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ABSTRACT We use the fractional aerosol optical depth (AOD) values derived from Multiangle Imaging Spectroradiometer (MISR) aerosol component measurements, along with aerosol transport model constraints, to estimate ground-level concentrations of [...]

This work aims at quantifying the relative contribution of secondary organic aerosol (SOA) precursors emitted by wildfires to organic aerosol (OA) formation, during summer 2007 over the Euro-Mediterranean region, where intense wild-fires occurred. A new SOA formation mechanism, H²Oaro, including recently identified aromatic volatile organic compounds (VOCs) emitted from wildfires is developed based on smog chamber experiment measurements, under low and high-NOx regimes. The aromatic VOCs included in the mechanism are toluene, xylene, benzene, phenol, cresol, catechol, furan, naphthalene, methylnaphthalene, syringol, guaiacol and structurally assigned and unassigned compounds with at least 6 carbon atoms per molecule (USC>6). This mechanism H²Oaro is an extension of the H²O (Hydrophilic/Hydrophobic organic) aerosol mechanism: the oxidation of the precursor forms surrogate species with specific thermodynamic properties (volatility, oxidation degree, affinity to water). The SOA concentrations over the Euro-Mediterranean region in summer 2007 are simulated using the chemistry transport model (CTM) Polair3D of the air-quality plateform Polyphemus, where the mechanism H²Oaro was implemented. To estimate the relative contribution of the aromatic VOCs, intermediate, semi and low volatile organic compounds (I/S/L-VOCs) to wildfires OA concentrations, different estimations of the gaseous I/S/L-VOC emissions (from primary organic aerosol (POA) using a factor of 1.5 or from non-methanic organic gas (NMOG) using a factor of 0.36) and their ageing (one-step oxidation vs multi-generational oxidation), are also tested in the CTM. Most of the particle organic aerosol (OA) concentrations are formed from I/S/L-VOCs. In average during the summer 2007 and over the Euro-Mediterranean domain, they are about 10 times higher than the OA concentrations formed from VOCs. However, locally, the OA concentrations formed from VOCs can represent up to 30 % of the OA concentrations from biomass burning. Amongst the VOCs, the main contributors to SOA formation are phenol, benzene and catechol (47 %), USC>6 compounds (23 %), and toluene and xylene (12 %). Sensitivity studies of the influence of the VOCs and the I/S/L-VOCs emissions and chemical ageing mechanisms on PM2.5 concentrations show that surface PM2.5 concentrations are more sensitive to the parameterization used for gaseous I/S/L-VOCs emissions than for ageing. Estimating the gaseous I/S/L-VOCs emissions from POA or from NMOG has a high impact on local surface PM2.5 concentrations (reaching -30 % in Balkans, -8 to -16 % in the fire plume and +8 to +16 % in Greece). Considering the VOC emissions results in a moderate increase of PM2.5 concentrations mainly in Balkans (up to 24 %) and in the fire plume (+10 %). [ABSTRACT FROM AUTHOR]

ough comparisons to observations from surface networks and satellite remote sensing, we evaluate the abilities of two CTMs, Polyphemus/Polair3D and CHIMERE, to simulate the impact of fires on the regional particulate matter (PM) concentrations and optical properties. During the two main fire events, fire emissions may contribute up to 90 % of surface PM2.5 concentrations, with a significant regional impact associated with long-range transport. Good general performances of the models and a clear improvement of PM2.5 and aerosol optical depth (AOD) are shown when fires are taken into account in the models with high correlation coefficients. Two sources of uncertainties are specifically analysed in terms of surface PM concentrations and AOD using sensitivity simulations: secondary organic aerosol (SOA) formation from intermediate and semi-volatile organic compounds (I/S-VOCs) and emissions' injection heights. The analysis highlights that surface PM2.5 concentrations are highly sensitive to injection heights (with a sensitivity that can be as high as 50 % compared to the sensitivity for I/S-VOCs emissions which is lower than 30 %). However, AOD which is vertically integrated is less sensitive to the injection heights (mostly below 20 %), but highly sensitive to I/S-VOCs emissions (with sensitivity that can be as high as 40 %). The maximum dispersion, which quantifies uncertainties related to fire emissions modeling, is up to 75 % for PM2.5 in Balkan and Greece, and varies between 36 and 45 % for AOD above fire regions. The simulated number of daily exceedance of World Health Organization (WHO) recommendations for PM2.5 over the considered region reaches 30 days in regions affected by fires and ∼ 10 days in fire plumes which is slightly underestimated compared to available observations. The maximum dispersion (σ) on this indicator is also large (with σ reaching 15 days), showing the need for better understanding of the transport and evolution of fire plumes in addition to fire emissions. [ABSTRACT FROM AUTHOR]

In the framework of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) project, the tropospheric chemical composition in the megacities along the Guinean Gulf is studied using the Weather and Research Forecast and CHIMERE regional models. Simulations are performed for the May-July 2014 period, without and with biomass burning emissions. Model results are compared to satellite data and surface measurements. Using numerical tracer release experiments, it is shown that the fire emissions in Central Africa are impacting the surface aerosol and gaseous species concentrations in the Guinean Gulf cities, such as Lagos (Nigeria) and Abidjan (Ivory Coast). Depending on the altitude of injection of these emissions, the pollutants follow different pathways: directly along the coast or over land towards the Sahel before to be vertically mixed in the convective boundary layer and transported to the south-west and over the cities. In July 2014, the maximum increase in surface concentrations is ~ 150 μg m-3 for CO, ~ 10 to 20 μg m-3 for O3 and ~ 5 μg m-3 for PM10. The analysis of the PM10 chemical composition shows that this increase is mainly related to an increase of Particulate Primary Matter and Particulate Organic Matter. [ABSTRACT FROM AUTHOR]

We use observations from two aircraft during the ICARTT campaign over the eastern United States and North Atlantic during summer 2004, interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem) to test current understanding of regional sources, chemical evolution, and export of NOx. The boundary layer NOx data provide top-down verification of a 50% decrease in power plant and industry NOx emissions over the eastern United States between 1999 and 2004. Observed NOx concentrations at 8–12 km altitude were 0.55 ± 0.36 ppbv, much larger than in previous U.S. aircraft campaigns (ELCHEM, SUCCESS, SONEX) though consistent with data from the NOXAR program aboard commercial aircraft. We show that regional lightning is the dominant source of this upper tropospheric NOx and increases upper tropospheric ozone by 10 ppbv. Simulating ICARTT upper tropospheric NOx observations with GEOS-Chem requires a factor of 4 increase in modeled NOx yield per flash (to 500 mol/flash). Observed OH concentrations were a factor of 2 lower than can be explained from current photochemical models, for reasons that are unclear. A NOy-CO correlation analysis of the fraction f of North American NOx emissions vented to the free troposphere as NOy (sum of NOx and its oxidation products) shows observed f = 16 ± 10% and modeled f = 14 ± 9%, consistent with previous studies. Export to the lower free troposphere is mostly HNO3 but at higher altitudes is mostly PAN. The model successfully simulates NOy export efficiency and speciation, supporting previous model estimates of a large U.S. anthropogenic contribution to global tropospheric ozone through PAN export., Earth and Planetary Sciences, Engineering and Applied Sciences

The summer of 2004 was one of the largest fire seasons on record for Alaska and western Canada. We construct a daily bottom-up fire emission inventory for that season, including consideration of peat burning and high-altitude (buoyant) injection, and evaluate it in a global chemical transport model (the GEOS-Chem CTM) simulation of CO through comparison with MOPITT satellite and ICARTT aircraft observations. The inventory is constructed by combining daily area burned reports and MODIS fire hot spots with estimates of fuel consumption and emission factors based on ecosystem type. We estimate the contribution from peat burning using drainage and peat distribution maps for Alaska and Canada; 17% of the reported 5.1 × 106 ha burned were located in peatlands in 2004. Our total estimate of North American fire emissions during the summer of 2004 is 30 Tg CO, including 11 Tg from peat. Including peat burning in the GEOS-Chem simulation improves agreement with MOPITT observations. The long-range transport of fire plumes observed by MOPITT suggests that the largest fires injected a significant fraction of their emissions in the upper troposphere., Earth and Planetary Sciences, Engineering and Applied Sciences

Collocated measurements of tropospheric ozone (O3) and carbon monoxide (CO) from the Tropospheric Emission Spectrometer (TES) aboard the EOS Aura satellite provide information on O3-CO correlations to test our understanding of global anthropogenic influence on O3. We examine the global distribution of TES O3-CO correlations in the middle troposphere (618 hPa) for July 2005 and compare to correlations generated with the GEOS-Chem chemical transport model and with ICARTT aircraft observations over the eastern United States (July 2004). The TES data show significant O3-CO correlations downwind of polluted continents, with dO3/dCO enhancement ratios in the range 0.4–1.0 mol mol−1 and consistent with ICARTT data. The GEOS-Chem model reproduces the O3-CO enhancement ratios observed in continental outflow, but model correlations are stronger and more extensive. We show that the discrepancy can be explained by spectral measurement errors in the TES data. These errors will decrease in future data releases, which should enable TES to provide better information on O3-CO correlations., Earth and Planetary Sciences, Engineering and Applied Sciences

Aircraft measurements of water-soluble organic carbon (WSOC) aerosol over NE North America during summer 2004 (ITCT-2K4) are simulated with a global chemical transport model (GEOS-Chem) to test our understanding of the sources of organic carbon (OC) aerosol in the free troposphere (FT). Elevated concentrations were observed in plumes from boreal fires in Alaska and Canada. WSOC aerosol concentrations outside of these plumes average 0.9 ± 0.9 μg C m−3 in the FT (2–6 km). The corresponding model value is 0.7 ± 0.6 μg C m−3, including 42% from biomass burning, 36% from biogenic secondary organic aerosol (SOA), and 22% from anthropogenic emissions. Previous OC aerosol observations over the NW Pacific in spring 2001 (ACE-Asia) averaged 3.3 ± 2.8 μg C m−3 in the FT, compared to a model value of 0.3 ± 0.3 μg C m−3. WSOC aerosol concentrations in the boundary layer (BL) during ITCT-2K4 are consistent with OC aerosol observed at the IMPROVE surface network. The model is low in the boundary layer by 30%, which we attribute to secondary formation at a rate comparable to primary anthropogenic emission. Observed WSOC aerosol concentrations decrease by a factor of 2 from the BL to the FT, as compared to a factor of 10 decrease for sulfate, indicating that most of the WSOC aerosol in the FT originates in situ. Despite reproducing mean observed WSOC concentrations in the FT to within 25%, the model cannot account for the variance in the observations (R = 0.21). Covariance analysis of FT WSOC aerosol with other measured chemical variables suggests an aqueous-phase mechanism for SOA generation involving biogenic precursors., Earth and Planetary Sciences, Engineering and Applied Sciences

We use the fractional aerosol optical depth (AOD) values derived from Multiangle Imaging Spectroradiometer (MISR) aerosol component measurements, along with aerosol transport model constraints, to estimate ground-level concentrations of fine particulate matter (PM2.5) mass and its major constituents in the continental United States. Regression models using fractional AODs predict PM2.5 mass and sulfate (SO4) concentrations in both the eastern and western United States, and nitrate (NO3) concentrations in the western United States reasonably well, compared with the available ground-level U.S. Environment Protection Agency (EPA) measurements. These models show substantially improved predictive power when compared with similar models using total-column AOD as a single predictor, especially in the western United States. The relative contributions of the MISR aerosol components in these regression models are used to estimate size distributions of EPA PM2.5 species. This method captures the overall shapes of the size distributions of PM2.5 mass and SO4 particles in the east and west, and NO3particles in the west. However, the estimated PM2.5 and SO4 mode diameters are smaller than those previously reported by monitoring studies conducted at ground level. This is likely due to the satellite sampling bias caused by the inability to retrieve aerosols through cloud cover, and the impact of particle hygroscopicity on measured particle size distributions at ground level. [ABSTRACT FROM AUTHOR]