[1] Fine particulate matter collected at two urban, four near-urban, and six remote sites throughout the United States were analyzed for total carbon (TC) and radiocarbon ( 14 C). Samples were collected at most sites for both a summer and winter season. The radiocarbon was used to partition the TC into fossil and contemporary fractions. On average, contemporary carbon composed about half of the carbon at the urban, $70-97% at near-urban, and 82-100% at remote sites. At Phoenix, Arizona, and Seattle, Washington, one monitor was located within the urban center and one outside to assess the urban excess over background concentrations. During the summer the urban and rural sites had similar contemporary carbon concentrations. However, during the winter the urban sites had more than twice the contemporary carbon measured at the neighboring sites, indicating anthropogenic contributions to the contemporary carbon. The urban fossil carbon was 4-20 times larger than the neighboring rural sites for both seasons. Organic (OC) and elemental carbon (EC) from TOR analysis were available. These and the radiocarbon data were used to estimate characteristic fossil and contemporary EC/TC ratios for the winter and summer seasons. These ratios were applied to carbon data from the Interagency Monitoring of Protected Visual Environments network to estimate the fraction of contemporary carbon at mostly rural sites throughout the United States. In addition, the ratios were used to develop a semiquantitative, lower bound estimate of secondary organic carbon (SOC) contribution to fossil and contemporary carbon. SOC accounted for more than one-third of the fossil and contemporary carbon.

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Particulate matter aerosols contribute to haze diminishing vistas and scenery at national parks and wilderness areas within the United States. To increase understanding of the sources of carbonaceous aerosols at these settings, the total carbon loading and 14C/C ratio of PM 2.5 aerosols at nine Interagency Monitoring for Protection of Visual Environments (IMPROVE) network sites were measured. Aerosols were collected weekly in the summer and winter at one rural site, two urban sites, five sites located in national parks and one site located in a wildlife preserve. The carbon measurements together with the absence of 14C in fossil carbon materials and known 14C/C levels in contemporary carbon materials were used to derive contemporary and fossil carbon contents of the particulate matter. Contemporary and fossil carbon aerosol loadings varied across the sites and suggest different percentages of carbon source inputs. The urban sites had the highest fossil carbon loadings that comprised around 50% of the total carbon aerosol loading. The wildlife preserve and national park sites together with the rural site had much lower fossil carbon loading components. At these sites, variations in the total carbon aerosol loading were dominated by nonfossil carbon sources. This suggests that reduction of anthropogenic sources of fossil carbon aerosols may result only in little decrease in carbonaceous aerosol loading at many national parks and rural areas. Examination of the major sources of uncertainty that might cause contemporary carbon contents to be artificially high indicates that potential errors and biases in the methodology do not change the fundamental conclusions of this study.

Rate constants for the reaction of four different butyl radicals with molecular oxygen have been measured at room temperature. The radicals were generated by flash photolysis and their time decay was followed with a photoionization mass spectrometer. The radical concentrations were kept low to avoid complications from radical–radical reactions. Radical lifetimes were long, up to 50 msec, thus assuring that thermalized radicals were being studied. The rate constants, in units of 10−11 cm3 molecule−1 sec−1, are: n-butyl (0.75±0.14); s-butyl (1.66±0.22); t-butyl (2.34±0.39); 3-hydroxy s-butyl (2.8±1.8). No pressure dependence of the rate constants was observed over the range 1 to 4 Torr. In the absence of O2, the butyl radicals decay mainly by loss on the quartz surface of the reaction cell, with sticking coefficients in the range of 10−2 to 10−3. The Adiabatic Channel Model can predict the approximate absolute values of these rate constants using reasonable molecular parameters, but it fails to reproduce the observed trend of rate constants with radical ionization potential.