Because of increased interest in the marine and atmospheric sciences in elemental carbon (EC), or black carbon (BC) or soot carbon (SC), and because of the difficulties in analyzing or even defining this pervasive component of particulate carbon, it has become quite important to have appropriate reference materials for intercomparison and quality control. The NIST “urban dust” Standard Reference Material® SRM 1649a is useful in this respect, in part because it comprises a considerable array of inorganic and organic species, and because it exhibits a large degree of (14C) isotopic heterogeneity, with biomass carbon source contributions ranging from about 2 % (essentially fossil aliphatic fraction) to about 32 % (polar fraction).A primary purpose of this report is to provide documentation for the new isotopic and chemical particulate carbon data for the most recent (31 Jan. 2001) SRM 1649a Certificate of Analysis. Supporting this is a critical review of underlying international intercomparison data and methodologies, provided by 18 teams of analytical experts from 11 institutions. Key results of the intercomparison are: (1) a new, Certified Value for total carbon (TC) in SRM 1649a; (2) 14C Reference Values for total carbon and a number of organic species, including for the first time 8 individual PAHs; and (3) elemental carbon (EC) Information Values derived from 13 analytical methods applied to this component. Results for elemental carbon, which comprised a special focus of the intercomparison, were quite diverse, reflecting the confounding of methodological-matrix artifacts, and methods that tended to probe more or less refractory regions of this universal, but ill-defined product of incomplete combustion. Availability of both chemical and 14C speciation data for SRM 1649a holds great promise for improved analytical insight through comparative analysis (e.g., fossil/biomass partition in EC compared to PAH), and through application of the principle of isotopic mass balance.

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[1] In the boreal forest, high-intensity crown fires account for an overwhelming proportion of the area burned yearly. Quantifying the amount of black carbon (BC) from boreal crown fires in Canada is essential for assessing the effect on regional climate from natural wildfire aerosol emissions versus that from anthropogenic activities. This is particularly relevant because climate change will likely lead to increased wildfire activity in northern Canada. During 4 -5 July 1998, two controlled fires in Northwest Territories, Canada, were conducted as part of the International Crown Fire Modeling Experiment. We report here the BC and organic carbon (OC) compositions of aerosols produced during the flaming and smoldering stages of burning. Particles were collected on back-to-back quartz-fiber filters by helicopter with a hi-vol sampler and at ground level with a dichotomous sampler to separate the fine ( 2.5 mm diameter) and coarse (2.5 -10 mm diameter) particle fractions. An analysis of the back filter in relation to the front filter from the dichot sampler for both the fine and coarse fractions provided a means to correct for the adsorption of gas-phase organic compounds on filters (positive artifact) and for the loss of particulate carbon from filters by volatilization (negative artifact). BC and OC masses, which combine here to give total carbon (TC), were determined by the thermal-optical method. The BC to TC ratio for the flaming stage was 0.085 ± 0.032 (x ± ksn À1/2 , k = 2, n = 2), based on aerial sampling of the dark plume 300 -500 m above the flame front. BC/TC for the smoldering stage was 0.0087 ± 0.0046 from ground-based sampling. Uncertainties consist of the combined variances in measurement and sampling and in emissions from different fires. These averages and uncertainties serve as important aerosol data input for predictions of climate change on both global and regional scales.

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[1] Chemical, physical, and radiative properties of surface and vertical column aerosols were measured at a rural site in southern New Hampshire from July 2000 to September 2001. The primary objective was to determine how intensive and extensive aerosol properties vary in air masses originating in different upwind regions. The data set also allows for an investigation of some of the relationships between surface and column aerosol properties at the site, and provides an estimate of direct radiative forcing by aerosols during the study period. Extensive properties (e.g., optical depth and chemical concentration) were at maximum values during times of south-southwest (S-SW) transport, while minimum values were seen during north-northeast (N-NE) transport. Certain intensive properties such as fine particle mass scattering efficiency did not vary significantly between times of transport from different source regions. Mean optical depth (wavelength = 500 nm) was 0.24 during S-SW transport, compared to 0.10 during N-NE transport. The study period average scattering efficiency for (NH 4 ) 2 SO 4 was 6.54 ± 0.26 m 2 g À1 (± standard error) and 3.36 ± 0.49 m 2 g À1 for organic carbon, while the absorption efficiency of elemental carbon was 12.85 ± 0.80 m 2 g À1 . Top of the atmosphere aerosol direct radiative forcing was À0.35 ± 0.83 Wm À2 (±1 standard deviation) in winter 2000-2001 and À9.06 ± 3.77 Wm À2 in summer 2001, differences that can be primarily attributed to seasonal changes in surface reflectance (high in winter, low in summer) and the relatively low values of single scatter albedo observed in winter. The annual average direct radiative forcing was À5.14 ± 4.32 Wm À2 . We generally observed a moderate correlation between surface and column aerosol light extinction, suggesting that vertical column aerosol radiative properties measured by surface-based radiometers should be supplemented by boundary layer measurements of aerosol chemical, physical, and radiative properties to help understand the mechanisms contributing to global aerosol variability. Citation: Slater, J. F., and J. E. Dibb (2004), Relationships between surface and column aerosol radiative properties and air mass transport at a rural New England site,