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1 Aerosol Science and Technology ISSN: 7- (Print) (Online) Journal homepage: Estimating the Secondary Organic Aerosol Contribution to PM.5 Using the EC Tracer Method Special Issue of Aerosol Science and Technology on Findings from the Fine Particulate Matter Supersites Program Juan C. Cabada, Spyros N. Pandis, Ramachandran Subramanian, Allen L. Robinson, Andrea Polidori & Barbara Turpin To cite this article: Juan C. Cabada, Spyros N. Pandis, Ramachandran Subramanian, Allen L. Robinson, Andrea Polidori & Barbara Turpin () Estimating the Secondary Organic Aerosol Contribution to PM.5 Using the EC Tracer Method Special Issue of Aerosol Science and Technology on Findings from the Fine Particulate Matter Supersites Program, Aerosol Science and Technology, 3:S1, 1-155, DOI: 1.1/7399 To link to this article: Published online: 1 Jun 1. Submit your article to this journal Article views: 131 View related articles Citing articles: 9 View citing articles Full Terms & Conditions of access and use can be found at

2 Aerosol Science and Technology, 3(S1):1 155, Copyright c American Association for Aerosol Research ISSN: 7- print / online DOI: 1.1/7399 Estimating the Secondary Organic Aerosol Contribution to PM.5 Using the EC Tracer Method Juan C. Cabada, 1 Spyros N. Pandis, 1 Ramachandran Subramanian, Allen L. Robinson, Andrea Polidori, 3 and Barbara Turpin 3 1 Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 3 Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey The EC tracer method is applied to a series of measurements by different carbonaceous aerosol samplers in the Pittsburgh Air Quality Study (PAQS) in order to estimate the concentration of secondary organic aerosol. High-resolution measurements ( h) and daily averaged concentrations were collected during the summer 1 intensive (1 July to August 1) and are used for the analysis. The various samplers used during PAQS show differences in the measured concentrations of OC and EC due to the different sampling artifacts and sampling periods. A systematic approach for the separation of periods where SOA contributes significantly to the ambient OC levels from the periods where organic and elemental carbon concentrations are dominated by primary emissions is proposed. Ozone is used as an indicator of photochemical activity to identify periods of probable secondary organic aerosol production in the area. Gaseous tracers of combustion sources (CO, NO, and NO x ) are used to identify periods where most of the OC is primary. Periods dominated by primary emissions are used to establish the relationship between primary OC and EC, a tracer for primary combustion-generated carbon for the different sets of measurements for July 1. Around 35% of the organic carbon concentration in Western Pennsylvania during July of 1 is estimated to be secondary in origin. INTRODUCTION Carbonaceous aerosol is an important constituent of the PM.5 (particulate matter with aerodynamic diameters less than.5 mi- Received October 3; accepted 1 May 3. This research was conducted as part of the Pittsburgh Air Quality Study that was supported by US Environmental Protection Agency under contract R11 and the US Department of Energy National Energy Technology Laboratory under contract DE-FC-1NT117. This article has not been subject to EPA s required peer and policy review and therefore does not necessarily reflect the views of the Agency. No official endorsement should be inferred. Address correspondence to Spyros N. Pandis, Department of Chemical Engineering, Carnegie Mellon University, 5 Forbes Avenue, Pittsburgh, PA spyros@andrew.cmu.edu crons) mass in most of the U.S. Between 1 5% of the fine particulate mass has been identified as carbonaceous material for various regions of the country (Gray et al. 19; Turpin et al. 1991; Seinfeld and Pandis 199; Tolocka et al. 1; Lim and Turpin ). Aerosol carbon is commonly classified as organic carbon (OC) and elemental carbon (EC). OC can be directly emitted to the atmosphere in the particulate form (primary) or can be produced by gas-to-particle conversion processes (secondary). EC is emitted from combustion sources. Since primary OC and EC are mostly emitted from the same sources, EC can be used as a tracer for primary combustion-generated OC (Gray et al. 19; Turpin and Huntzicker 1995; Strader et al. 1999). The formation of secondary organic aerosol (SOA) increases the ambient concentration of OC and the ambient OC/EC ratio. OC-to-EC ratios exceeding the expected primary emission ratio are an indication of SOA formation. For Southern and Central California, 3 % of the total OC has been identified as secondary in summer (Gray et al. 19; Pandis et al. 199; Hildemann et al. 1993; Turpin and Huntzicker 1995; Schauer et al. 199). The relationship between primary OC and EC depends also on the sampling and analyses techniques used to determine the ambient OC and EC concentrations. Sample collection (i.e., the use or not of denuders, filter face velocities, etc.) and different analysis techniques (i.e., thermal optical transmittance versus thermal optical reflectance) affect the reported concentrations for OC and EC (Countess 199; Birch 199; Chow et al. 1; Schmid et al. 1). Sampling carbonaceous particulate matter from the atmosphere is challenging because of interferences from gaseous material that is adsorbed on the filters and evaporation of the collected organic material during sampling (Turpin and Huntzicker 199; Fitz 199; Hering et al. 199). Different sampling arrangements (e.g., using backup quartz filters, placing denuders upstream of the filter to remove organic gases, etc.) have been proposed in order to reduce, measure, and correct for the positive and negative artifacts that 1

3 SECONDARY ORGANIC AEROSOL CONTRIBUTION TO PM.5 11 affect the measured carbonaceous concentrations (Turpin et al. ). Primary ratios of OC to EC vary from source to source and show temporal and diurnal patterns (Gray 19; Cabada et al. a), but since EC is only emitted by combustion sources, gaseous tracers of combustion (CO, NO, NO x ) can be used to determine periods dominated by primary aerosol emissions. Ozone is an indicator of photochemical activity, and it also can be used as a tracer for periods where secondary organic aerosol production is expected. In this case, increases in the OC-to-EC ratio correlated to ozone episodes are indicative of SOA production. In this work a relationship between primary OC and EC is established for each of the different types of measurements and artifact estimation approaches. An algorithm is proposed for the determination of the primary OC-to-EC ratio and secondary organic aerosol concentrations are estimated. SOA results based on high-resolution and daily-averaged samples are compared. The effect of sampling frequency on the estimates of the primary ratios is also discussed. EXPERIMENTAL METHODS AND EQUIPMENT The Pittsburgh Air Quality Study (PAQS) main site was located in Schenley park on the top of a hill just outside of Carnegie Mellon University campus, around three miles to the east of downtown Pittsburgh. The Pittsburgh supersite operated three different samplers for collecting carbonaceous aerosol (one undenuded and two denuded samplers). The undenuded sampler and a denuded in situ analyzer collected samples every h, and the denuder-based sampler collected daily samples, during the 1 summer intensive (1 July to August, 1). Quartz fiber filters (7 mm Tissuquartz 5 QAO-UP), Teflon filters ( µm pore, Whatman 759-1), and carbonimpregnated filters (Schleicher and Schuell, GF-39) were used to sample carbonaceous material in three different samplers. Quartz fiber filters were baked at 55 C for more than 1 h and stored in previously cleaned glass jars until sampling and analysis. Carbon-impregnated filters were baked at 37 C for more than3hinanitrogen atmosphere. Undenuded Sampler PM.5 carbonaceous aerosol samples were collected on quartz fiber filters using filter packs in a nondenuded line. This sampler consisted of two parallel lines, the first line holding a quartz fiber filter followed by a backup quartz filter and the second line having a Teflon filter followed by a backup quartz fiber filter (Figure 1). The two backup quartz fiber filters are used to estimate the positive and negative artifact (Turpin et al. ). Five samples a day, with intervals between sampling times of h, were collected during the summer intensive. Samples were collected during, 1, 1 1, 1 1, and 1 h (all in EST). The filter configuration allows two different estimates of the adsorption artifacts on the front quartz fiber filter (Q F ). The first correction is done by subtracting the OC collected in the backup quartz fiber filter behind the front quartz (Q B,F ) from the OC collected by the Q F. This approach assumes that the Q F collects 1% of the carbonaceous particulate matter (no evaporation) and that both the Q F and the Q B,F adsorb organic gases and reach equilibrium with them during the sampling period. The second correction approach subtracts the OC collected in the Q B,F behind the Teflon filter (Q B,T ) from the OC in the Q F. This approach assumes that the Teflon filter collects all of the particles from the sampled flow with 1% efficiency and that the Q B,F from this line adsorbs the same quantity of gases as the Q F. The EC concentration reported by the Q F is used for all datasets from the undenuded sampler. More details on the undenuded sampler and its operation are described in Subramanian (). Undenuded Sampler Denuded Sampler Denuded In-situ Sampler-Analyzer PM.5 Cyclone PM.5 Cyclone Denuder PM.5 Cyclone Denuder Sunset Labs Instrument T F Q F Q Fden Q B,T Q B,Q CIF B,Qden Q Fsitu 1.7 lpm 1.7 lpm 1.7 lpm. lpm Figure 1. Schematic of the Pittsburgh Air Quality Study carbonaceous aerosol samplers. Subscript F denotes the front filter in the samplers. Backup filters are indicated as a subscript B followed by the type of filter they are after (T, Teflon; Q, quartz).

4 1 J. C. CABADA ET AL. Denuded Sampler Filter packs holding a quartz filter in front of a carbonimpregnated filter (CIF) were used to collect carbonaceous material from a denuded sampling line (Figure 1). A carbon annular denuder (Novacarb monolith synthetic carbon, Mast Carbon Ltd., Guilford, UK) was used to remove organic gases and minimize the positive artifact in the quartz filter. The CIF organic carbon concentration was intended to correct for evaporation of semivolatile material from the quartz filter (negative artifact). Sampling frequency for this unit was h, from midnight to midnight (EST). More details on the undenuded sampler and its operation are described in Subramanian (). Semicontinuous Denuded in situ Analyzer An in situ semicontinuous carbon analyzer (Sunset Labs, Carbon Aerosol Analysis Field Instrument), similar in design to that described by Turpin et al. (199), was used to collect and analyze carbonaceous aerosol with sampling periods of h (1 to min sampling time plus min for analysis). Instrument performance and PAQS protocols are described in detail by Lim et al. (). A parallel plate diffusion denuder (CIF; Schleicher Schuell, Keene, NH, USA) was placed upstream of a quartz filter, which was mounted inside the analyzer (Figure 1). Cycles of sampling and analysis were alternated in order to determine the ambient concentrations of OC and EC (Lim et al. ). Quartz filters from the filter-pack based samplers were analyzed using a Thermal/Optical transmittance carbon analyzer (Sunset Laboratory Inc., OC-EC Aerosol Carbon Analyzer Model-3) using the temperature steps of the NIOSH protocol (Birch and Cary 199; NIOSH 1999) for the determination of OC and EC. Table 1 shows the experimental parameters for the analysis of the quartz and carbon-impregnated filters during Table 1 Temperature programs used by the Thermal/Optical Transmittance methods for the analysis of carbonaceous material during PAQS PAQS analysis method Filter In situ Carrier pack-based carbon Carbon impregnated gas samplers analyzer filters He 3 C, 1 s 3 C, s C/min up to 33 C He 5 C, 1 s 5 C, s 33 C, 3 s He 15 C, 1 s 15 C, s He 7 C, 1 s 7 C, 9 s He/O 575 C, 5 s 575 C, 5 s He/O 5 C, 5 s 5 C, 5 s He/O 75 C, 5 s 75 C, 5 s He/O C, 5 s C, 5 s He/O 91 C, 1 s 91 C, 1 s PAQS. The time length of the different temperatures steps in the method was modified to get a better split between OC and EC (Yu et al. ). Carbon-impregnated filters were analyzed using a temperature ramp up to 3 C during 5 min under a helium atmosphere. All concentrations of OC and EC were corrected for field blanks. Concentrations reported from the in situ carbon analyzer were corrected for dynamic blanks generated by sampling with a Teflon filter upstream of the denuder. CARBONACEOUS AEROSOL MEASUREMENTS Differences exist among the measured concentrations of organic and elemental carbon collected by the different samplers. For example, adsorption of organic gases on the Q F of the undenuded lines (positive artifact) is evident as the OC measured by this line is higher than that of the denuded samplers. The magnitudes of the positive and negative artifacts depend not only on the sampling method and the atmospheric composition but also on the length of the sampling period. For the high-time resolved samples the most adequate correction for the concentration measured with the Q F in the undenuded sampler is the subtraction of the OC concentration measured in the backup quartz filter behind the Teflon filter. A detailed discussion of the artifacts using these datasets is presented by Subramanian et al. (). Figure shows time-resolved concentrations for the different samplers during a six-day period. Overall samplers indicate similar patterns of OC and EC concentrations, but the OC and EC concentrations from the undenuded line are almost always higher than those of the other two samplers. The summer intensive averaged concentrations of OC and EC for all types of samplers and all artifact correction approaches at the Pittsburgh supersite is shown in Table. Subtracting the measured OC concentration on the Q B,F and Q B,T from the Q F reduces the OC concentrations by % on average for the summer intensive. Subtracting the OC concentration of the Q B,F behind the Teflon filter in the parallel line of the undenuded sampler results in an average correction of around 5% on average for the summer intensive. The reported OC concentrations of the two Table Monthly averaged OC and EC concentrations for the summer intensive 1 at PAQS Measurement type OC (µg C/m 3 ) EC(µg C/m 3 ) Undenuded (Q F ).1. Undenuded (Q F Q B,Q ) Undenuded (Q F Q B,T ) 1.. Denuded in situ analyzer.. (Q F,situ ) Denuded sampler The undenuded sampler OC concentrations are corrected by subtracting the OC collected in the backup quartz filter behind the front quartz or by subtracting the OC collected in the backup quartz filter behind the Teflon filter from the parallel sampling line.

5 SECONDARY ORGANIC AEROSOL CONTRIBUTION TO PM.5 13 EC (µg/m 3 ) OC (µg C/m 3 ) Denuded In-situ OC Denuded In-situ EC Denuded OC July Denuded EC OC Undenuded OC (Q F ) EC Undenuded EC (Q F ) Figure. Time-resolved concentrations for the different organic samplers at the Pittsburgh supersite project. The filter-based undenuded samples and the denuded in situ analyzer samples were collected in high resolution time periods ( h). The filter-based denuded samples were collected in h periods. denuded samplers agree within 1% and give particulate OC concentrations between the Q F,B and Q B,T corrected undenuded sampler concentrations. However, the reported average EC concentrations differ by 5%. EC concentrations reported by the undenuded and denuded in situ analyzer agree within 1%. A detailed discussion of the potential reasons for these differences is provided by Subramanian et al. (). In this work we examine the effect of the difference in sampler configuration and sampling periods of OC and EC measurements on the SOA estimates by applying the EC tracer method. THE EC TRACER METHOD The ratio of the ambient concentrations of particulate OC to EC includes information about the extent of secondary OC formation. Ambient OC/EC ratios greater than those characteristic of the primary emissions for a specific area are an indication of secondary aerosol formation. The EC tracer method takes advantage of the fact that primary OC and EC are mostly emitted by the same combustion sources. Primary ratios of OC to EC can be determined from a subset of ambient measurements if a large data set is available and conditions to produce SOA are unlikely (Turpin and Huntzicker 1995; Strader et al. 1999), or by developing an emissions inventory of the principal sources for an area of interest (Gray 19; Cabada et al. a). Assuming that OC primary can be defined by, [ ] OC [OC] p = [EC] + b, [1] EC the contribution of secondary OC can be estimated as p [OC] S = [OC] [OC] p, [] where [OC] p is the primary organic aerosol concentration, [OC/EC] p is the ratio of OC to EC for the primary sources affecting the site of interest, and b is the noncombustion contribution to the primary OC and sampling artifacts (Turpin and Huntzicker 1995; Strader et al. 1999). Sources that emit mainly OC and would contribute to this intercept b include meatcooking operations, biogenic sources (i.e., plant detritus, resuspension of other biogenic material), etc. [EC] is the measured EC concentration, [OC] S is the secondary organic aerosol contribution to the total OC, and [OC] is the measured OC concentration. All of these parameters are time dependent because of the temporal variations in anthropogenic emissions and meteorology. The application of this method requires measurements of [OC], [EC], and the determination of the [OC/EC] p ratio, as well as the noncombustion primary OC contribution (b) for the area and period of interest (Turpin and Huntzicker 1995).

6 1 J. C. CABADA ET AL. Calculation of the Primary OC-to-EC Ratio and Intercept Diurnal variations of the ambient OC-to-EC ratio were observed for all the high-resolution measurements taken during the summer intensive of 1 at PAQS. Photochemical activity, meteorology, and primary emissions all contribute to these variations. Ozone concentration can be used as an indicator of photochemical activity. Carbon monoxide (CO) and nitrogen oxides (NO and NO x ) can be used as tracers of combustion-related primary emissions. The primary ratio and intercept are determined from a dataset by identifying the periods where the ambient concentrations are dominated by primary emissions. The first step in the determination of the primary OC/EC ratio is the subtraction from the original dataset of the points where rain and the corresponding storms cause significant changes to the OC/EC ratio (Figure 3a). These changes have a variety of causes (removal of aged particles and increased importance of the locally produced ones, preferential removal of secondary OC, etc.). These periods are excluded from the analysis to avoid unnecessary complications. The second step consists of identifying the OC and EC concentrations where there is high probability of SOA production. The OC-to-EC ratio usually showed a strong correlation with ozone, but a lag time between the ozone peak and the actual OC-to-EC ratio was observed. In an effort to account for those events, the history of the ozone peak was taken into account, and the peak ozone concentration for the period preceding the sample was compared to the OC-to-EC ratio to evaluate its influence. For example, Figure 3b shows the measured OC-to-EC ratio and O 3 concentrations during a -day period in July. Periods of significant photochemical activity are evident during the afternoon of each day. The corresponding afternoon OC and EC measurements are deleted from the dataset because they probably include some SOA contribution. This does not mean that the other periods have only primary OC. Secondary aerosol can be produced elsewhere, maybe even during the previous day, and transported into the site. Figures 3c and d show periods where combustion-related sources were dominating over the area. The last step in this methodology consists of identifying these periods. As the NO x, NO, and CO concentrations peaked during the night and early morning of 3 July and 1 August, the OC-to-EC ratios decreased, indicating the influence of primary sources over the area of analysis. The corresponding samples during these periods are kept in the dataset and are used to estimate the primary OC-to-EC ratio for the period of analysis. OC/EC Ratio (a) Rainfall July Rainfall (mm) OC/EC Ratio (b) Ozone July 1 1 O 3 (ppb) OC/EC Ratio (c) NOx NO NO, NOx (ppb) OC/EC Ratio (d) 31 1 July July August CO 3 1 CO (ppm) Figure 3. Time series of OC/EC ratio (undenuded Q F ) and gaseous tracers of photochemical activity and primary emissions for different periods in July 1 during the summer intensive at the Pittsburgh Supersite. (a) OC/EC ratio affected by rain. (b) Ozone and OC/EC ratio. (c) Nitrogen oxides and OC/EC ratio. (d) OC/EC ratio and carbon monoxide.

7 SECONDARY ORGANIC AEROSOL CONTRIBUTION TO PM.5 15 OC (µg C/m 3 ) 1 (a) OC (µg C/m 3 ) 1 (b) OC (µg C/m 3 ) 1 EC (µg/m 3 ) (c) OC (µg C/m 3 ) 1 EC (µg/m 3 ) (d) EC (µg/m 3 ) EC (µg/m 3 ) Figure. Scatter plot of OC versus EC for all samples collected during the period (summer intensive 1). Concentrations shown are from the undenuded sampler, front quartz data. Hollow circles represent concentrations that have not been classified according to the criteria used. (a) OC versus EC for all samples collected during the period of analysis. (b) Concentrations eliminated from the dataset because they are affected by rain (solid rhombus). (c) Concentrations strongly influenced by photochemical activity are deleted from the dataset (solid squares). (d) Final set of concentrations influenced by primary emissions (hollow triangles). OC/EC primary ratio and intercept, b, are estimated by a linear fit of these data. Figure shows the sequence of how the OC versus EC plot is evolving during the different steps of the analysis. Once all the points that are dominated by primary OC are determined (those influenced by combustion sources as described by the algorithm), a linear regression by least squares minimization is fitted to the primary concentrations. The slope of the fit represents the OC-to-EC primary ratio, and the intercept represents the noncombustion organic carbon contribution to the primary OC concentration (see Equations (1) and ()). Table 3 shows the classification of concentrations between primary and secondary influenced for a two-day period during the summer intensive using the undenuded Q F data. Most of the OC during the first morning (: 1:) appears to be primary, as the area was heavily influenced by primary emissions showing higher values of NO x and CO. OC-to-EC ratios increased rapidly during the day as the ozone concentration increased, suggesting that the formation of SOA was probably taking place. A period where SOA material is probably transported into the area can be observed during the late hours of 1 August and the first hours of August. These periods are characterized by relatively high ozone and NO x, leading to the classification of this period as SOA dominated. Periods when most of the OC is primary are characterized by average concentrations of 5 ppb of NO x, ppb of NO,.3 ppm of CO, and 3 ppb of ozone. The remaining periods where there may be significant amounts of SOA present have average concentrations of 17 ppb of NO x, ppb of NO,. ppm of CO, and 5 ppb of ozone. For the analysis of the daily samples (or daily-averaged concentrations) the above algorithm needs to be modified. Daily averages of the O 3, CO, and NO x concentrations are used to determine the periods when the OC concentrations are influenced by primary sources. Primary-dominated concentrations for the daily-averaged concentrations show an average concentration of 17 ppb of NO x, ppb of NO,. ppm of CO, and 3 ppb of ozone. Secondary-dominated concentrations show and average concentration of ppb of NO x, ppb of NO,. ppm of CO, and 5 ppb of ozone. Estimated [OC/EC] p and b Figure 5 summarizes the classification of points between primary and secondary influenced for all high-resolution datasets. The estimated primary ratio of OC to EC ([OC/EC] p ) and the intercept, b, vary depending on the dataset analyzed and the averaging period used (Table ). A consistent set of concentrations of OC and EC should be used in order to estimate the

8 1 J. C. CABADA ET AL. Table 3 Selection criteria for OC and EC concentrations (undenuded Q F dataset) influenced by primary emissions or SOA formation EC OC OC/EC O 3 avg. O 3 peak O 3 peak CO NO NO x Source Date µg/m 3 µg C/m 3 Ratio (ppb) (ppb) (i-1) (ppb) (ppm) (ppb) (ppb) influence /1/1 : 1: Primary 1: 1: Secondary 1: 1: Secondary 1: : Secondary //1 : : Secondary : 1: Primary 1: 1: Secondary 1: 1: Secondary 1: : Secondary i-1, corresponds to the average concentration of ozone during the previous sampling interval of carbonaceous material. Undenuded OC Q F (µg C/m 3 ) Undenuded OC Q F Q B,T (µg C/m 3 ) 1 1 (a) OC=.3*EC+1. O OC=.*EC (c) Undenuded OC Q F Q B,F (µg C/m 3 ) Undenuded EC, Q F (µg/m 3 ) Undenuded EC, Q F (µg/m 3 ) Denuded In-situ analyzer OC (µg C/m 3 ) OC=.9*EC+. O OC=1.7*EC Undenuded EC, Q F (µg/m 3 ) 1 1 (d) (b) Denuded In-situ analyzer EC (µg/m 3 ) Figure 5. Estimated carbonaceous concentrations influenced by primary emissions and SOA production for high time resolution measurements ( h samples) during the summer intensive at PAQS. Solid squares correspond to SOA-influenced concentrations and hollow triangles to primary-dominated concentrations. (a) Front quartz OC (Q F ) and EC concentrations. (b) Correcting the OC measurement with the backup quartz filter behind the front quartz (Q F Q B,F ). (c) Correcting the OC concentrations with the backup quartz filter behind the Teflon filter in the parallel undenuded line (Q F Q B,T ). (d) Denuded in situ analyzer.

9 SECONDARY ORGANIC AEROSOL CONTRIBUTION TO PM.5 17 Table Estimated parameters for the linear fitofthe primary OC and EC concentrations: High-resolution data ( h) and daily averages and low-resolution measurements ( h) from the July 1 summer intensive at Pittsburgh Air Quality Study Noncombustion Correlation Measurement type [OC/EC]p primary OC, b (µg/m 3 ) coefficient (R ) Undenuded (Q F h samples).3 ±. 1. ±.. Undenuded (Q F Q B,Q h samples). ±..3 ±..7 Denuded in situ analyzer ( h samples) 1.7 ±..9 ±..7 Undenuded (Q F Q B,T h samples).9 ±.. ±..53 Denuded sampler ( h samples) 3.1 ±. 1. ±.. Undenuded (Q F h averages).7 ± ±..9 Undenuded (Q F Q B,Q h averages).3 ±.5 1. ±.3. Undenuded (Q F Q B,T h averages) 1. ±.5.5 ±.. Denuded in situ analyzer ( h averages) 1.9 ±.5.7 ±.. SOA concentration using the approach proposed in the previous sections. Estimates of the primary ratio vary from.9 to 3.1, and the intercept, b, varies from.3 to 1. µg C/m 3. Variations in the estimated parameters for the different sets of measurements are due to the different characteristics of the samplers (Subramanian et al. 3). In general primary ratios and intercepts from the fitting of the daily-averaged concentrations influenced by primary emissions are higher than those calculated from the high time resolution measurements (Figure ). This can be explained by the fact that high-resolution measurements have the ability to more accurately identify and separate periods of secondary organic aerosol formation from those dominated by primary emissions. Higher correlations coefficients are achieved for the daily samples because the datasets show less variability among the points considered to be influenced by primary emissions (Table ). The effect of the sampling artifacts can also be seen in the estimated primary ratios for the different datasets. Those datasets with corrections of the artifact (using the backup quartz filters in the undenuded sampler) or having a denuded line have lower primary ratios. The exception to this rule is the denuded sampler. Higher contributions from the noncombustion primary OC are calculated for the undenuded front quartz datasets. This is due to the addition of the adsorbed organic gases to the actual primary noncombustion OC, and the calculated values (1. 1. µg C/m 3 ) are probably overestimates of the real noncombustion primary OC. Lower intercepts are calculated for the undenuded datasets where corrections are done for the front quartz artifact, and these values are probably closer to the true concentrations (Table ). Since the primary ratio and intercept are expected to vary temporally, primary ratios and intercepts were estimated for the different high time resolution samplers, segregating the data for the different times of the day sampled. This analysis showed no significant diurnal variations for the estimated ratios and intercepts. This lack of temporal dependence is associated with the characteristics of the organic PM.5 in the Northeast U.S. The Northeast U.S. is characterized by long-range transport of PM and shorter residence times over the large urban areas. The dominance of regional sources reduces the effects of local emissions and their temporal patterns. Studies done in Southern and Central California, where most of these types of analyses have been performed, are able to identify different patterns of emissions for different times of the day since there is little background transport into the area (Gray 19; Turpin and Huntzicker 1995). SOA CONCENTRATIONS Once the primary ratio and the noncombustion primary OC contribution are calculated for each of the different datasets of carbonaceous measurements, the primary and secondary components of the Pittsburgh organic aerosol can be determined by applying Equations (1) and (). SOA Based on High Time Resolution Measurements The calculated SOA concentrations for the different highresolution measurements are in a qualitative agreement (Figure 7), predicting the same periods of SOA production for the summer intensive. For example, from 15 July to 5 July a high pressure system dominated the area, allowing high production of SOA. The effect of two other high-pressure systems can be seen from July to 11 July and in the beginning of August. Production of SOA can be observed during the mid-afternoon of each of those days, and significant transport of secondary material into the area occasionally occurs during the nighttime. The first days of July were characterized by a strong contribution of SOA during the daylight hours. Ozone and solar radiation (UV) are triggers of secondary organic aerosol production. Estimated SOA concentrations show the same behavior as the ozone and UV radiation during the day (Figure ). This qualitative agreement provides some additional confidence on the estimated concentrations. The first hours of August show a period of pollution transport to the area. SOA concentration increases and a peak in the ozone concentration is observed during the middle of the night. The effect of ambient

10 1 J. C. CABADA ET AL. Undenuded OC Q F (µg C/m 3 ) 1 (a) OC=.7*EC Undenuded EC, Q F (µg/m 3 ) Undenuded OC Q F Q B,F (µg C/m 3 ) 1 (b) OC=.3*EC Undenuded EC, Q F (µg/m 3 ) Undenuded OC Q F Q B,T (µg C/m 3 ) 1 (c) OC=1.*EC Undenuded EC, Q F (µg/m 3 ) Denuded In-situ analyzer OC (µg C/m 3 ) 1 (d) OC=1.9*EC Denuded In-situ analyzer EC (µg/m 3 ) Denuded sampler OC (µg C/m 3 ) 1 (e) OC=3.1*EC Denuded sampler EC (µg/m 3 ) Figure. Estimated carbonaceous concentrations influenced by primary emissions and SOA production for daily averaged and h measurements during the summer intensive at PAQS. Solid squares correspond to SOA-influenced concentrations and hollow triangles to primary-influenced concentrations. (a) Front quartz OC (Q F ) and EC concentrations. (b) Correcting the OC measurement with the backup quartz filter behind the front quartz (Q F Q B,F ). (c) Correcting the OC concentrations with the backup quartz filter behind the Teflon filter in the parallel undenuded line (Q F Q B,T ). (d) Denuded in situ analyzer. (e) Denuded sampler. temperature can also be observed as the temperature increases more SOA is produced in the area (Figure c). Temperature increases are associated with high-pressure systems over the Pittsburgh area during the summer. In general these periods provide the ideal conditions for the production of SOA, like stagnant air masses and high photochemical activity. Little correlation exists between relative humidity and SOA production (Figure d). For the summer intensive 1, in Pittsburgh low relative humidity periods are associated with high temperatures at the middle of the day, and high relative humidity periods correspond to fronts entering the area and lowering the SOA production. Figure 9 shows the average daily concentrations of the estimated SOA and primary OC for all high-resolution datasets analyzed. In order to make comparisons between the datasets, average concentrations for each are normalized to the daily average primary OC and SOA for the period. The diurnal variation of primary OC is relatively small because during the summer intensive 1 the majority of the OC is transported to the region from elsewhere (Cabada et al. b). SOA patterns

11 SECONDARY ORGANIC AEROSOL CONTRIBUTION TO PM.5 19 (a) SOA (µg C/m 3 ) (b) (c) (d) July August Figure 7. Estimated SOA concentrations based on the higher resolution measurements of OC and EC. (a) Undenuded sampler, Q F. (b) Undenuded sampler, Q F Q B,F. (c) Undenuded sampler, Q F Q B,T. (d) Concentrations from the denuded in situ analyzer. (Figure 9) show some differences among the datasets used. Overall, all samplers show a minimum concentration of SOA in the early morning hours (: 9: EST) and SOA increase as the photochemical activity increases during the day. The SOA production peak varies depending on the dataset analyzed, but it follows the ozone average daily peak occurring around 15: EST (Figure 9). All datasets suggest that a significant amount of SOA is due to long-range transport during the late afternoon and night hours. Minimum production of SOA coincides with periods where the ozone concentration is at its minimum (: : EST). The daily averaged SOA concentrations for all high-resolution datasets show high episodes of SOA formation in the middle of July and at the beginning on August (Figure 1). Practically all days during the summer intensive show some contribution of SOA material to the total OC concentrations. Table 5 summarizes the monthly averaged SOA contribution for all the high-resolution measurements. Estimates of SOA vary from 3% for the denuded in situ analyzer to a high of 5% for the undenuded sampler doing the correction to the front quartz artifact with the backup filter behind the Teflon filter. Error bars are estimated from the uncertainties in the linear fit (95% confidence level) from the point influenced by primary emissions determined for each dataset. Taking into account these uncertainties all methods agree in the average contribution of SOA around 35%.

12 15 J. C. CABADA ET AL. SOA (µg/m 3 ) July Ozone (a) August 1 1 O 3 (ppb) SOA (µg/m 3 ) 1 (b) July Solar Radiation August 3 1 Solar radiation SOA (µg/m 3 ) 1 (c) Temperature July Temperature (C) SOA (µg/m 3 ) 1 RH July (d) RH (%) Figure. Hourly SOA production patterns for various periods during the summer intensive 1 at PAQS. SOA concentrations calculated from the undenuded front quartz dataset (Q F ). (a) Ozone concentration and SOA production. (b) Solar radiation (UV) and SOA production. (c) Ambient temperature influences over the SOA production in Pittsburgh. (d) Relative humidity has a minor role on the production of SOA. SOA Based on Daily Results, Summer Intensive 1 Figure 11 shows the estimated organic carbon composition (primary and secondary) for all different carbonaceous concentrations datasets using daily concentrations averages to estimate the primary ratio and the primary OC intercept. The SOA concentration variation during the period is qualitatively similar to that estimated using the high-resolution measurements (Figure 1). Periods of high SOA production are evident in the middle of July and the beginning of August. Unlike the estimates of SOA from the high-resolution measurements, primary Table 5 Summer intensive average SOA fraction of the Pittsburgh organic aerosol for all high-resolution datasets ( h sampling times) Measurement type SOA (%) Undenuded, Q F ( h) 3± 1 Undenuded, Q F Q B,Q ( h) 7± 17 Undenuded, Q F Q B,T ( h) 3± 1 Denuded in situ analyzer, Q F,situ ( h) 9± 1 ratios and intercepts calculated with the daily averaged concentrations show a number of days when no SOA is produced. SOA is probably present in all days but the estimated primary ratio and intercept is probably too high for this dataset, so these results can be viewed as a lower bound for the SOA concentrations. On average from all the methods, % of the total OC concentrations are estimated to be SOA (Table ). All estimates agree within 5% from each other. The higher estimate is given from the denuded Table Summer intensive average SOA fraction of the Pittsburgh organic aerosol for all datasets ( h averaged concentrations) Measurement type SOA (%) Undenuded, Q F ( h) 19 ± 1 Undenuded, Q F Q B,Q ( h) ± 1 Undenuded, Q F Q B,T ( h) 3 ± 15 Denuded In-situ analyzer, Q F,situ ( h) ± 19 Denuder sampler ( h) 5 ± 1

13 SECONDARY ORGANIC AEROSOL CONTRIBUTION TO PM OC pri/avg. OC pri undenuded Q F and Q F Q B,F undenuded Q F Q B,T Denuded in-situ Time of Day (EST) (a) SOA/Avg. SOA undenuded Q F and Q F Q B,F undenuded Q F Q B,T Time of Day (EST) (b) Denuded in-situ O 3 (ppb) Ozone (c) NOx (ppb) 3 NOx (d) Time of Day (EST) Time of Day (EST) CO (ppm) CO (e) Time of Day (EST) Figure 9. Average daily pattern of SOA and primary OC concentrations (normalized) during the summer intensive at PAQS, plotted along with daily averaged concentrations of various atmospheric variables. (a) Daily pattern primary OC estimated from high-resolution samplers. (b) Daily pattern SOA estimated from high-resolution samplers (c) Ozone averaged daily concentrations. (d) NO x averaged daily concentrations. (e) CO averaged daily concentrations. sampler dataset, where SOA contributes 5%. The lower estimate is 1% from the undenuded front quartz dataset. Error bars are calculated from the uncertainties in the linear fitting of the point identified as influenced by primary emissions for each dataset. Figure 1 shows a scatter plot of the daily fraction of the different samplers for both high-resolution and daily-averaged samplers. For both cases all samplers agree within % on the estimated SOA fraction for each day. Higher differences are shown at the lower fractions. The reason for this could be that the lower fractions correspond to lower concentrations of OC, magnifying small variations over the SOA estimations. CONCLUSIONS AND DISCUSSION Application of the EC tracer method analysis to the different types of high-resolution measurements suggests an average of 35% SOA contribution to the monthly average OC concentration during the summer intensive of 1. Overall estimates range from a low of % to a high of 5%. A preliminary study trying to identify the sources of carbonaceous aerosol for western Pennsylvania estimated a SOA contribution of 3 5% to the total OC concentration during the summer of 1995 (Cabada et al. b). These previous results are in good agreement with the estimates that are obtained applying this new method, giving some confidence about the results.

14 15 J. C. CABADA ET AL. Figure 1. Daily-averaged SOA and primary OC concentrations during the summer intensive, estimated from the high-resolution measurements. (a) Undenuded sampler, Q F. (b) Undenuded sampler, Q F Q B,F. (c) Undenuded sampler, Q F Q B,T. (d) Concentrations from the denuded in situ analyzer. Higher time resolution measurements result in the highest estimation of SOA. Events that trigger SOA production have a strong diurnal dependence (i.e., ozone and sunlight daily cycles), so high-resolution measurements are more likely to identify periods of primary or secondary production dominance. The use of daily-averaged measurements probably tends to underpredict the SOA concentration, especially for relatively small datasets, because it may be impossible to find days without any SOA present. On average, the SOA concentrations are around 5 1% higher if high-resolution measurements are used compared to the daily-averaged concentrations. The EC tracer method is a simple approach for the determination of contribution of SOA to the total OC concentration measured in a sampling site. It relies on simultaneous measurements of gaseous pollutants that could be indicators of primary emissions or secondary aerosol production. The major weakness of the method is its reliance on the assumption of a constant primary OC/EC and constant b during the analysis period

15 SECONDARY ORGANIC AEROSOL CONTRIBUTION TO PM Figure 11. Daily-averaged SOA and primary OC concentrations during the summer intensive, estimated from the daily-averaged concentrations ( h averages). (a) Undenuded sampler, Q F. (b) Undenuded sampler, Q F Q B,F. (c) Undenuded sampler, Q F Q B,T. (d) Concentrations from the denuded in situ analyzer. (e) Concentrations from the denuder sampler.

16 15 J. C. CABADA ET AL. SOA, all other samplers (µg C/m 3 ) 1% % (a) % % % % % % % % % 1% SOA, all other samplers (µg C/m 3 ) 1% % % % % % (b) % % % % % 1% Undenuded SOA, Q F (µg C/m 3 ) Undenuded SOA, Q F (µg C/m 3 ) + % SOA, Undenuded Sampler (Q F -Q B,F ) % SOA, Undenuded Sampler (Q F -Q B,T ) % SOA, Denuded in-situ analyzer % SOA, Denuded sampler Figure 1. Correlation of daily percentage SOA contribution to the total OC for the different samplers for the summer intensive at PAQS for all datasets. Solid line represents the 1:1 fit and dashed lines are % error lines. (a) % SOA contributions from high-resolution measurements. (b) % SOA contributions from daily-averaged measurements. (the whole month, or the few hours of the measurement period). Variations of sources strengths, meteorology, etc. are expected to change the (OC/EC) p even for the same -h period from day to day. This variability introduces significant uncertainties (see Tables 5 and ). REFERENCES Birch, M. E., and Cary, R. A. (199). Elemental Carbon-Based Method for Monitoring Occupational Exposures to Particulate Diesel Exhaust, Aerosol Sci. Technol. 5:1 1. Birch, E. M. (199). Analysis of Carbonaceous Aerosols: Interlaboratory Comparison, Analyst. 13: Cabada, J. C., Pandis, S. N., Davidson, C. I., Robinson, A. L., Subramanian, R., Tang, W., and Raymond, T., (b). The Contribution of Long-Range Transport and Secondary Organic Aerosol to PM.5 in Pittsburgh. In DOE-NETL PM.5 and Electric Power Generation: Recent Findings and Implications. NETL Publications, Pittsburgh, PA. Cabada, J. C., Pandis, S. N., and Robinson, A. L. (a). Sources of Atmospheric Carbonaceous Particulate Matter in Pittsburgh, Pennsylvania, J. Air Waste Manag. Assoc. 5: Chow, J. C., Watson, J. G., Crow, D., Lowenthal, D. H., and Merrifield, T. (1). Comparison of IMPROVE and NIOSH Carbon Measurements, Aerosol Sci. Technol. 3:3 3. Countess, R. J. (199). Interlaboratory Analyses of Carbonaceous Aerosol Samples, Aerosol Sci. Technol. 1: Fitz, D. R. (199). Reduction of the Positive Artifact in Quartz Filters, Aerosol Sci. Technol. 1:1 1. Gray, H. A. (19). Ph.D. Thesis, California Institute of Technology, Pasadena, CA. Gray, H. A., Cass, G. R., Huntzicker, J. J., Heyerdahi, E. K., and Rau, J. A. (19). Characteristics of Atmospheric Organic and Elemental Carbon Particle Concentrations in Los Angeles, Environ. Sci. Technol. :5 59. Hering, S. V., Appel, B. R., Cheng, W., Salaymeh, F., Cadle, S. H., Mulawa, P. A., Cahill, T. A., Eldred, R. A., Surovik, M., Fitz, D., Howes, J. E., Knapp, K. T., Stockburger, L., Turpin, B. J., Huntzicker, J. J., Zhang, X. Q., and McMurry, P. H. (199). Comparison of Sampling Methods for Carbonaceous Aerosol in Ambient Air, Aerosol Sci. Technol. 1: 13. Hildemann, L. M., Cass, G. R., Mazurek, M. A., and Simoneit, B. R. T. (1993). Mathematical Modeling of Urban Organic Aerosol Properties Measured by High Resolution Chromatography, Environ. Sci. Technol. 7:5 55. Lim, H. J., and Turpin, B. J. (). Origins of Primary and Secondary Organic Aerosol in Atlanta: Results of Time-Resolved Measurements During the Atlanta Supersite Experiment, Environ. Sci. Technol. 3:9 9. NIOSH. (1999). Method 5 Issue 3 (Interim): Elemental Carbon (diesel exhaust). In NIOSH Manual of Analytical Methods. th edition, Paula Fey O Connor, ed., National Institute of Occupational Safety and Health, Cincinnati, OH. Pandis, S. N., Seinfeld, J. H., Harley, R., and Cass, G. (199). Secondary Organic Aerosol Formation and Transport, Atmos. Environ. A:9. Schauer, J. J., Rogge, W. F., Hildemann, L. M., Mazurek, M. A., Cass, G. R., and Simoneit, B. T. (199). Source Apportionment of Airborne Particulate Matter Using Organic Compounds as Tracers, Atmos. Environ. 3: Schmid, H., Laskus, L., Abraham, J. H., Baltensperger, U., Lavanchy, V., Bizjak, M., Burba, P., Cachier, H., Crow, D., Chow, J., Gnauk, T., Even, A., ten Brink, H. M., Giesen, K. P., Hitzenberger, R., Hueglin, C., Maenhaut, W., Pio, C., Carvalho, A., Putaud, J. P., and Toom-Sauntry D. (1). Results of the Carbon Conference International Aerosol Carbon Round Robin Test Stage I, Atmos. Environ. 35: Seinfeld, J. H., and Pandis, S. N. (199) Atmospheric Chemistry and Physics: From Air Pollution to Global Change, John Wiley and Sons Inc., New York. Strader, R., Lurmann, F., and Pandis, S. (1999). Evaluation of Secondary Organic Aerosol Formation in Winter, Atmos. Environ. 33:9 3. Subramanian, R., Khlystov, A., Cabada, J. C., and Robinson, A. L. (). Measurement of Positive and Negative Artifacts with Denuded and Undenuded Sampler Configurations, Aerosol Sci Technol. 3:7. Tolocka, M. P., Solomon, P. A., Mitchel, W., Norris, G. A., Gemmill, D. B., Weiner, R. W., Vanderpool, R. W., Homolya, J. B., and Rice, J. (1). East Versus West in the US: Chemical Characteristics of PM.5 During the Winter of 1999, Aerosol Sci. Technol. 3: 9. Turpin, B. J., and Huntzicker, J. J. (199). Investigation of Organic Aerosol Sampling Artifacts in the Los Angeles Basin, Atmos. Environ. : Turpin, B. J., and Huntzicker, J. J. (1995). Identification of Secondary Organic Aerosol Episodes and Quantitation of Primary and Secondary Organic Aerosol Concentrations During SCAQS, Atmos. Environ. 9:

17 SECONDARY ORGANIC AEROSOL CONTRIBUTION TO PM Turpin, B. J., Cary, R. A., and Huntzicker, J. J. (199). An In Situ, Time Resolved Analyzer for Aerosol Organic and Elemental Carbon, Aerosol Sci. Technol. 1: Turpin, B. J., Huntzicker, J. J., Larson, S. M., and Cass, G. R. (1991). Los Angeles Summer Midday Particulate Carbon: Primary and Secondary Aerosol, Environ. Sci. Technol. 5: Turpin, B. J., Saxena P., and Andrews, E. (). Measuring and Simulating Particle Organics in the Atmosphere: Problems and Prospects. Atmos. Environ. 3: Yu, J. Z., Xu, J., and Yang, H. (). Charring Characteristics of Atmospheric Organic Particulate Matter in Thermal Analysis, Environ. Sci. Technol. 3:75 71.