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Wildland fires represent the major source of fine aerosols, i.e., atmospheric particles with diameters <1 μm. The largest numbers of these fires occur in Africa, Asia and South America, but a not negligible fraction also occurs in Eastern Europe and former USSR countries, particularly in the Russian Federation, Ukraine and Kazakhstan. Besides the impact of large forest fires, recent studies also highlighted the crucial role played by routine agricultural fires in Eastern Europe and Russia on the Arctic atmosphere. An evaluation of the impact of these fires over Europe is currently not available. The assessment of the relative contribution of fires to the European aerosol burden is hampered by the complex mixing of natural and anthropogenic particle types across the continent. In this study we use long term (2002–2007) satellite-based fires and aerosol data coupled to atmospheric trajectory modelling in the attempt to estimate the wildfires contribution to the European aerosol optical thickness (AOT). Based on this dataset, we provide evidence that fires-related aerosols play a major role in shaping the AOT yearly cycle at the continental scale. In general, the regions most impacted by wildfires emissions and/or transport are Eastern and Central Europe as well as Scandinavia. Conversely, a minor impact is found in Western Europe and in the Western Mediterranean. We estimate that in spring 5 to 35% of the European fine fraction AOT (FFAOT) is attributable to wildland fires. The estimated impact maximizes in April (20–35%) in Eastern and Central Europe as well as in Scandinavia and in the Central Mediterranean. An important contribution of wildfires to the FFAOT is also found in summer over most of the continent, particularly in August over Eastern Europe (28%) and the Mediterranean regions, from Turkey (34%) to the Western Mediterranean (25%). Although preliminary, our results suggest that this fires-related, continent-wide haze plays a not negligible role on the European radiation budget, and possibly, on the European air quality, therefore representing a clear target for mitigation.
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Large-eddy simulations of a pocket of open cells (POC) based on VOCALS Regional Experiment (REx) NSF C-130 Research Flight 06 are analyzed and compared with aircraft observations. A doubly-periodic domain 192 km × 24 km with 125 m horizontal and 5 m vertical grid spacing near the capping inversion is used. The POC is realized in the model as a fixed 96 km wide region of reduced cloud droplet number concentration (<i>N</i><sub>c</sub>) based on observed values; initialization and forcing are otherwise uniform across the domain. The model reproduces aircraft-observed differences in boundary-layer structure and precipitation organization between a well-mixed overcast region and a decoupled POC with open-cell precipitating cumuli, although the simulated cloud cover is too large in the POC. A sensitivity study in which <i>N</i><sub>c</sub> is allowed to advect following the turbulent flow gives nearly identical results over the 16 h length of the simulation (which starts at night and goes into the next afternoon). <br><br> The simulated entrainment rate is nearly a factor of two smaller in the less turbulent POC than in the more turbulent overcast region. However, the inversion rises at a nearly uniform rate across the domain because powerful buoyancy restoring forces counteract horizontal inversion height gradients. A secondary circulation develops in the model that diverts subsiding free-tropospheric air away from the POC into the surrounding overcast region, counterbalancing the weaker entrainment in the POC with locally weaker subsidence.
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Atmospheric aging and processing appears to alter physical and chemical properties of mineral dust aerosol and thus its role as reactive surface in the troposphere. Yet, previous studies in the atmosphere have mainly focused on the clean surfaces of mineral dust aerosol, and the reactivity of aged mineral aerosol toward atmospheric trace gases is still poorly recognized. This work presents the first laboratory investigation of heterogeneous reactions of gaseous hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), an important atmospheric oxidant, on the surface of HNO<sub>3</sub> and SO<sub>2</sub>-processed alumina particles as surrogates of mineral dust aerosol aged by acidic trace gases as a function of relative humidity (RH) and surface coverage of coatings. Pretreatment of the alumina surfaces with HNO<sub>3</sub> and SO<sub>2</sub> has a strong impact on its reactivity toward H<sub>2</sub>O<sub>2</sub> uptake. On HNO<sub>3</sub>-processed particles, because of the dual role of the nitrate coating in modifying the reactivity of the particle surface, namely blocking oxide active sites but altering surface hygroscopicity, H<sub>2</sub>O<sub>2</sub> uptake seems to decrease in some cases whereas increase in other cases, largely depending on RH and surface coverage of nitrate. On SO<sub>2</sub>-processed particles, the presence of adsorbed S(IV) species appears to enhance the intrinsic reactivity of the alumina particles due to its affinity for H<sub>2</sub>O<sub>2</sub>, and the uptake of H<sub>2</sub>O<sub>2</sub> increases by 40–80% in the range of RH from 25% to 92% relative to the unprocessed particles. However, when S(IV) is completely oxidized to S(VI), the alumina surface is significantly deactivated and the measured uptake of H<sub>2</sub>O<sub>2</sub> decreased markedly. The mechanisms for heterogeneous reactions of H<sub>2</sub>O<sub>2</sub> with these processed particles are discussed, as well as its potential implications on tropospheric chemistry. The results of our study suggest that the reactivity of mineral dust aerosol toward H<sub>2</sub>O<sub>2</sub> and maybe other atmospheric trace gases will depend on the chemical nature and coverage of the coatings as well as ambient RH, and thus will vary considerably in different polluted atmosphere, which should be taken into account in current atmospheric models.
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This paper presents results from 240-member ensemble simulations of aerosol indirect effects on tropical deep convection and its thermodynamic environment. Simulations using a two-dimensional cloud-system resolving model are run with pristine, polluted, or highly polluted aerosol conditions and large-scale forcing from a 6-day period of active monsoon conditions during the 2006 Tropical Warm Pool – International Cloud Experiment (TWP-ICE). Domain-mean surface precipitation is insensitive to aerosols primarily because the large-scale forcing is prescribed and dominates the water and static energy budgets. The spread of the top-of-atmosphere (TOA) shortwave and longwave radiative fluxes among different ensemble members for the same aerosol loading is surprisingly large, exceeding 25 W m<sup>−2</sup> even when averaged over the 6-day period. This variability is caused by random fluctuations in the strength and timing of individual deep convective events. The ensemble approach demonstrates a small weakening of convection averaged over the 6-day period in the polluted simulations compared to pristine. Despite this weakening, the cloud top heights and anvil ice mixing ratios are higher in polluted conditions. This occurs because of the larger concentrations of cloud droplets that freeze, leading directly to higher ice particle concentrations, smaller ice particle sizes, and smaller fall velocities compared to simulations with pristine aerosols. Weaker convection in polluted conditions is a direct result of the changes in anvil ice characteristics and subsequent upper-tropospheric radiative heating and weaker tropospheric destabilization. Such a conclusion offers a different interpretation of recent satellite observations of tropical deep convection in pristine and polluted environments compared to the hypothesis of aerosol-induced convective invigoration. Sensitivity tests using the ensemble approach with modified microphysical parameters or domain configuration (horizontal gridlength, domain size) produce results that are similar to baseline, although there are quantitative differences in estimates of aerosol impacts on TOA radiative fluxes.
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We provide satellite-based evidence of the spectral dependence of absorption in biomass burning aerosols over South America using near-UV measurements made by the Ozone Monitoring Instrument (OMI) during 2005–2007. In the current near-UV OMI aerosol algorithm (OMAERUV), it is implicitly assumed that the only absorbing component in carbonaceous aerosols is black carbon whose imaginary component of the refractive index is wavelength independent. With this assumption, OMI-derived aerosol optical depth (AOD) is found to be significantly over-estimated compared to that of AERONET at several sites during intense biomass burning events (August-September). Other well-known sources of error affecting the near-UV method of aerosol retrieval do not explain the large observed AOD discrepancies between the satellite and the ground-based observations. A number of studies have revealed strong spectral dependence in carbonaceous aerosol absorption in the near-UV region suggesting the presence of organic carbon in biomass burning generated aerosols. A sensitivity analysis examining the importance of accounting for the presence of wavelength-dependent aerosol absorption in carbonaceous particles in satellite-based remote sensing was carried out in this work. The results convincingly show that the inclusion of spectrally-dependent aerosol absorption in the radiative transfer calculations leads to a more accurate characterization of the atmospheric load of carbonaceous aerosols. The use of a new set of aerosol models assuming wavelength-dependent aerosol absorption in the near-UV region (Absorption Angstrom Exponent λ<sup>−2.5 to −3.0</sup>) improved the OMAERUV retrieval results by significantly reducing the AOD bias observed when gray aerosols were assumed. In addition, the new retrieval of single-scattering albedo is in better agreement with those of AERONET within the uncertainties (ΔSSA = ±0.03). The new colored carbonaceous aerosol model was also found to reproduce the ground-based AOD observations over the biomass burning region of central Africa and northern India. Together with demonstrating a significant improvement in the retrieval of aerosol properties from OMI, the present study highlights the greater sensitivity of the near-UV measurements to the varying spectral aerosol absorption. This capability can be explored further for the use in the identification of the black carbon and organics in the biomass burning aerosols.
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The transformation process that a carbon backbone undergoes in the atmosphere is complex and dynamic. Understanding all these changes for all the species in detail is impractical; however, choosing different molecules that resemble progressively higher stages of oxidation or aging and studying them can give us an insight into general characteristics and mechanisms. Here we determine secondary organic aerosol (SOA) mass yields of two sequences of molecules reacting with the OH radical at high NO<sub>x</sub>. Each sequence consists of species with similar vapor pressures, but a succession of oxidation states. The first sequence consists of <i>n</i>-pentadecane, <i>n</i>-tridecanal, 2-, 7-tridecanone, and pinonaldehyde. The second sequence consists of <i>n</i>-nonadecane, <i>n</i>-heptadecanal and <i>cis</i>-pinonic acid. Oxidized molecules tend to have lower relative SOA mass yields; however, oxidation state alone was not enough to predict how efficiently a molecule forms SOA. Certain functionalities are able to fragment more easily than others, and even the position of these functionalities on a molecule can have an effect. <i>n</i>-Alkanes tend to have the highest yields, and <i>n</i>-aldehydes the lowest. <i>n</i>-Ketones have slightly higher yields when the ketone moiety is located on the side of the molecule and not in the center. In general, oxidation products remain efficient SOA sources, though fragmentation makes them less effective than comparable alkanes.
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The West African region is characterized by large concentrations of smoke and biomass burning aerosols, which could significantly modify the regional radiative budget and the hydrological cycle. Here, we propose long-term (2001–2006) RegCM simulations of aerosol optical properties over West Africa together with their spectral dependences. Results of simulations are evaluated at local and regional scale by using surface network (AERONET/PHOTON) and remote sensing observations (MODIS, MISR, OMI) especially during the dry season, December-January-February, DJF. New original satellite retrievals are tested and compared to RegCM simulations. Concerning AOD, we obtain values in agreement with AERONET/PHOTON observations at the local scale but some differences clearly appear between simulated AOD and regional MISR, OMI and MODIS view, especially over (1) the central Africa and (2) the gulf of Guinea during DJF. Concerning simulated SSA (for visible wavelengths), our results display (1) comparable values with level 2 AERONET/PHOTON local observations together with (2) non negligible differences with satellite (MODIS Deep blue, OMI and MISR products) observations. In most cases, satellite SSA is found to be higher than those simulated by RegCM and retrieved through AERONET/PHOTON network. In parallel, we also note significant differences on retrieved SSA from each satellite (OMI, MISR, MODIS Deep Blue) remote sensing techniques over this specific region. Finally, our work highlights that the spectral dependence of aerosol optical properties is a useful parameter to adapt so that modeled simulations should be be better evaluated and constrained.
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Past studies have shown that a clear relationship exists between the field of a passive tracer and the Probability Distribution Function (PDF) of tracer concentrations, which can be exploited to identify the position and variability of stratospheric barriers to isentropic mixing. <br><br> In the present study, we focus on the dynamical barrier located in the subtropics. We calculate PDFs of the long-lived tracers nitrous oxide (N<sub>2</sub>O) and methane (CH<sub>4</sub>) from different satellite instruments: the Microwave Limb Sounder (MLS) on board Aura, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board Envisat, the Sub-Millimetre Radiometre (SMR) on board Odin and the Halogen Occultation Experiment (HALOE) on board UARS, overall covering the time period of 1992–2009. <br><br> An analysis of the consistency among the different sets of data and their capability of identifying mixing regions and barrier-to-transport regions in the stratosphere and the subtropical barrier location is a prime aim of the present study. This is done looking at the morphological structure of the one- and two-dimensional PDFs of tracer concentrations measured by the different instruments. The latter differ in their spatial and temporal sampling and resolution, and there are some systematic differences in the determination of the subtropical barrier position that have been highlighted. However, the four satellite instruments offer an overall consistent picture of the subtropical barrier annual cycle. There is a strong seasonality consistently represented, characterized by the wintertime shift of the subtropical edge toward the summer hemisphere. However, the influence of the Quasi Biennial Oscillation (QBO) on isentropic transport and mixing, and by consequence, on the position of the subtropical barrier, is not equally represented in all satellite data using the methodology proposed.
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We report the first observations of formaldehyde (HCHO) flux measured via eddy covariance, as well as HCHO concentrations and gradients, as observed by the Madison Fiber Laser-Induced Fluorescence Instrument during the BEACHON-ROCS 2010 campaign in a rural, Ponderosa Pine forest northwest of Colorado Springs, CO. A median noon upward flux of ~80 μg m<sup>−2</sup> h<sup>−1</sup> (~24 ppt<sub>v</sub> m s<sup>−1</sup>) was observed with a noon range of 37 to 131 μg m<sup>−2</sup> h<sup>−1</sup>. Enclosure experiments were performed to determine the HCHO branch (3.5 μg m<sup>-2</sup> h<sup>−1</sup>) and soil (7.3 μg m<sup>−2</sup> h<sup>−1</sup>) direct emission rates in the canopy. A zero-dimensional canopy box model, used to determine the apportionment of HCHO source and sink contributions to the flux, underpredicted the observed HCHO flux by a factor of 6. Simulated increases in concentrations of species similar to monoterpenes resulted in poor agreement with measurements, while simulated increases in direct HCHO emissions and/or concentrations of species similar to 2-methyl-3-buten-2-ol best improved model/measurement agreement. Given the typical diurnal variability of these BVOC emissions and direct HCHO emissions, this suggests that the source of the missing flux is a process with both a strong temperature and radiation dependence.
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Measurements of submicron aerosol chemical composition were made over the central Arctic Ocean from 5 August to 8 September 2008 as a part of the Arctic Summer Cloud Ocean Study (ASCOS) using an aerosol mass spectrometer (AMS). The median levels of sulphate and organics for the entire study were 0.051 and 0.055 μ g m<sup>−3</sup>, respectively. Positive matrix factorisation was performed on the entire mass spectral time series and this enabled marine biogenic and continental sources of particles to be separated. These factors accounted for 33% and 36% of the sampled ambient aerosol mass, respectively, and they were both predominantly composed of sulphate, with 47% of the sulphate apportioned to marine biogenic sources and 48% to continental sources, by mass. Within the marine biogenic factor, the ratio of methane sulphonate to sulphate was 0.25 ± 0.02, consistent with values reported in the literature. The organic component of the continental factor was more oxidised than that of the marine biogenic factor, suggesting that it had a longer photochemical lifetime than the organics in the marine biogenic factor. The remaining ambient aerosol mass was apportioned to an organic-rich factor that could have arisen from a combination of marine and continental sources. In particular, given that the factor does not correlate with common tracers of continental influence, we cannot rule out that the organic factor arises from a primary marine source.
