Relative contributions of fossil and contemporary carbon sources to PM 2.5 aerosols at nine Interagency Monitoring for Protection of Visual Environments (IMPROVE) network sites

Abstract

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.

Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol

“…tracers for anthropogenic emissions, such as acetylene, but that budgets of organic aerosol production are poorly represented by current understanding based on yields for individual VOCs measured in smog chambers (de Gouw et al, 2005Quinn et al, 2006;Sullivan et al, 2006). The contribution of biogenic VOCs to organic aerosol production remains uncertain, although recent analysis of radiocarbon in organic aerosol has suggested that a large fraction comes from biogenic sources, even in areas with large anthropogenic VOC emissions (Bench et al, 2007;Schichtel et al, 2008;Weber et al, 2007). Because the yield of SOA from the NO 3 +isoprene reaction (Ng et al, 2008) is some 5-8× larger than from photochemical isoprene oxidation (Kroll et al, 2005), its nighttime oxidation is a mechanism by which biogenic VOCs can more efficiently form SOA in polluted air masses.…”

“…The reaction is known to produce unsaturated organic nitrates with a yield of approximately 80% as first-generation products (Barnes et al, 1990). These nitrates consist of unsaturated hydroxy-, hydroperoxy-and carbonyl nitrates (Kwok et al, 1996;Skov et al, 1992Skov et al, , 1994Tuazon et al, 1999;Berndt and Boge, 1997), with a recent study also showing evidence for formation of dinitrates and oligomers (Ng et al, 2008).…”

“…Because N 2 O 5 hydrolysis is unimportant in comparison to the reaction of NO 3 with isoprene for large isoprene levels, the rate of NO 2 consumption, which is explicitly incorporated into the expression below, is equal to the rate of reaction (R2) (rather than being twice this rate, as would be the case if N 2 O 5 hydrolysis were dominant, . However, the NO 3 +isoprene reaction regenerates a fraction of NO 2 with a yield, NO 2 , of 20%; the remainder of the nitrogen goes to organic nitrates (Barnes et al, 1990).…”

“…Figure 8 shows a time series of different components of reactive nitrogen, including HNO 3 , PAN (shown here as the sum of speciated PANs measured with a chemical ionization mass spectrometer, see Table 1), NO x , N 2 O 5 and isoprene nitrates from isoprene+NO 3 . The isoprene nitrate trace is the product of the estimate for total isoprene reacted after dark and the laboratory organic nitrate yield of 80% (Barnes et al, 1990) and includes only production of these nitrates. Potential loss of these nitrates due to further reaction with NO 3 or with O 3 could reduce the amount of reactive nitrogen stored in this form if it were to result in release of NO 2 from the nitrate, or increase the amount of organic nitrate if, for example, further reaction with NO 3 were to result in the production of di-nitrates.…”

Biogenic carbon and anthropogenic pollutants combine to form a cooling haze over the southeastern United States

“…(e.g., Georgia), whereas the maxima sulfate and nitrate occur much further north (e.g., Ohio) based on rural IMPROVE network and urban Speciation Trends Network (STN) surface observations (19). The organic aerosol in the SE U.S. is mostly secondary, with radiocarbon ( 14 C) measurements indicating 70% or more of the carbon is modern (rather than fossil) in rural locations and therefore likely of biogenic origin (35,36).…”