Contribution Of Secondary Organic Aerosol Research Articles

Characteristics of Carbonaceous Aerosol at Taichung Harbor, Taiwan during Summer and Autumn Period of 2005

In recent years, suspended particle pollution has become a serious problem in Taiwan. The carbonaceous materials EC and OC are play important roles in various atmospheric processes. The primary OC/EC ratio approach is applied to assess the contribution of secondary organic aerosol (SOA) to the PM(2.5) and PM(10) mass at the Taichung harbor sampling site. The results indicated that the average EC and OC concentration were 1.06 and 6.50 microg m(-3), respectively, in fine particulate. And the average EC and OC concentration were 4.04 and 40.32 microg m(-3), respectively, in coarse particulate at Taichung Harbor sampling site. In addition, and the average EC/OC rations was 8.72 in fine particle, respectively, at Taichung Harbor, Taiwan during summer and autumn period of 2005. The fine particle exhibited high particulate concentrations in October, and lower concentration particulate occurred in August. And in this study OC and EC concentrations in this study are compared with those in other cities. The results of EC and OC concentration in this study are also compare with those other cities.

Reply to comment by R. L. Tanner and D. J. Eatough on “Aerosol organic carbon to black carbon ratios: Analysis of published data and implication for climate forcing”

[1] We appreciate the opportunity to reply to comments and concerns raised by Tanner and Eatough [2007, hereinafter referred to as TE] on our paper [Novakov et al., 2005]. In the following we address the “major flaws” that they assert. [2] 1. TE state that our work was motivated by a need to reduce OC/EC ratios to match predominantly urban-based emissions inventories. [3] Our objective was to analyze a large number of measured OC/BC values, obtained at different locations and published in a variety of contexts. Our main finding is that ambient OC/BC ratios, taken at face value as published, decrease with BC mass concentrations for all locations. [4] 2. TE claim that we do not consider negative artifact. [5] We concluded that the positive sampling artifact may, at least qualitatively, explain the OC/BC versus BC trend. We provide proof of this explanation using data from SAFARI 2000, which unequivocally show that the OC/BC dependence on BC concentration observed with uncorrected data is removed with positive artifact-corrected values. Regarding the negative artifact, we stated that the data considered are insufficient to evaluate the effect of a negative sampling artifact on OC/BC ratios. [6] 3. TE comment that we used predominantly urban emission data for global estimates of the artifact-induced errors in reported OC/EC ratios. This, they think, implicitly assumes that the correction for gaseous organics adsorbed on the filter is the same for urban as for rural. [7] This inference is incorrect. After saturation of the filter material with organic gases is achieved, the positive artifact diminishes with continued sampling of carbonaceous particles. Thus the magnitude of the positive artifact may tend to be smallest in (urban) areas where carbon particle concentrations are highest, and is certainly not “clearly dependent of the relative amounts…of gaseous organics vis à vis particulate carbon” as TE assert. [8] 4. TE state that we have made the extraordinary assumption that OC/EC ratios should remain constant during aerosol transport from urban or source-rich regions to rural and background locations. [9] We make no assumption that OC/BC ratios are constant during aerosol transport from urban to rural areas, nor do we ignore the frequently large contribution of secondary organic aerosols. In the GISS model we account for most secondary OC, which is derived from natural biogenic emissions by assuming it to be proportional to terpene emissions. We report ratios for a variety of locations, both urban and rural, but we do not report data as a function of time after emission. Thus inferences regarding transport or changes of the OC/BC ratio as a function of time since emission cannot be made. [10] We have shown that OC/BC ratios that can be “substitutes” for the artifact-corrected values. We compared these values with OC/BC ratios calculated from published OC and BC emission inventories. This is important because compiled OC and BC emission inventories and, therefore, OC/BC ratios may be significantly uncertain especially for some global regions.

Modeling Secondary Organic Aerosol Formation via Multiphase Partitioning with Molecular Data

A new model for atmospheric secondary organic aerosol (SOA) is presented for biogenic compounds. It is based to the extent possible on experimental molecular SOA data, and it is compatible with any existing gas-phase chemical kinetic mechanism. Six SOA precursors or groups of precursors are used to represent biogenic monoterpenes and sesquiterpenes. SOA formation is modeled using five SOA surrogates to represent classes of compounds with different partitioning properties, e.g., hydrophobicity, aqueous solubility, acid dissociation, and saturation vapor pressure. Model simulations are evaluated against smog chamber data for SOA yields and some adjustments are made to uncertain stoichiometric coefficients and saturation vapor pressure parameters to improve model performance. The model is applied undertypical atmospheric conditions to exemplify the effect of relative humidity on SOA formation and the relative contributions of hydrophilic and hydrophobic SOA.

Interpretation of particulate elemental and organic carbon concentrations at rural, urban and kerbside sites

Concentration measurements are reported for particulate organic and elemental carbon, measured using an R&P 5400 ambient particulate carbon monitor at four sites in the United Kingdom: one roadside (London, Marylebone Road), two urban (London, North Kensington, and Belfast, Centre) and one rural (Harwell). The measurements were collected on a continuous three hourly average basis between January 2002 and mid-2004. The concentrations show no obvious seasonal cycle, except for an increase in OC/EC ratio at London, North Kensington, during the summer months consistent with a possibly greater relative contribution of secondary organic aerosol. Perhaps surprisingly this is not, however, seen at the rural Harwell site. At Belfast, both organic and elemental carbon show elevated winter concentrations, consistent with important local primary sources. Only at the roadside Marylebone Road site is there a high correlation ( r 2 = 0.59 ) between organic carbon and elemental carbon concentrations which persists when simultaneously measured urban background concentrations from the nearby North Kensington site are subtracted; the OC/EC ratio in the traffic-related concentration increment is 0.88, well below the ratios typical of the urban background. Directional analysis of the data from the Marylebone Road street canyon shows that whilst elemental carbon concentrations are determined primarily by on-road traffic emissions, both organic carbon and PM 10 derive primarily from inputs from outside the street canyon. It therefore appears that at all times of year non-traffic sources of particulate organic carbon, be they primary or secondary, are dominant over traffic emissions in the urban background. Organic compounds account for about 22% of aerosol mass, irrespective of site.

Stable estimate of primary OC/EC ratios in the EC tracer method

In fine particulate matter studies, the primary OC/EC ratio plays an important role in estimating the secondary organic aerosol contribution to PM2.5 concentrations using the EC tracer method. In this study, numerical experiments are carried out to test and compare various statistical techniques in the estimation of primary OC/EC ratios. The influence of random measurement errors in both primary OC and EC measurements on the estimation of the expected primary OC/EC ratios is examined. It is found that random measurement errors in EC generally create an underestimation of the slope and an overestimation of the intercept of the ordinary least-squares regression line. The Deming regression analysis performs much better than the ordinary regression, but it tends to overcorrect the problem by slightly overestimating the slope and underestimating the intercept. Averaging the ratios directly is usually undesirable because the average is strongly influenced by unrealistically high values of OC/EC ratios resulting from random measurement errors at low EC concentrations. The errors generally result in a skewed distribution of the OC/EC ratios even if the parent distributions of OC and EC are close to normal. When measured OC contains a significant amount of non-combustion OC Deming regression is a much better tool and should be used to estimate both the primary OC/EC ratio and the non-combustion OC. However, if the non-combustion OC is negligibly small the best and most robust estimator of the OC/EC ratio turns out to be the simple ratio of the OC and EC averages. It not only reduces random errors by averaging individual variables separately but also acts as a weighted average of ratios to minimize the influence of unrealistically high OC/EC ratios created by measurement errors at low EC concentrations. The median of OC/EC ratios ranks a close second, and the geometric mean of ratios ranks third. This is because their estimations are insensitive to questionable extreme values. A real world example is given using the ambient data collected from an Atlanta STN site during the winter of 2001–2002.

Sources of Atmospheric Carbonaceous Particulate Matter in Pittsburgh, Pennsylvania

The organic carbon (OC)/elemental carbon (EC) tracer method is applied to the Pittsburgh, PA, area to estimate the contribution of secondary organic aerosol (SOA) to the monthly average concentration of organic particu-late matter (PM) during 1995. An emissions inventory is constructed for the primary emissions of OC and EC in the area of interest. The ratio of primary emissions of OC to those of EC ranges between 2.4 in the winter months and 1.0 in the summer months. A mass balance model and ambient measurements were used to assess the accuracy of the emissions inventory. It is estimated to be accurate to within 50%. The results from this analysis show a strong monthly dependence on SOA contribution to the total organic PM concentration, varying from near zero during winter months to 50% or more of the total OC concentration in the summer.

Organic and black carbon in PM 2.5 and PM 10: 1 year of data from an urban site in Helsinki, Finland

The results from a 1-year measurement period concerning the diurnal PM 2.5 and PM 10 organic carbon (OC) and black carbon (BC) concentrations are presented for a traffic-influenced site in the Helsinki metropolitan area. The measurements were based on aerosol sampling using a virtual impactor and the subsequent thermal–optical analysis to distinguish between OC and BC. Backup filters were used to estimate and correct for the positive sampling artefact. Daily-average concentrations in PM 2.5 varied between 1.0 and 8.5 μg C m −3 for OC, and between 0.3 and 5.7 μg C m −3 for BC. Annual-average concentrations of OC and BC were 3.0 and 1.2 μg C m −3, respectively, in PM 2.5, and 4.2 and 1.3 μg C m −3 in PM 10. On an annual level, particulate organic matter (POM=1.6×OC) accounted for 50±14% and 36±8% (average±1 σ) of the total PM 2.5 and PM 10, respectively, whereas BC stayed lower at 14±8% and 7±4%. Typically more than 90% of BC resided in the PM 2.5 size fraction. The contribution of coarse particles (>2.5 μm) to the overall OC varied between the 0% and 67% (median 27%). The effect of meteorological conditions on the variability of OC and BC concentrations was examined, and the contribution of secondary organic aerosol to the total fine organic aerosol was estimated.

Regional and local influences on the nature of airborne particulate organic matter at four sites in New Jersey during summer 1981

As part of the New Jersey Project on Airborne Toxic Elements and Organic Substances, daily variations in the concentrations of three fractions of extractable organic matter (EOM) in samples of inhalable particulate matter were investigated at three urban sites and one rural site in New Jersey. During three episodes of elevated concentrations of airborne particulate matter, SO 2− 4 and O 3, the concentrations of EOM increased by 50–100 % at all sites. Lower ratios of elemental to organic carbon and higher proportions of the acetone-soluble organic fraction of EOM, which contains more polar, oxidized hydrocarbons, were also observed at all sites during these episodes, indicating the formation of secondary particulate organic matter. Local emissions plus Northeast regional background organic aerosol were estimated to contribute about 10μg m −3 to the total EOM at the urban sites during these periods. The contribution of secondary organic aerosol to EOM was estimated to be 2 μg m −3 and this seemed to be distributed over the New Jersey region. At the Newark site, an additional 4μg m −3 of secondary aerosol appeared to be locally generated.