Atmospheric Environment (v.42, #27)
Editorial board (i).
Review of ammonia emission factors for United States animal agriculture by W.B. Faulkner; B.W. Shaw (6567-6574).
Ammonia emissions from agricultural industries are a significant source of atmospheric reactive nitrogen, which can lead to negative environmental consequences such as ecosystem change and formation of fine particulate. While a number of emission factors (EFs) have been proposed for developing ammonia emissions inventories for the US, most are based on European research with little discussion of their applicability to US production systems. Recently developed ammonia EFs from literature for animal feeding operations (AFOs), including production facilities for beef and dairy cattle, swine, and poultry, are presented. Tentative EFs for US animal agriculture are suggested until further research can be conducted. Currently, there is a dearth of EFs developed specifically for agricultural production practices in the US.
Keywords: Ammonia; Emission factor; Cattle; Dairy; Swine; Poultry;
Traffic-related PM2.5 aerosol in residential houses located near major highways: Indoor versus outdoor concentrations by Dainius Martuzevicius; Sergey A. Grinshpun; Taekhee Lee; Shaohua Hu; Pratim Biswas; Tiina Reponen; Grace LeMasters (6575-6585).
PM2.5 mass concentration and its elemental composition were measured inside and outside of six houses located at a distance of approximately 30–300 m from major highways. To investigate the amount of traffic particles penetrating from indoor to the outdoor (I/O) environment, the data were analyzed by three-way factor method PARAFAC. The PM2.5 I/O ratio ranged from 0.5 ± 0.2 to 2.9 ± 1.2 in spring and from 0.7 ± 0.1 to 4.7 ± 6.9 in fall. The traffic component was very pronounced in the ambient air of the six houses. Although the distance from the highway and traffic intensity are generally important for assessing the indoor concentration of traffic-related aerosols, the data collected in this study suggest that – specifically for houses located in a close proximity to major highways – these two factors may not necessarily play the most important role in differentiating exposure levels between houses.
Keywords: Aerosol; PM2.5; Traffic; Indoor; Outdoor; Receptor; PARAFAC;
Investigation of nitrous acid concentration in an indoor environment using an in-situ monitoring system by Seung Shik Park; Jin H. Hong; Jai H. Lee; Young J. Kim; Sung Y. Cho; Seung J. Kim (6586-6596).
An in-situ measurement system for the determination of nitrous acid (HONO) was developed and used at an indoor residential environment. The system uses a diffusion scrubber to sample gaseous HONO and the peroxynitrite-induced luminol chemiluminescent method to quantify the amount of HONO. In this system, the detection limit of HONO, estimated as three times the noise level of the scrubbing solution blank, was 120 pptv for a 2-min integrated sample. Indoor HONO and NO x concentrations were determined for 7 days in the living room of an apartment with a gas range for cooking in the kitchen. Close examination of the relationships among HONO, NO, and NO2 concentrations during both the background and combustion periods confirm that the observed HONO was formed not only by direct emission from gas combustion, but also from heterogeneous reactions of NO2 with H2O on indoor surfaces. The average ratio of HONO to NO2 over the study period was 0.12 ± 0.05. The HONO/NO2 concentration ratio was 0.04–0.08 during the combustion period, whereas it was 0.10–0.25 after combustion had stopped. This suggests that HONO was generated through different production processes, both during combustion and after the completion of combustion. The controlled combustion experiments indicate that the burning rate is an important factor to determine the peak HONO concentration. In darkness, HONO had a nearly constant removal rate for all of the combustion experiments, whereas the removal rates of NO and NO2 depended on the burning rates of the gas range. Combustion experiments conducted at the fixed burning rate setting show also that ventilation decreased HONO concentration. This indicates that the airflow rate of the range hood fan is an important factor to control the concentration of indoor air pollutants.
Keywords: HONO; Nitrogen oxides; Luminol chemiluminescent method; Heterogeneous chemistry;
Chemical and morphological properties of particulate matter (PM10, PM2.5) in school classrooms and outdoor air by H. Fromme; J. Diemer; S. Dietrich; J. Cyrys; J. Heinrich; W. Lang; M. Kiranoglu; D. Twardella (6597-6605).
Studies have shown high concentrations of particulate matter (PM) in schools. Further insights into the sources and the composition of these particles are needed.During school hours for a period of 6 weeks, outdoor air and the air in two classrooms were sampled. PM was measured gravimetrically, and PM filters were used for the determination of the elemental and organic carbon, light absorbance, and 10 water-soluble ions. Some filters were further analyzed by scanning electron microscopy (SEM) and energy dispersive microanalysis (EDX).The median PM10 concentrations were 118.2 μg m−3 indoors and 24.2 μg m−3 outdoors; corresponding results for PM2.5 were 37.4 μg m−3 indoors and 17.0 μg m−3 outdoors. Using PM10 and PM2.5 data, we calculated the following indoor/outdoor ratios: 0.3 and 0.4 (sulfate), 0.1 and 0.2 (nitrate), 0.1 and 0.3 (ammonium), and 1.4 and 1.6 (calcium).Using the measured sulfate content on PM filters as an indicator for ambient PM sources, we estimated that 43% of PM2.5 and 24% of PM10, respectively, were of ambient origin. The composition of the classrooms' PM (e.g., high calcium concentrations) and the findings from SEM/EDX suggest that the indoor PM consists mainly of earth crustal materials, detrition of the building materials and chalk.Physical activity of the pupils leads to resuspension of mainly indoor coarse particles and greatly contributes to increased PM10 in classrooms. The concentration of fine particles caused by combustion processes indoors and outdoors is comparable.We conclude that PM measured in classrooms has major sources other than outdoor particles. Assuming that combustion-related particles and crustal materials vary in toxicity, our results support the hypothesis that indoor-generated PM may be less toxic compared to PM in ambient air.
Keywords: Indoor air; School; PM; Sulfate; Elemental carbon; Exposure;
Dry atmospheric deposition rates of metals along a coastal transect in southern California by Lisa D. Sabin; Kenneth C. Schiff (6606-6613).
While recent studies indicate atmospheric deposition is a significant source of metals to the Santa Monica Bay and coastal river systems of the Los Angeles area, the spatial extent of the atmospheric source along the entire southern California coast has not been measured in 30 years. This study provides measurements of dry atmospheric deposition of chromium, copper, lead, nickel and zinc at eight sites located along the coast between Santa Barbara and San Diego, and compares these data to historic measurements from the 1970s. Median dry deposition fluxes across sites ranged between 0.23 and 3.6 (chromium), 0.21 and 5.4 (nickel), 0.52 and 14 (lead), 0.89 and 29 (copper), and 4.8 and 160 (zinc) μg m−2 day−1. Differences in metal dry deposition rates observed between sites were dominated by proximity to urban areas and/or other nearby sources, with the highest metal fluxes observed near the Los Angeles Harbor (LAH) and San Diego Bay (SDB) sites. Compared with data from the 1970s, lead fluxes were typically one to two orders of magnitude lower in the present study (2006), indicating atmospheric sources of this metal have decreased over the past three decades in southern California. Chromium fluxes were also lower in 2006 compared with the 1970s, although to a lesser extent than for lead. In contrast, copper and zinc fluxes were typically within the same order of magnitude between the two time periods, with some higher measurements observed in 2006 compared with the 1970s at the LAH and SDB sites. This result indicates atmospheric sources of copper and zinc have not decreased over the past three decades in southern California, and have increased near our harbor/urban sites. Differences in sampling conditions (e.g., Santa Ana winds) and measurement techniques may also explain, in part, the differences observed in metal flux rates for these time periods. However, these limitations were most important for those metals with the smallest difference in flux rates measured in the 1970s vs. 2006 (e.g., chromium).
Keywords: Dry atmospheric deposition; Metals; Southern California; Los Angeles; San Diego;
An experimental study for rate constants of the gas phase reactions of CH3CH2OOH with OH radicals, O3, NO2 and NO by Caixia Wang; Zhongming Chen (6614-6619).
The rate constants for ethyl hydroperoxide (EHP, CH3CH2OOH) reactions with OH radicals, O3, NO2 and NO in the gas phase have been investigated, by use of in situ long-path Fourier transform infrared spectroscopy. At 293 ± 2 K temperature and 760 Torr pressure, the measured values are k EHP+OH = (5.92 ± 0.36) × 10−12, k EHP + O 3 ≤ ( 8.4 ± 0.1 ) × 10 − 21 , k EHP + NO 2 ≤ ( 9.7 ± 0.3 ) × 10 − 21 and k EHP+NO ≤ (1.2 ± 0.1) × 10−21 cm3 molecule−1 s−1. The atmospheric lifetimes of EHP with respect to these reactions have been estimated from these values, and the atmospheric implications have been briefly discussed.
Keywords: Ethyl hydroperoxide; Rate coefficients; Loss reaction; Atmospheric lifetime; FTIR;
Anthropogenic mercury emissions in South Africa: Coal combustion in power plants by James M. Dabrowski; Peter J. Ashton; Kevin Murray; Joy J. Leaner; Robert P. Mason (6620-6626).
South Africa is regarded as a country with the 2nd highest mercury (Hg) emissions in the world. This assumption is based on estimates of total Hg emissions derived primarily from gold mining and coal combustion. The potential sources of Hg to the South African environment were assessed by focussing particularly on coal combustion at the country's coal-fired power plants. Mercury emission estimates were based on the total amount of coal burned in all power plants per year (112.3 Mt y−1), the Hg content of South African coals (0.2 ppm) and the emission control devices used in each power plant. Results indicate that Hg emissions arising from South Africa's coal-fired power plants (ranging between 2.6 and 17.6 tonnes y−1, with an estimated average emission of 9.8 tonnes y−1) are significantly lower than suggested in the literature (approximately 50 tonnes y−1). The calculated emission factor (ranging between 0.02 and 0.16 g Hg tonne−1 coal burned) and per capita estimates (0.24 g Hg person−1 y−1 R −1, where R is the fraction of total electricity generated from coal) fall within the range of values reported for Hg inventories derived in other countries and indicate that Hg emission estimates for coal-fired power plants presented in this paper are more reliable than those published previously.
Keywords: Mercury emissions; South Africa; Coal combustion; Power plants;
QSPR-based estimation of the atmospheric persistence for chloronaphthalene congeners by Tomasz Puzyn; Aleksandra Mostrąg; Noriyuki Suzuki; Jerzy Falandysz (6627-6636).
Gas phase reaction with OH• radicals is the major atmospheric loss process for the aromatic hydrocarbons. Thus, we used the half-live values (t 1/2) derived based on this mechanism for characterization of environmental persistence and long-range atmospheric transport (LRAT) potential of all 75 chloronaphthalene congeners (CNs). We applied the single-media LRAT criterion from the Stockholm Convention (t 1/2 in air > 2 days and vapor pressure < 1000 Pa). The values of t 1/2 were estimated from a robust and predictive quantitative structure–property relationship (QPSR) model. The average half-lives for particular homologue groups of CNs were as following: 2 days for mono-CNs, 5 days for di-CNs, 10 days for tri-CNs, 19 days for tetra-CNs, 39 days for penta-CNs, 79 days for hexa-CNs, 163 days for hepta-CNs, and 343 days for octa-CN. Main factors affecting the persistence of chloronaphthalenes were the degree of chlorination and the substitution pattern. The congeners having the –C(β)Cl–C(α)H–C–C(α)Cl– fragment, in which relatively low electron density was observed on the α hydrogen and carbon atoms, were characterized by significantly lower half-lives than the rest of the compounds within the individual homologue groups.
Keywords: PCNs; Chloronaphthalenes; OH• radical; Half-live; QSPR;
Estimate of annual NH3 dry deposition to a fumigated ombrotrophic bog using concentration-dependent deposition velocities by J.N. Cape; M.R. Jones; I.D. Leith; L.J. Sheppard; N. van Dijk; M.A. Sutton; D. Fowler (6637-6646).
Estimates of the dry deposition of ammonia (NH3) gas in a field fumigation experiment on an ombrotrophic bog have been made using the inferential technique, with measured wind speed at 2 m, and air concentrations at two heights above the vegetation. The parameters for a concentration-dependent surface resistance term have been derived from flux measurements over the same vegetation in a chamber study, separating stomatal from non-stomatal resistances.Annual NH3-N deposition in each of the 4 years 2003–2006 was estimated to increase from 3.0 ± 0.2 kg N ha−1 y−1 in ambient air, with an NH3 concentration at 0.5 m above the canopy of 0.7 μg m−3, to 50–70 kg N ha−1 y−1 where annual average air concentrations were 70–90 μg m−3 and concentrations during fumigation were up to 1600 μg m−3. The equivalent deposition velocities (at z = 0.5 m) were 0.016 m s−1 in ambient air and 0.003 m s−1 at 100 μg m−3. The differences between annual deposition estimates made from independent air concentration data at 0.1 m and 0.5 m above the canopy were small for distances more than 10 m from the source, after vertical mixing was complete. Over 4 years (2003–2006) and at eight sampling points more than 10 m from the NH3 source, the mean difference between the dry deposition estimates, using NH3 concentrations measured independently at 0.1 m and 0.5 m above the canopy, was 2%.Use of a constant surface resistance, with no concentration dependence, as commonly used in inferential models of dry deposition, would have predicted deposition up to eight times too large.
Keywords: Surface resistance; Dry deposition; Ammonia; Field release; Point source;
Quantification of indoor and outdoor volatile organic compounds (VOCs) in pubs and cafés in Pamplona, Spain by M.A. Parra; D. Elustondo; R. Bermejo; J.M. Santamaría (6647-6654).
Indoor and outdoor concentrations of volatile organic compounds (VOCs) were measured in 30 pubs and cafés (13 smoking, 13 non-smoking and 4 mixed atmospheres) in Pamplona city, Spain. The samples were obtained using a sampling pocket pump connected to stainless steel tubes filled with Tenax TA, and subsequently analysed by means of GC–MS coupled to a thermal desorption unit. The levels registered were found to be generally higher indoors. Smoking, cleaning products and the entrance of outdoor pollutants were identified as the main sources of these compounds, the later being especially relevant in non-smoking areas. BTEX concentrations were higher during the winter months and higher in smoking areas also.
Keywords: VOCs; Indoor air; Indoor/outdoor ratio; Seasonal variability; Pubs and cafés;
Semi-continuous measurement of PM2.5 ionic composition at several rural locations in the United States by Taehyoung Lee; Xiao-Ying Yu; Sonia M. Kreidenweis; William C. Malm; Jeffrey L. Collett (6655-6669).
To improve understanding of the nature and variability of the ionic fraction of atmospheric fine aerosol particles in non-urban environments, one to two month measurement campaigns were conducted at several rural locations in the United States. Study sites included Yosemite National Park (NP) (July–September 2002), Bondville, Illinois (February 2003), San Gorgonio Wilderness Area, California (April and July 2003), Grand Canyon National Park, Arizona (May 2003), Brigantine National Wildlife Refuge (NWR), New Jersey (November 2003), and Great Smoky Mountains National Park, Tennessee (July/August 2004). PM2.5 ion composition was measured at 15 min intervals using a Particle-Into-Liquid-Sampler (PILS) coupled to two ion chromatographs. Comparisons of PILS measurements with parallel traditional 24 h denuder/filter-pack measurements reveal generally good agreement between the two techniques for major species, although PILS measurements of PM2.5 NH4 + are biased low by approximately 4–20%. High time resolution PILS aerosol concentration measurements provide better estimates of the range of aerosol concentrations at the rural locations than the 24 h integrated filter data. Ratios of peak 15 min to 24 h nitrate concentrations, for example, ranged from 1.7 at Brigantine NWR to 7.0 at Great Smoky Mountains NP. A strong influence of diurnal upslope/downslope transport patterns was observed on aerosol concentrations at several locations, including Yosemite NP, San Gorgonio Wilderness Area, and Great Smoky Mountains NP, with peak concentrations typically occurring during afternoon upslope transport. High time resolution aerosol composition measurements also provide new insight into relationships between individual aerosol species and the influence of environmental conditions on aerosol composition. Observations at several locations revealed important information about mechanisms of particle nitrate formation. At Yosemite and Grand Canyon National Parks, for example, evidence was observed for reaction of nitric acid or its precursors with sea salt or soil dust. Observations from several sites also revealed the importance of aerosol acidity (Great Smoky Mountains NP, Bondville) and temperature/humidity (San Gorgonio) on fine particle ammonium nitrate formation.
Keywords: PM2.5 composition; Particle-Into-Liquid-Sampler; Continuous measurements; Temporal variability;
Thermogravimetric characteristics of water-soluble organic matter from atmospheric aerosols collected in a rural–coastal area by Regina M.B.O. Duarte; Armando C. Duarte (6670-6678).
The influence of chemical composition on the thermal degradation behaviour of water-soluble organic compounds (WSOC) from fine atmospheric aerosols collected over a period of 12 months was investigated by using thermogravimetric (TG) analysis. The obtained results were further compared to those of Suwannee River Humic Acid (SRHA) and Fulvic Acid (SRFA) standards, which were obtained from the International Humic Substances Society (IHSS). Prior to the TG analyses, the WSOC samples were separated into hydrophobic acids and hydrophilic acids fractions by using a XAD-8/XAD-4 isolation procedure. The data reported in this study were illustrative of the very complex nature of aerosol WSOC hydrophobic acids samples (three to six peaks of thermal degradation) in comparison to that of SRHA and SRFA standards (two degradation peaks). A pseudo-first order mathematical approach was applied to estimate the apparent activation energy of the different multi-step degradation regions. It was found that the degradation processes of the SRHA and SRFA standards occurred with lower apparent activation energies than those obtained for the aerosol WSOC hydrophobic acids samples, although these latter samples appear to exhibit the most thermally labile structural component. The WSOC hydrophobic acids showed a trend towards high apparent activation energies at high temperatures, thus suggesting the decomposition of very stable structural units with relatively strong bond energies above ca. 460 °C. The thermal profiles presented in this study are unique in showing the annual evolution of the thermal-oxidative properties of the fine aerosol WSOC hydrophobic acids samples, and in providing important supplementary information on the structural stability of the bulk aerosol WSOC.
Keywords: Atmospheric aerosols; Water-soluble organic compounds; Thermogravimetric analysis; Apparent activation energy; Humic substances;
Effects on aerosol size distribution of polycyclic aromatic hydrocarbons from the heavy-duty diesel generator fueled with feedstock palm-biodiesel blends by Yuan-Chung Lin; Cheng-Hsien Tsai; Chi-Ru Yang; C.H. Jim Wu; Tzi-Yi Wu; Guo-Ping Chang-Chien (6679-6688).
Biodiesels are promoted as alternatives to fossil fuels and their applications in diesel engine have been studied extensively. However, the size distribution of polycyclic aromatic hydrocarbons (PAHs) and generator particulate material (GPM) emitted from heavy-duty diesel generator fueled with biodiesel blends has seldom been addressed. Seven different biodiesel blends with volume fractions of biodiesel ranging from 0% to 30% were studied. Experimental results indicate that the mean reductions of sum of PAHi/GPM0.056–18 (generator particulate material with aerodynamic diameter 0.056–18 μm) and BaPeqi [=(benzo[a]pyrene equivalent)i]/GPM0.056–18 of B5, B10, B15, B20, B25 and B30 are (−8.21%, −5.72%), (−36.7%, −29.7%), (−1.25%, 2.32%), (16.2%, 18.6%), (33.4%, 35.0%) and (40.5%, 42.4), respectively, compared with B0. Both PAHi/GPMi and BaPeqi/GPMi in stage 1 (0.056 – 0.166 μm) and stage 2 (0.166 – 0.31 μm) of all test fuels are higher than those in the other stages due to higher specific surface area of smaller particles. It is also observed that there are more highly toxic PAHs in stage 2. It should be noticed that the trend of particle-phase PAH contents is different from the trend of particle-phase PAH concentration and opposite to the trend of total GPM0.056–18 emission. The differences are due to a higher number of particles with diameters between 0.056 and 0.31 μm. The above results indicate that fuel blends with less than 15% biodiesel would increase PAH content at particle size between 0.056 and 0.31 μm. Therefore, the blending fraction should be between 15% and 30%. Moreover, particle-size control is needed in future emission regulations which would necessitate further improvements in combustion quality. Besides, researches on health effects of biodiesel blends are needed as well.
Keywords: PAH; Size distribution; Biodiesel; Engine; Emission;
α-Terpineol reactions with the nitrate radical: Rate constant and gas-phase products by Brian T. Jones; Jason E. Ham (6689-6698).
The bimolecular rate constant of k NO 3 • + α-terpineol (16 ± 4) × 10−12 cm3 molecule−1 s−1 was measured using the relative rate technique for the reaction of the nitrate radical (NO3 •) with α-terpineol (2-(4-methyl-1-cyclohex-3-enyl)propan-2-ol) at 297 ± 3 K and 1 atmosphere total pressure. To more clearly define part of α-terpineol's indoor environment degradation mechanism, the products of α-terpineol + NO3 • reaction were investigated. The identified reaction products were: acetone, glyoxal (HC(＝O)C(＝O)H), and methylglyoxal (CH3C(＝O)C(＝O)H). The use of derivatizing agents O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) and N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) were used to propose the other major reaction products: 6-hydroxyhept-5-en-2-one, 4-(1-hydroxy-1-methylethyl)-1-methyl-2-oxocyclohexyl nitrate, 5-(1-hydroxy-1-methylethyl)-2-oxocyclohexyl nitrate, 1-formyl-5-hydroxy-4-(hydroxymethyl)-1,5-dimethylhexyl nitrate, and 1,4-diformyl-5-hydroxy-1,5-dimethylhexyl nitrate. The elucidation of these products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible α-terpineol + NO3 • reaction mechanisms based on previously published volatile organic compound + NO3 • gas-phase mechanisms. The additional gas-phase products (2,6,6-trimethyltetrahydro-2H-pyran-2,5-dicarbaldehyde and 2,2-dimethylcyclohexane-1,4-dicarbaldehyde) are proposed to be the result of cyclization through a reaction intermediate.
Keywords: α-Terpineol; Nitrate radical; Kinetics; Reaction products; Oxygenated organic compounds;
The historical residue trends of DDT, hexachlorocyclohexanes and polycyclic aromatic hydrocarbons in an ice core from Mt. Everest, central Himalayas, China by Xiao ping Wang; Bai qing Xu; Shi chang Kang; Zhi yuan Cong; Tan dong Yao (6699-6709).
High mountains may serve as “cold traps” for persistent organic pollutants (POPs) and ice cores can provide long-term records of atmospheric deposition of pollutants. In this study, DDT, hexachlorocyclohexanes (HCHs) and polycyclic aromatic hydrocarbons (PAHs) in an ice core from East Rongbuk Glacier were analyzed and the deposition fluxes of these pollutants were investigated. Concentrations of total DDTs reached maxima of approximately 2 ng l−1 in mid-1970s, which is corresponding to the peak of malaria cases in India (in 1976). The decrease of DDT concentration after 1990s was in-line with the ban of DDT in India (in 1989). High level of α-HCH was observed in early 1970s and it showed a decrease to undetectable level at the end of 1990s, which is in agreement with the period when India banned the usage of HCH (in 1997). Concentrations of total PAHs sharply increased after 1990 and the peak (approximately 100 ng l−1) was found at the end of 1990s, when India entered the rapid industrialization (urbanization). PAHs in the ice core are dominantly pyrogenic in source, and are mainly from incomplete combustion of coal and biomass burning. Good correlations among concentrations of PAHs, nssSO4 2− and microparticles in snow pit samples showed that the origin of the PAHs and nssSO4 2− is often the same and they may be absorbed by particles and transported to high mountain regions by atmospheric circulation.
Keywords: Ice core; Historical trend; DDT; HCH; PAHs; Mt. Everest;
Carbon in southeastern U.S. aerosol particles: Empirical estimates of secondary organic aerosol formation by Charles L. Blanchard; George M. Hidy; Shelley Tanenbaum; Eric Edgerton; Benjamin Hartsell; John Jansen (6710-6720).
Fine particles in the southeastern United States are rich in carbon: Southeastern Aerosol Research and Characterization (SEARCH) network measurements from 2001 through 2004 indicate that fine particles less than 2.5 μm aerodynamic diameter (PM2.5) at two inland urban sites, Atlanta, GA and Birmingham, AL, contain 9 and 11% black carbon (BC) by mass, respectively, on average. For neighboring rural or urban Gulf Coast sites, the range is 4–7% BC. Organic carbon (OC) ranges from 25 to 27% in the inland cities, and 19–24% at the rural or Gulf Coast locations. Evidence in the literature suggests that a substantial fraction of the OC found in the Southeast is produced by atmospheric chemical reactions of volatile organic compounds (VOCs). Estimation of the fraction of OC from primary and secondary sources is difficult from first principles, because the chemistry is complex and incompletely understood, and the emission sources are both anthropogenic and natural. As an alternative, measurement-based models can be used to estimate empirically the primary and secondary source contributions. Three complementary empirical models are described and applied using the SEARCH database. The methods include (a) a multiple regression model employing markers for primary and secondary carbon using gas and particle data, (b) a carbon mass balance using carbon and CO data, along with certain assumptions about the OC/CO ratios in primary emissions for different urban and rural conditions, and (c) exploitation of isotopic measurements of carbon along with the BC and OC data. Secondary organic carbon (SOC) represents ∼20–60% of mean OC, depending upon location and season. The results are sensitive to estimates of emissions of primary OC and BC.
Keywords: Secondary organic aerosol; Southeastern United States; SEARCH network; Organic carbon; Black carbon;