonly used for many years, this technique still suffers from a lack of information on the comparability of the different analytical protocols (temperature protocols, type of optical correction) currently applied in the laboratories. To better evaluate the EC–OC data set quality and related uncertainties, the French National Reference Laboratory for Ambient Air Quality Monitoring (LCSQA) organised an EC–OC comparison exercise for French laboratories using different thermal-optical methods (five laboratories only). While there is good agreement on total carbon (TC) measurements among all participants, some differences can be observed on the EC / TC ratio, even among laboratories using the same thermal protocol. These results led to further tests on the influence of the optical correction: results obtained from different European laboratories confirmed that there were higher differences between OC_TOT and OC_TOR measured with NIOSH 5040 in comparison to EUSAAR-2. Also, striking differences between EC_TOT / EC_TOR ratios can be observed when comparing results obtained for rural and urban samples, with EC_TOT being 50% lower than EC_TOR at rural sites whereas it is only 20% lower at urban sites. The PM chemical composition could explain these differences but the way it influences the EC–OC measurement is not clear and needs further investigation. Meanwhile, some additional tests seem to indicate an influence of oven soiling on the EC–OC measurement data quality. This highlights the necessity to follow the laser signal decrease with time and its impact on measurements. Nevertheless, this should be confirmed by further experiments, involving more samples and various instruments, to enable statistical processing. All these results provide insights to determine the quality of EC–OC analytical methods and may contribute to the work toward establishing method standardisation

DMS atmospheric concentrations and sulphate aerosol indirect radiative forcing: a sensitivity study to the DMS source representation and oxidation

The global sulphur cycle has been simulated using a general circulation model with a focus on the source and oxidation of atmospheric dimethylsulphide (DMS). The sensitivity of atmospheric DMS to the oceanic DMS climatology, the parameterisation of the sea-air transfer and to the oxidant fields have been studied. The importance of additional oxidation pathways (by O₃ in the gas- and aqueous-phases and by BrO in the gas phase) not incorporated in global models has also been evaluated. While three different climatologies of the oceanic DMS concentration produce rather similar global DMS fluxes to the atmosphere at 24-27 Tg S yr ^-1, there are large differences in the spatial and seasonal distribution. The relative contributions of OH and NO₃ radicals to DMS oxidation depends critically on which oxidant fields are prescribed in the model. Oxidation by O₃ appears to be significant at high latitudes in both hemispheres. Oxidation by BrO could be significant even for BrO concentrations at sub-pptv levels in the marine boundary layer. The impact of such refinements on the DMS chemistry onto the indirect radiative forcing by anthropogenic sulphate aerosols is also discussed