[1] Forty-four small-scale experimental fires were conducted in a combustion chamber to examine the relationship between biomass consumption, smoke production, convective energy release, and middle infrared (MIR) measurements of fire radiative energy (FRE). Fuel bed weights, trace gas and aerosol particle concentrations, stack flow rate and temperature, and concurrent thermal images were collected during laboratory-controlled burns of vegetative fuels. Using two different MIR thermal imaging systems, measurements of FRE taken at polar angles of ff48°and ff60°were found not to be significantly different from each other (p < 0.05), but were significantly different from those obtained at ff76°. A simple linear regression revealed that less than 12% of the variation in biomass consumption remained unexplained by the measured FRE regardless of MIR sensor characteristics, fuel type, or viewing angle. Measurements of FRE detected per unit of dry organic material consumed ranged from 1.29 to 4.18 MJ/kg, corresponding to an average of 12 ± 3% of the higher heating value of the biomass. Whole-fire emission factors agreed with previously reported values, and emission ratios relating total mass production to FRE were determined for CO 2 , CO, NO, NO 2 , and particulate matter less than 2.5 mm in aerodynamic diameter. A heat balance performed on the system showed that the release of convective energy could be predicted from a measurement of FRE (r 2 ! 0.84), and together these two modes of heat transfer accounted for 61 ± 13% of the total, potential heat of combustion available in the preburn solid fuel.Citation: Freeborn, P. H., M. J. Wooster, W. M. Hao, C. A. Ryan, B. L. Nordgren, S. P. Baker, and C. Ichoku (2008), Relationships between energy release, fuel mass loss, and trace gas and aerosol emissions during laboratory biomass fires,
[1] To reduce uncertainties in the quantitative assessment of aerosol effects on regional climate and environmental changes, extensive measurements of aerosol optical properties were made with handheld Sun photometers in the Chinese Sun Hazemeter Network (CSHNET) starting in August 2004. Regional characteristics of the aerosol optical depth (AOD) at 500 nm and Å ngström exponent (a) computed using 405, 500, and 650 nm were analyzed for the period of August 2004 to September 2005. The smallest mean AOD ($0.15) was found in the Tibetan Plateau where a showed the largest range in value (0.06-0.9). The remote northeast corner of China was the next cleanest region with AODs ranging from 0.19 to 0.21 and with the largest a (1.16-1.79), indicating the presence of fine aerosol particles. The forested sites exhibited moderate values of AOD (0.19-0.51) and a (0.97-1.47). A surprising finding was that the AOD measured at a few desert sites in northern China were relatively low, ranging from 0.24 to 0.36, and that a ranged from 0.42 to 0.99, presumably because of several dustblowing episodes during the observation period. The AOD observed over agricultural areas ranges from 0.38 to 0.90; a ranges from 0.55 to 1.11. These values do not differ much from those observed at the inland urban and suburban sites where AOD ranges from 0.50 to 0.69 and a ranges from 0.90 to 1.48. Given the geographic heterogeneity and the rapid increase in urbanization in China, much longer and more extensive observations are required.
[1] The Moderate Resolution Imaging Spectroradiometer (MODIS) currently provides the most extensive aerosol retrievals on a global basis, but validation is limited to a small number of ground stations. This study presents a comprehensive evaluation of Collection 4 and 5 MODIS aerosol products using ground measurements from the Chinese Sun Hazemeter Network (CSHNET). The Collection 5 product uses a new method to determine the surface reflectance and a new aerosol model to retrieve aerosol optical thickness (AOT). Substantial improvement was found in the Collection 5 AOT product relative to the Collection 4 AOT product. Overall, the correlation coefficient of regression with ground measurements increased from 0.66 to 0.84 for all data points. The offset is reduced from 0.179 to 0.047, and the slope is improved from 0.74 to 0.98. At individual sites, the improvement varies with surface and atmospheric conditions. In general, both versions of the MODIS aerosol product tend to overestimate AOT over deserts/ semideserts, tend to underestimate AOT over forests, and are more accurate over agricultural and suburban sites. The poorest retrievals occur over urban areas. The land cover dependence of aerosol retrievals was traced to the estimation of surface reflectance. The selection of the aerosol type is another major factor contributing to the discrepancies. Errors caused by both factors are subject to considerable variations with season and location due to land cover changes and varying fractions of coarse and fine mode aerosols, as well as the changing amount of scattering and absorbing aerosols.
Biomass burning emission inventories serve as critical input for atmospheric chemical transport models that are used to understand the role of biomass fires in the chemical composition of the atmosphere, air quality, and the climate system. Significant progress has been achieved in the development of regional and global biomass burning emission inventories over the past decade using satellite remote sensing technology for fire detection and burned area mapping. However, agreement among biomass burning emission inventories is frequently poor. Furthermore, the uncertainties of the emission estimates are typically not well characterized, particularly at the spatio-temporal scales pertinent to regional air quality modeling. We present the Wildland Fire Emission Inventory (WFEI), a high resolution model for non-agricultural open biomass burning (hereafter referred to as wildland fires, WF) in the contiguous United States (CONUS). The model combines observations from the MODerate Resolution Imaging Spectroradiometer (MODIS) sensors on the Terra and Aqua satellites, meteorological analyses, fuel loading maps, an emission factor database, and fuel condition and fuel consumption models to estimate emissions from WF. <br></br> WFEI was used to estimate emissions of CO (ECO) and PM<sub>2.5</sub> (EPM<sub>2.5</sub>) for the western United States from 2003–2008. The uncertainties in the inventory estimates of ECO and EPM<sub>2.5</sub> (<i>u</i><sub>ECO</sub> and <i>u</i><sub>EPM<sub>2.5</sub></sub>, respectively) have been explored across spatial and temporal scales relevant to regional and global modeling applications. In order to evaluate the uncertainty in our emission estimates across multiple scales we used a figure of merit, the half mass uncertainty, <i>ũ</i><sub>EX</sub> (where X = CO or PM<sub>2.5</sub>), defined such that for a given aggregation level 50% of total emissions occurred from elements with <i>u</i><sub>EX</sub> <i>ũ</i><sub>EX</sub>. The sensitivity of the WFEI estimates of ECO and EPM<sub>2.5</sub> to uncertainties in mapped fuel loading, fuel consumption, burned area and emission factors have also been examined. <br></br> The estimated annual, domain wide ECO ranged from 436 Gg yr<sup>−1</sup> in 2004 to 3107 Gg yr<sup>−1</sup> in 2007. The extremes in estimated annual, domain wide EPM<sub>2.5</sub> were 65 Gg yr<sup>−1</sup> in 2004 and 454 Gg yr<sup>−1</sup> in 2007. Annual WF emissions were a significant share of total emissions from non-WF sources (agriculture, dust, non-WF fire, fuel combustion, industrial processes, transportation, solvent, and miscellaneous) in the western United States as estimated in a national emission inventory. In the peak fire year of 2007, WF emissions were ~20% of total (WF + non-WF) CO emissions and...
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