JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D18305, doi: /2006jd007215, 2006

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2006jd007215, 2006 Size-selective nonrefractory ambient aerosol measurements during the Particulate Matter Technology Assessment and Characterization Study New York 2004 Winter Intensive in New York City Silke Weimer, 1,2,3 Frank Drewnick, 4 Olga Hogrefe, 1 James J. Schwab, 1 Kevin Rhoads, 1,5 Douglas Orsini, 1,5 Manjula Canagaratna, 6 Douglas R. Worsnop, 6 and Kenneth L. Demerjian 1 Received 20 February 2006; revised 22 May 2006; accepted 8 June 2006; published 21 September [1] During the Particulate Matter Technology Assessment and Characterization Study New York (PMTACS-NY) in January/February 2004 a variety of modern aerosol measurement instruments were deployed at a field site on the campus of Queens College in Queens, New York. These instruments included the Aerodyne Aerosol Mass Spectrometer (Q-AMS), R&P Particulate Ambient Sulfate and Nitrate monitors (8400S&N), a Particle-into-liquid Sampler equipped with ion chromatographs (PILS-IC), an R&P FDMS TEOM, and several other techniques to measure physical aerosol properties. The PMTACS-NY 2004 winter campaign was designed to replicate measurements performed at this site during the PMTACS-NY 2001 summer campaign. The field campaign experienced significant meteorological variability and extreme weather conditions, which together with local and regional aerosol and precursor emissions generated a wide range of PM concentrations at the site, with significant changes in physical aerosol properties as well as aerosol composition as a function of particle size. Q-AMS quantification procedures and intercomparison studies of nonrefractory species mass concentrations measured with the Q-AMS and other colocated instruments (PILS-IC, R&P 8400 S/N, FDMS TEOM, and Sunset Labs OC Analyzer) are presented. The size-resolved aerosol composition over the measurement period and its variation with time of the day, regional meteorology and local source impacts are discussed. Results obtained during this field campaign are compared to data from the PMTACS-NY 2001 summer campaign, where aerosol composition was observed to have higher sulfate, significantly lower nitrate content and overall mean aerosol mass size distributions with larger mode diameters. Citation: Weimer, S., F. Drewnick, O. Hogrefe, J. J. Schwab, K. Rhoads, D. Orsini, M. Canagaratna, D. R. Worsnop, and K. L. Demerjian (2006), Size-selective nonrefractory ambient aerosol measurements during the Particulate Matter Technology Assessment and Characterization Study New York 2004 Winter Intensive in New York City, J. Geophys. Res., 111,, doi: /2006jd Introduction [2] An increasing number of clinical and epidemiological studies [e.g., National Research Council, 1998] associating adverse health effects in humans and fine particle mass (PM 2.5 ) provided the basis for the promulgation of a new 1 Atmospheric Sciences Research Center, State University of New York, Albany, New York, USA. 2 Now at Eidgenössische Materialprüfungs- und Forschungs-Anstalt, Dübendorf, Switzerland. 3 Also at Paul Scherrer Institute, Villigen, Switzerland. 4 Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany. 5 Also at Chemistry Department, Siena College, Loudonville, New York, USA. 6 Aerodyne Research, Inc., Billerica, Massachusetts, USA. Copyright 2006 by the American Geophysical Union /06/2006JD National Ambient Air Quality Standard (NAAQS) for PM 2.5 mass (15 mgm 3 annual and 65 mgm 3 24-hour average) in the United States [U.S. Environmental Protection Agency, 1997]. Additional studies since the establishment of the 1997 PM 2.5 standard have provided further evidence that adverse health outcomes are associated with PM exposure [Pope et al., 2002; Samet et al., 2000; Wichmann et al., 2000; Health Effects Institute, 2003]. Significant scientific and technical issues surround the characterization of the PM 2.5 /copollutant complex and the processes that drive fine PM formation and removal. The interdependences between PM 2.5 and O 3 air quality through coupled photochemical pathways and common precursors, as well as their seasonal/meteorological dependencies, must be addressed if effective control strategies are to be implemented to meet the PM 2.5 NAAQS. In addition particulate matter plays important roles in (1) climate change through its impact on direct and indirect radiative 1of17

2 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 forcing, (2) heterogeneous chemistry, (3) as well as cloud and fog formation [Andreae and Crutzen, 1997; Jacob, 2000]. This enhanced interest in ambient particulate matter has resulted in a considerable increase in aerosol research efforts and the development of improved aerosol measurement instrumentation. Nevertheless, many questions concerning particle formation, transport and transformation remain largely unanswered and available data on ambient aerosol mass and chemical composition as a function of particle size are not adequate [McMurry, 2000]. This is especially true for the organic aerosol fraction. [3] An important step toward the resolution of these questions is the development and characterization of adequate instrumentation that is capable of providing near realtime measurements of particle composition and chemically resolved size distributions, as well as enhanced information on organic aerosols. Traditionally off-line methods have been used for characterization of atmospheric aerosols based on filter and impactor sampling and subsequent analysis in the laboratory. These methods tend to suffer from low time resolution, long turnaround times, high postcollection analysis costs and sampling artifacts [Seinfeld and Pandis, 1998; McMurry, 2000]. [4] In recent years a significant improvement of chemical analysis of atmospheric aerosols was accomplished by the development of multiple online aerosol measurement techniques that provide chemical information on single aerosol particles [Hinz et al., 1994; Gard et al., 1997; Murphy and Thomson, 1997; Carson et al., 1997] or relatively small aerosol samples [Weber et al., 2001; Stolzenburg and Hering, 2000; Jayne et al., 2000]. One of the recently developed instruments is the Aerosol Mass Spectrometer (Q-AMS), developed and manufactured by Aerodyne Research, Inc. [Jayne et al., 2000; Jimenez et al., 2003]. The Q-AMS provides quantitative information on mass concentration and species-resolved mass size distributions for nonrefractory aerosol components like nitrate, sulfate, ammonium, chloride, and total organics in near real-time, using thermal evaporation of the particles and subsequent electron impact ionization and quadrupole mass spectrometry of the vapor. [5] The Q-AMS, together with other state-of-the-art online aerosol measurement instruments including the Rupprecht & Patashnick ambient sulfate and nitrate monitors (R&P 8400 S/N), the R&P Filter Dynamics Measurement System (FDMS TEOM), and the Particle-into-liquid sampler with ion chromatograph (PILS-IC) were deployed during the PM 2.5 Technology Assessment and Characterization Study New York (PMTACS-NY) in January/ February The PMTACS-NY program is one of several U.S. EPA Supersites intended to provide enhanced measurement data on chemical and physical parameters of the fine particulate matter and its associated precursors. One of the primary objectives of this study is to test and evaluate new measurement techniques for particulate matter through laboratory measurements and field intercomparison studies. In pursuit of this goal, a wide variety of state-of-the-art online and off-line techniques for physical and chemical aerosol analysis were deployed in common field intensives at Queens College in New York City during July 2001 and January/February The winter field campaign, the subject of this paper, was performed within 50 m of the location of the summer campaign [Drewnick et al., 2004a, 2004b]. Data from these two campaigns are compared to better understand summer/winter differences in ambient aerosol composition and size in New York City and the processes that affect those differences. [6] The paper briefly describes the operation of the Q-AMS during this campaign and data processing procedures applied to its data for calculation of mass concentrations from the Q-AMS raw data. The resulting Q-AMS aerosol mass concentration data and mass distribution measurements are presented and compared with measurements from other colocated instrumentation. Analyses of the winter aerosol composition are presented and these results are contrasted with those found in the summer campaign. 2. Q-AMS Instrument Description and Operation 2.1. Instrument Description [7] The Q-AMS has been described in detail in other publications [Jayne et al., 2000; Jimenez et al., 2003; Drewnick et al., 2004a], thus only a brief description will be given here. The vacuum system of the instrument consists of three major parts: an aerosol sampling chamber, a particle sizing chamber and an analysis chamber. The aerosol is introduced into the vacuum system through an aerodynamic particle beam-forming lens. The flow into the inlet system is set to 0.1 l min 1 by a 100 mm critical orifice before the lens. Particles in the range of approximately nm are focused with almost 100% efficiency into a narrow beam that passes a skimmer and the particle sizing chamber, before it impacts onto the vaporizer heated to approximately C. Nonrefractory particle components are flash vaporized and the generated vapor is ionized by electron impact (70 ev). Positive ions are extracted into a quadrupole mass spectrometer (QMS, Balzers QMG 422) for mass analysis and subsequent detection with a calibrated electron multiplier. [8] The AMS measures the vacuum aerodynamic diameter (d va ), defined as d va =(r p /c v r 0 )d ve,, where r p is the particle density, c v is the dynamic shape factor in the free molecular regime, r 0 is the unity density (=1 g cm 3 ), and d ve, is the volume-equivalent diameter [DeCarlo et al., 2004; Takegawa et al., 2005]. The aerodynamic lens [Zhang et al., 2002, 2004] transmits particles with 100% efficiencies in the d va range from 50 to 600 nm. Particle transmission of the lens drops off significantly for D va > 1 providing similar characteristics to PM 1 size selective inlets used in filter based measurements. [9] As in the summer campaign, the Q-AMS operated in two modes during the winter campaign, the Particle Timeof-Flight Mode (P-TOF) and the Mass Spectra Mode (MS). The P-TOF mode is used for the measurement of species resolved aerosol mass size distributions. For particle sizing a mechanical chopper wheel, which is situated right after the skimmer at the beginning of the particle sizing chamber (l = 39 cm), is moved into the particle beam to chop the beam with a frequency of about 120 Hz. The chopper wheel has two radial slits, covering 1% of the wheel area. An equivalent fraction of particles pass the chopper when the flight path is open, resulting in a common starting time of all particles at the position of the chopper. During the expansion of the sampled air into the vacuum, aerosol particles are 2of17

3 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 accelerated according to their aerodynamic properties and reach the vaporizer at different times. By setting the QMS on a fixed mass and measuring the time resolved ion signal, the particle velocities can be mapped out. The velocity distribution can be transformed into a particle size distribution for the selected species using a P-TOF calibration. [10] In the MS mode the average composition of the nonrefractory aerosol components is determined by scanning the complete mass spectrum (1 300 amu) with the QMS at a frequency of 3 Hz. In order to maximize particle transmission the chopper is moved completely out of the particle beam ( beam open position). The instrument background signal is measured routinely by moving the chopper wheel far into the particle beam to completely block it ( beam closed position). The difference of the signal at beam open and beam closed position is used for the calculation of aerosol mass concentrations for nonrefractory species that are extracted from the averaged mass spectra. [11] During the PMTACS-NY 2004 campaign, the Q-AMS was equipped with a particle beam width probe (BWP) [Huffman et al., 2005] to measure the width of the particle beam near the vaporizer. This experimental probe was introduced to explore possible sources of error in the quantification of the Q-AMS as a result of particle beam divergence. The BWP consists of a thin wire (here: d = 0.5 mm), mounted vertically on a mechanical device that is designed to precisely step it across the particle beam. The resulting attenuation of the particle signal measured for the individual wire positions yields information on the width of the beam in front of the vaporizer and hence information on losses of aerosol signal due to incomplete focusing into the particle beam of some or all of the aerosol components Field Operation During PMTACS-NY 2004 [12] During the PM 2.5 Technology Assessment and Characterization Study New York (PMTACS-NY) 2004 winter campaign the Q-AMS was operated at the Queens College measurement site in Queens, New York, from 6 January through 6 February The measurement site was located on campus of Queens College (40.74 N, W, 25 m a.m.s.l.), adjacent to parking field 6, which is approximately 300 m south of Long Island Expressway (I-495) and 1 km east of Van Wyck Expressway (I-678), two high-traffic highways in the New York City metropolitan area. [13] The Q-AMS was situated in a one-story building complex, together with several other aerosol instruments. The sampling inlet of the Q-AMS was at a height 6.5 m above ground level, 1 m above the roof of the building. Here, ambient air was sampled through a PM 2.5 cyclone (URG EN) and transported into the building to the Q-AMS using 14.1 mm ID copper tube. Inside the room the tube was thermally insulated with foam tube insulation to minimize heating of the aerosol while flowing to the Q-AMS inlet. The Q-AMS shared a common inlet line with the recently developed Aerodyne Time-of-Flight Aerosol Mass Spectrometer (TOF-AMS) [Drewnick et al., 2005]. The sampling tube itself was approximately 6 m of length. The tube diameter (ID 14.1 mm) was chosen to minimize losses by impaction and gravitational settling for the given flow rate. The total flow through the copper tube was 10 l min 1. From this flow 0.1 l min 1 was isokinetically extracted from the center of the tube by the Q-AMS and at a different position of the transport tube the same flow was diverted to the TOF-AMS. The rest of the flow (9.8 l min 1 ) was exhausted by a makeup flow pump. Particle losses in the sampling line for the relevant particle size range for Q-AMS measurements (i.e., from 20 nm to 1 mm), were estimated to be below 3%. [14] During the whole campaign the Q-AMS was operated in the alternate mode, where it automatically switches between the P-TOF and the MS mode. From 6 January until 14 January and from 4 February until 6 February the dwell time in each of the measurement modes was 20 s. Every 10 min the averaged mass spectrum and size distribution data were stored to disc. During the period 14 January to 4 February the BWP was operated on the Q-AMS to map out the particle beam width. The beam-attenuating wire was stepped between 5 locations within the beam as well as outside the particle beam to measure the unattenuated aerosol signal. The measurements altered in and out of the beam and measurement time for each wire position was 1 min, resulting in a 10 min BWP cycle time. Q-AMS switched between MS and P-TOF mode every 10 s, making three measurements in both modes within each of the 1-min averaging intervals. [15] To determine species-resolved size distributions from the P-TOF mode data, a select number of ion masses are chosen for which the time-resolved ion signal is measured. [16] The following m/z were selected in the P-TOF mode for the entire campaign: m/z 15 (NH + ) and 16 (NH + 2 ) for ammonium; m/z 18 (H 2 O + ) for particulate water; m/z 28 (N + 2 ) for monitoring the air signal intensity; m/z 30 (NO + ) and 46 (NO + 2 ) for nitrate; m/z 48 (SO + ) and 64 (SO + 2 ) for sulfate; and m/z 43, 44, 55, 57, 69, and 71 for various fragments of organic particle components, including markers for hydrocarbon-like (m/z 57) and oxygenated (m/z 44) organics. [17] Quality assurance procedures performed in accordance with the standard operation procedure (SOP) for the instrument included: routine checks and calibrations of the electron multiplier (at least every fourth day); periodic calibrations of the ionization efficiency (6 calibrations); calibration of the P-TOF particle vacuum aerodynamic diameter conversion; and calibration and check of the inlet flow rate into the instrument. Further detail regarding the calibration procedures is provided by Drewnick et al. [2004a]. In addition the inlet flow rate as well as the multiplier gain were continuously monitored by the Q-AMS Data Processing [18] The PMTACS-NY 2004 data were evaluated using the AMS Analysis Toolkit, developed by Allan et al. at University of Manchester, UK, that calculates mass concentrations or size distributions from the Q-AMS raw data using the calibrations performed during the campaign and a well defined methodology for extracting species mass concentrations from the mass spectra that accounts for the fragmentation patterns of the individual species as well as for isotope ratios [Allan et al., 2004a]. [19] During most of the campaign Q-AMS data were collected in 1-min intervals to allow for fast scanning of the particle beam with the beam width probe (BWP). The 3of17

4 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 processing of BWP data was performed independent of the other data. The calculation of particle beam width from the attenuation information at the individual BWP positions is used to determine the collection efficiency as a consequence of particle beam broadening. In this application the BWP data were averaged over 4 hours to increase the signal-to-noise ratio. The beam width was determined from the attenuation data by calculating the two-dimensional Gaussian shaped particle beam density function that fits the measured data best, using the BWP Toolbox, a separate software package for beam width probe data analysis [Huffman et al., 2005]. [20] During the BWP experiments, only the data with the BWP positioned outside the particle beam were used in the calculation of 10-min average mass concentration and size distribution data for the individual species. Overall min cycles were measured during the campaign. This represents a data capture of 97% during the PMTACS-NY 2004 experiment. Instrument calibration and maintenance accounted for the remaining 3% of lost cycles. 3. Results and Discussion 3.1. Quantification of the Q-AMS Measurements [21] The determination of species concentrations from Q-AMS mass spectra depends primarily on the calibration of Q-AMS ionization efficiency (IE). The IE is calibrated routinely using particulate ammonium nitrate of known particle diameter and is based on the most intense NO 3 fragments at m/z 30 (NO + ) and 46 (NO 2 + ). The IEs for other species are determined on the basis of Relative Ionization Efficiency (RIE) factors. The RIE is the ratio of the electron impact ionization efficiency (IE x ) of a given species x to the calibrated ionization efficiency for nitrate (IE NO3 ) on a per unit mass basis. For this study, RIE factors of 4 for ammonium, 1.2 for sulfate, 1.4 for organics and 1.1 for nitrate have been used. The 1.1 RIE factor for nitrate accounts for signals from less intense fragments of nitrate that are not taken into account in the calibration. These factors are empirically determined on the basis of previous Q-AMS measurements by many groups in the lab and in field applications [Jimenez et al., 2003; Alfarra et al., 2004]. [22] Field and laboratory measurement comparisons of Q-AMS mass concentrations for NO 3 and SO 4 with those obtained from other instruments indicate that a Collection Efficiency (CE) factor is needed to account for the fact that not all the nonrefractory PM 2.5 mass is measured by the Q-AMS. Three possible contributions to the CE have been suggested: (1) beam broadening by nonspherical particles results in the incomplete collection of the diffuse particle beam at the oven, which is addressed in this study and shown not to be a contributing factor; (2) a fraction of the particles bounce upon impacting the oven and thus are not vaporized and detected; and (3) the Q-AMS lens transmits particles in the 30 nm to 1 mm size range, but the field measure comparisons are with instruments that have 2.5 mm size cut inlets, thus contributing to mass differences as a result of different size ranges sampled. [23] The determination of the CE of 0.42 for the PMTACS-NY 2004 Q-AMS data set is based on a comparison of the Q-AMS and the Particle-into-liquid sampler with IC (PILS-IC) [Weber et al., 2001; Orsini et al., 2003] sulfate mass concentrations for the period 23 January to 31 January This CE is quite similar to the CE of 0.43 determined for the PMTACS NY 2001 summer campaign also by comparison of Q-AMS sulfate data with PILC-IC data [Drewnick et al., 2004a]. For the last week of the campaign, 31 January to 5 February, a new CE factor had to be calculated as a result of a misalignment of the Q-AMS aerodynamic lens that occurred when the system was accidentally jarred. The revised CE of 0.34 for this period was also based on comparison with PILS-IC sulfate data. The CE factors for the respective periods were applied to NH 4 +, NO 3, SO 4 2, organics and chloride data assuming internally mixed particles. During the winter campaign the mass size distributions of the individual species suggest that for some periods a partial external mixture of nitrate may be present with other species. According to laboratory measurements this could affect particle bounce and consequently the CE. Applying a higher CE to periods of this data set that were suggesting externally mixed nitrate particles did not yield the expected better comparison to other instruments. Therefore we used a constant CE as explained above. [24] Limits of Detection (LOD) of the Q-AMS are evaluated on the basis of Q-AMS background signals from 10 min data for each of the respective species. LODs are defined as 3 times the standard deviation of the background (beam closed) signals for 10 consecutive data points with the lowest concentrations. The 10-min LODs of sulfate, ammonium, nitrate, chloride and total nonrefractory organics during this study are estimated to be 0.07, 0.11, 0.01, 1.04 and 0.3 mg m 3, respectively. [25] Time series plots for sulfate (Figure 1, top) and nitrate (Figure 1, bottom) measured with the Q-AMS are shown in Figure 1 together with measurements performed with other colocated instruments. Sulfate was also measured with the PILS-IC and the R&P 8400S ambient sulfate monitor (Rupprecht & Patashnick Co., Inc., Albany, New York); nitrate was measured with the PILS-IC and with the R&P 8400 N ambient nitrate monitor. All four instruments were located in the same building with sampling inlets at similar heights within a distance of less than 5 m from each other. Each of the sampling inlets was equipped with a PM 2.5 cyclone to remove all particles larger than 2.5 mm in diameter. While PILS-IC and the R&P 8400 instruments report PM 2.5, the Q-AMS measures an aerosol fraction close to PM 1.0 due to the 1 mm cutoff of the Q-AMS instrument inlet assembly. Although it is conceivable that the aerosol chemical composition within the mm diameter range may be different than that for aerosol diameters 1 mm, previous measurements in New York city [Drewnick et al., 2003, 2004a] do not suggest significant variation in composition between the two size regions. Unlike recent results report on the ACE-Asia field campaign which report compositional difference in the PM 1 PM 2.5 fraction possible perturbed by mineral dust in the region [Topping et al., 2004]. [26] As shown in Figure 1 the sulfate and nitrate mass concentration measurements from the individual instruments have similar time trends for most of the time intervals during the campaign. Sulfate measured by the Q-AMS tracks the R&P 8400S data very well through the January period. Starting on 30 January and until the end of the field experiment, R&P 8400S sulfate measurements reported 4of17

5 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 Figure 1. One hour time series of mass concentrations of (top) sulfate and (bottom) nitrate. Shown are mass concentrations for PILS-IC, R&P 8400 instruments and Q-AMS. consistently higher values than those from the Q-AMS. While Q-AMS (and R&P 8400S) sulfate does not agree with sulfate mass concentrations measured with the PILS-IC for the time interval before 22 January, it agrees well with the data measured after this time, including the last week of the field measurement, where Q-AMS sulfate is lower than R&P 8400S sulfate. The PILS-IC measurements prior to 22 January were sufficiently noisy in comparison to previous applications that a modification of the PILS-IC system was made in the field (20 22 January). This included replacing the IC preconcentration columns with conventional sampling loops, the addition of a carbon denuder in series with the carbonate and citric acid denuders and moving the denuder cluster to the outside of the building. The introduction of the sampling loops was considered to eliminate some nonlinearity observed in the cation calibration curves with the preconcentration column; and the introduction of the carbon denuder was to improve the scrubbing efficiency of acid and basic gases. Post analyses of the PILS data indicate that premodification measurements were likely experiencing artifact sulfate and nitrite formation in the PILS as a result of ineffective scrubbing of NO 2 and SO 2 by the denuders, possibly as the result of the extreme cold ambient temperatures experienced during this measurement period. [27] Overall the relative time series trends of the four instruments are quite consistent with some systematic biases observed between instruments that will be discussed in a later section. One particularly interesting observation in the nitrate data (Figure 1, bottom) are the systematically high Q-AMS values during nitrate episodic events as compared to PILS and R&P measurements. While the general nitrate mass concentration trends measured by all three instruments are very similar, the R&P 8400N tends to measure slightly lower mass concentrations than the PILS-IC, the Q-AMS and PILS-IC nitrate show very similar mass concentrations for most of the measurement period. However, during these spiked high-concentration nitrate events, Q-AMS nitrate values are significantly above that measured with the PILS-IC and R&P 8400N instruments. The observed instrument behavior during the nitrate events suggests a change in aerosol properties may be occurring that results in a change in the detection efficiency for nitrate in one of the instruments. Since PILS-IC and R&P 8400N nitrate concentration measurements are in general agreement during most of these events, it is assumed that Q-AMS was the source of the difference and that difference relates to changes in the Q-AMS collection efficiency of aerosol nitrate due to changes in aerosol properties during these events. The Q-AMS nitrate measurements are discussed in further detail in a results section to follow BWP Experiments [28] As mentioned earlier a contributing component to the Q-AMS collection efficiency factor (CE) may be the result of incomplete focusing of the particle beam and can be estimated using the BWP data analysis [Huffman et al., 2005]. Typically under field measurement conditions a CE factor in the order of was applied to the data by a several groups using the Q-AMS [Alfarra et al., 2004; Allan et al., 2003, 2004b; Drewnick et al., 2004a]. Laboratory measurements suggest that reduced particle beam focusing only contributes significantly to CE for extremely irregularly shaped particles like fresh diesel soot [Huffman et al., 2005; Slowik et al., 2004]. Besides the contribution of the inlet transmission range, the remaining contribution to the CE, particle bounce to date remains unresolved on the basis of first principles, but quantification of the Q-AMS is 5of17

6 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 Figure 2. Four hour averaged time series of the particle beam width and the collection efficiency (CE) due to beam broadening, shown for 23 January until 31 January. typically determined by comparison of a specific species concentration (e.g., sulfate) from the Q-AMS with mass concentrations of this species measured by a different method. [29] In this study the contribution of particle beam broadening to collection efficiency was determined for every 4-hour period using the 1-min data collected with the beam width probe scanning the particle beam to measure its width. For this calculation all data collected with the BWP located at the same position were averaged for the 4-hour period. The particle beam width is determined on the basis of a best fit of a 2-D Gaussian distribution of particle density within the beam to the measured attenuations at the individual BWP positions. Time series of the particle beam width for the period 23 January until 1 February, determined from the BWP data for nitrate and sulfate individually are shown in Figure 2. Also shown in Figure 2 are the calculated collection efficiencies due to beam broadening as a function of time. For some periods the calculated CE for nitrate is slightly lower than for sulfate. This happens to be during low nitrate concentrations and is within the error bars. These time series show that for all times, including the time intervals where the differences between Q-AMS and PILS-IC nitrate concentrations were found, the particle beam was narrow enough to allow almost all particles to reach the vaporizer surface. These results indicate that beam broadening as a consequence of nonspherical particle shapes did not contribute significantly to CE for any time during this campaign. It also suggests that BWP measurements are likely unnecessary in most Q-AMS field measurement applications Intercomparison of Colocated Instruments [30] One of the main objectives of the PMTACS-NY study is instrument testing and evaluation. The performance evaluation of the Q-AMS, performed during the summer 2001 field campaign [Drewnick et al., 2004a] was extended as part of the winter 2004 campaign considering a similar complement of colocated instruments for comparison purposes. Q-AMS nitrate and sulfate mass concentration measurements were compared to PILS-IC and R&P 8400N and 8400S (Ruprecht & Patashnick, Albany, New York, USA) nitrate and sulfate concentration measurements. All instruments were located in the same building with sampling inlets within 5 m of each other. In addition, Q-AMS total organics mass concentrations were compared to organic carbon aerosol measurements using a Sunset Labs Carbon Analyzer (Sunset Laboratory, Tigard, Oregon), also located in the same building with an inlet approximately 15 m from the Q-AMS inlet. The Sunset Labs instrument used the same sampling setup as that used for the PILS-IC system and incorporated a PM 2.5 Very Sharp Cut Cyclone (VSCC) (BGI inc., Waltham, Massachusetts) inlet to size segregate the aerosol. The carbon analyzer was setup to report organic and elemental carbon concentrations every hour on the basis of a 44 min sample collection period followed by a 16 min analysis period. In addition the Q-AMS total nonrefractory aerosol mass concentrations (PM 1.0 ) were calculated by adding nitrate, sulfate, ammonium, total organics and chloride mass concentrations. These concentrations were compared to direct measurements of PM 2.5 mass concentrations, measured with an FDMS TEOM (Filter Dynamics Measurement System Tapered Element Oscillating Microbalance, Ruprecht & Patashnick, Albany, New York, USA), which was located within the same building complex housing the other instruments with an inlet located approximately 30 m from the Q-AMS inlet. EPA Speciation Trends Network (STN) filters sampled by an R&P Partisol Model 2300 Chemical Speciation Sampler (24-hour sampler) were located approximately 100 m from the Q-AMS on the roof of another building. Time series of Q-AMS nitrate, sulfate, 6of17

7 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 Figure 3. Comparison of one hour averaged Q-AMS mass concentration time series of (a) total (=nitrate + sulfate + ammonium + chloride + organics) to FDMS, (b) organics to OC Sunset Labs, (c) sulfate to 8400S, and (d) nitrate to 8400N. organics, and total nonrefractory mass concentrations together with corresponding data measured with the other instruments are presented in Figure 3. These time series show that there is a strong correlation between the species mass concentrations measured with the Q-AMS and the species concentrations measured with the other instruments. The general trends of the aerosol mass concentrations are reproduced by each pair of instruments. However, total organics and nitrate mass concentrations indicate significant differences in events with high mass loadings. The Q-AMS sulfate mass concentrations compare well with those of the R&P 8400 S over the entire period. Quantitative instrument comparisons were performed for Q-AMS and R&P 8400 S/N sulfate and nitrate, Sunset Labs organic carbon and FDMS total mass. Table 1 summarizes the linear correlation results for the paired instruments with the slope, intercept and R 2 for the linear regressions as well as the recovery (linear regression forced to a zero intercept) presented. With the exception of the 24-hour integrated filter based measurements all other correlations are based on hourly averaged data. [31] The measurement comparisons between Q-AMS sulfate and the R&P 8400 S sulfate monitor and the 24-hour filter based measurements are quite similar to those reported Table 1. Summary of Correlation Statistics of Q-AMS Mass Species Concentration Versus Colocated Instrument Data From 6 January to 5 February R 2 Slope Intercept, mg m 3 Recovery Sulfate Q-AMS versus 8400S Q-AMS versus 24 hour filter STN Nitrate Q-AMS versus 8400N Q-AMS versus 24 hour filter STN Organics Q-AMS Organics versus Sunset Labs OC Q-AMS Organics versus 24 hour filter STN OC Total Q-AMS versus FDMS Q-AMS versus 24 hour filter STN of17

8 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 for the 2001 summer campaign [Drewnick et al., 2003], with somewhat higher recoveries reported in the winter campaign for the filter based measurements (0.85 for summer versus 0.99 for winter). [32] The correlations of Q-AMS nitrate measurements with both the R&P 8400N and 24-hour filter data show generally higher Q-AMS concentrations than those of the other instruments with reported recoveries of 1.68 and 1.16 for the respective methods, suggesting significant losses of nitrate mass by these methods. These results are consistent with previous summer nitrate measurements [Hogrefe et al., 2004] that reported recoveries of 1.4 for the Q-AMS compared to the R&P nitrate monitor. It should also be noted that an increased temperature difference between ambient air and instrument temperature is expected to result in an increase of the Q-AMS-to-R&P 8400 N nitrate ratio as a consequence of increased evaporation from the sampling strip of the R&P instrument. [33] Another possible explanation for the positive bias in the Q-AMS nitrate measurements relates to the observed Q-AMS spiked nitrate concentrations (Figure 1), which are not observed by the other methods. It seems that these spikes are associated with the generation of new NH 4 NO 3 particles. These particles are likely formed by the reaction of ammonia and nitric acid condensed on preexisting small particles that remain externally mixed in the ambient aerosol. If so, laboratory data suggest that such particles have increased collection efficiency as a result of reduced particle bounce from the Q-AMS vaporizer. [34] The linear correlation of Q-AMS organics and OC measured with the Sunset Labs (SSL) instrument reports a correlation coefficient of R 2 = 0.66, with a slope of 3.28 and a rather large intercept of 4.18 mg m 3. The recovery for the direct comparison of the two instruments is It has been suggested by Zhang et al. [2005a, 2005b], Turpin and Lim [2001] and others that nominal particulate organic mass to carbon ratios will fall in the range 1.2 to 2.2 and that the upper end of this range is most likely to be associated with highly oxidized organic aerosol products. This is somewhat inconsistent with our winter time observation where photochemical oxidation is known to be very low [Ren et al., 2006]. Although a similar measurement comparison in Pittsburgh [Zhang et al., 2005a] reported a slope of 1.69 with an R 2 = 0.88, these measurements were performed during summer time. We believe the large differences in the recoveries for both the semicontinuous SSL and 24-hour filter based measurements compared to the Q-AMS organics maybe are related to evaporative losses experienced by these methods, especially for samples collected during these extreme cold winter conditions, but not experienced by the Q-AMS [Pang et al., 2002; Schwab et al., 2006]. [35] The last entry in Table 1 reports on the comparison of total Q-AMS nonrefractory mass (calculated as the sum of nitrate, sulfate, ammonium, organics (Org), and chloride) with total mass concentrations measured with a FDMS TEOM. The linear fit shows a correlation coefficient of R 2 = 0.85, a slope of 1.12 with a negative intercept of 1.28 mgm 3 and a recovery of 1.1, indicating that the Q-AMS measured mass is on average 10% higher than that of the FDMS TEOM measurement. It should be noted that total nonrefractory Q-AMS mass concentrations do not include metals, sea salt, (nonrefractory) carbonaceous material or Table 2. Data Summary of the PMTACS-NY 2004 Data, Obtained With the Q-AMS mg m 3 Nitrate Sulfate Ammonium Organics Chloride Total Mean Median Minimum <LOD <LOD <LOD 1.22 Maximum black carbon which, on the basis of comparable filter based measurements of these components, are likely to contribute of the order of 5% of the fine particulate aerosol mass. As mentioned earlier the Q-AMS measures only PM 1.0, whereas the FDMS TEOM measured PM 2.5. However, we also note that the mass differences reported between these instruments are well within the measurement uncertainties of the methods. In addition, it is likely that these differences will vary with season and aerosol composition. Regarding aerosol composition, for most of the periods for which Q-AMS overestimates FDMS measurements, the Q-AMS also shows higher values for nitrate concentrations compared to the 8400N measurements. Excluding the data for highnitrate periods in the correlation yielded a recovery of 0.96 with a R 2 of It seems that Q-AMS measurements are consistent with FDMS for periods with lower nitrate concentrations PMTACS-NY 2004 Data Set [36] A statistical summary of measured mass concentrations for nitrate, ammonium, sulfate, total nonrefractory organics, and chloride is shown in Table 2. During the PMTACS-NY 2004 winter campaign average measured nitrate mass concentrations were approximately the same as sulfate mass concentrations, with average organic mass concentrations significantly higher than those of sulfate and nitrate. The high nitrate and organic mass concentrations are in part likely due to the very low temperatures recorded during the field campaign that resulted in their enhanced partitioning to the condensed phase. Observed chloride mass concentrations are very low and included only nonrefractory chloride compounds, excluding sodium chloride from sea salt aerosol because of its negligible vaporization at the vaporizer temperature. [37] Q-AMS mass concentration time series for all measured species over the entire campaign are shown in Figure 4. As these time series show, the trends in the aerosol composition are dominated by clean and polluted episodes lasting several days in duration and with limited diurnal variation. The pollution episodes are mainly characterized by south-southwesterly wind flows under somewhat stagnant conditions, suggesting a regional contribution to these aerosol episodes with a significant local component under low wind conditions. [38] Analyzing diurnal patterns of aerosol mass concentrations and gaseous precursors is useful in understanding the contributions of local and regional emissions and secondary production pathways to observed ambient concentrations. Diurnal cycles of ambient concentrations are affected by (1) emissions, (2) boundary layer dynamics, and (3) photochemical activity. Atmospheric transport can also be associated with diurnal circulations (e.g., land-sea breeze or orographic winds), but these are the exception rather than 8of17

9 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 Figure 4. Q-AMS one hour averaged mass concentration time series of nitrate, sulfate, ammonium, organics and chloride for PMTACS-NY 2004 for the period 6 January until 6 February the rule. In many instances, diurnal patterns of concentration are transported into the region from upwind sources, introducing lagged diurnal patterns that can confound interpretation of local patterns. In major metropolitan areas with high emission densities over large areas such as in New York City, the diurnal patterns driven by these three components remain distinct even though substantial contributions from regional transport can and do affect the measured concentrations in the region. In the case of the winter time measurements presented here, the diurnal patterns are dominated by local emissions and boundary layer dynamics. Local photochemical production cycles are generally unresolved, since wintertime photochemical transformation processes are significantly reduced in northern climates because of low solar irradiances. However, it should be noted that regional air masses are capable of accumulating secondary oxidation products with time and transporting those products into the down wind regions. [39] Boxplots of the diurnal distribution of sulfate, nitrate, and organics concentrations presented in Figure 5 report the mean and median concentrations as a cross and horizontal bar respectively, the box covering the 25% and 75% percentile, and the whiskers, the 5% and 95% percentile concentrations, respectively. [40] The nitrate particle concentration (Figure 5a) has a clear diurnal pattern with maximum concentrations in the morning hours ranging from 0700 to 1000 LT followed by a continuous decline reaching a minimum concentration at 1600 to 1800 LT. The peak in the morning nitrate pattern is consistent with NH 3 measurements at this site [Li et al., 2006] suggesting that entrained HNO 3 formed aloft during night from the reaction of N 2 O 5 +H 2 O, reacts with surface NH3 emissions from mobile sources resulting in the morning product NH 4 NO 3. Although there is also an afternoon NH 3 traffic peak, the HNO3 reservoir no longer exists and only a slight perturbation in NH 4 NO 3 is observed. Sulfate mass concentrations also show a clear diurnal pattern with a broad maximum ranging from 0700 to 1200 LT (Figure 5b). This is consistent with the regional character of sulfate and the entrainment of air aloft with the rising boundary layer during this period. The diurnal pattern of total organics mass concentrations is less pronounced, showing a gradual increase in the morning from 0700 to 1100 LT (Figure 5c) and another weak maximum is found around 1700 to 2000 LT, during the evening rush hour. During the first 3 weeks of the field measurement campaign (9 29 January) Queens College was not in session and traffic activity associated with the adjacent parking lot where the sampling site was located was negligible. A comparison of the diurnal concentrations of Q-AMS organic aerosol for weekdays when Queens College was out of session and in session is shown in Figure 6. The high in-session concentrations during the morning hours (9 11) as well as the high levels through the afternoon and evening hours is consistent with the traffic activity patterns expected for the scheduled class for this commuting campus. Very similar patterns for NOx (not shown) were also observed. In addition, Li et al. [2006] have observed similar diurnal patterns of NH 3 and CO 2 concentrations from collocated measurements at this site reflecting similar differences in traffic activity patterns for sessions. [41] Further analysis of the aerosol measurements during PMTACS-NY 2004 considered separating the data set in clean and polluted periods on the basis of total PM 2.5 mass loadings. PM 2.5 mass loadings greater than or equal to 15 mg m 3 were binned in the polluted classification and mass loadings less than or equal to 5 mgm 3 were binned in the clean classification. Intermediate concentration values between 5 and 15 mg m 3 were not considered in this analysis. Clean and polluted periods accounted for 21% and 31% of the total measured data, respectively. [42] The analysis of clean and polluted periods is intended to provide insight into the contributions of local and regionally transported PM 2.5 species to the air quality in the New York City metropolitan area. Low mass loadings characterize the clean regional background and local source contributions. Higher mass loadings are typically associated with large-scale stagnations and are influenced by significant contributions from regional transport as well as local primary 9of17

10 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 Figure 5. Diurnal pattern of the mass concentrations of (a) nitrate, (b) sulfate, and (c) organics averaged over entire campaign. The upper and lower limits of the boxes indicate the 75th and the 25th percentile; the line within the box marks the median; and the whiskers above and below the box indicate the 90th and 10th percentiles, respectively. Crosses represent the mean. 10 of 17

11 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 Figure 6. Queens College Diurnal patterns of Q-AMS organic aerosol for weekdays during school in session and school out of session. particulate and precursor emissions leading to the production of secondary nitrates, sulfates and organics. [43] Figure 7 shows the average composition of the aerosol from nitrate, sulfate, ammonium and total nonrefractory organics for the clean and polluted periods. The average total Q-AMS mass concentrations for the clean and polluted classifications are 3.4 mg m 3 and 24.3 mg m 3, respectively. A comparison of the changes in fractional contributions in species composition between clean and polluted periods indicates similar percentage contributions in species composition, with the exception of the nitrates, which are enhanced during the polluted periods. Time series of NO x and CO concentrations with wind direction and the Q-AMS sulfate, nitrate and the markers for hydrocarbon-like (m/z 57) and oxygenated (m/z 44) organics concentrations, presented in Figure 8 indicate substantially higher pollution levels associated with winds from the south-southwest, flows impacted by high emission density regions within and adjacent to the metropolitan area. The high correlation of NO x with Q-AMS PM organic and m/z 57 fragment, numbers shown in Table 2, suggests that a significant portion of the PM organic is local and primary in origin. The m/z 57 fragment has been shown to be a dominant mass spectral feature of hydrocarbon-like organic aerosol typically found in primary exhaust emissions of mobile sources [Zhang et al., 2005b] as well as a portion of the m/z 44 fragment. Under wintertime conditions, the m/z 44 contributions from photochemical oxidation reactions forming oxygenated organic aerosols are significantly attenuated and as a result the two fragments show very similar patterns, in contrast to summertime observations. [44] The correlation of NO x with Q-AMS nitrate (Table 3) is also significant, but shows more variance (R 2 = 0.33), likely the result of the variability in the formation pathways of HNO 3 via the formation of N 2 O 5 and its hydrolysis and the temperature sensitive NH 3 + HNO 3 $ NH 4 NO 3 equilibrium reaction. Since the oxidation of NO x to nitric acid is very fast as compared to the sulfate generation reactions, the combination of low OH concentrations due to wintertime solar conditions minimize regional sulfate contributions to the overall PM 2.5 mass while low temperatures enhance nitrate contributions. The clean versus polluted periods reported mean temperatures of 6.9 C and 1.1 C, respectively, which will likely be a minor factor in the nitrate partitioning for the two periods. What is particularly significant is the major repartitioning that has taken place be- 11 of 17

12 WEIMER ET AL.: AEROSOL COMPOSITION IN PMTACS-NY 2004 Figure 7. Average composition of the aerosol (a) for clean and (b) for polluted periods (values in mg m 3 and wt%). tween sulfate and nitrate in comparing the winter clean versus polluted air masses, where the sulfate/nitrate mass ratio goes from 2.4 to 0.8 for clean versus polluted periods, respectively. No significant changes in the relative contribution of total nonrefractory organics to the aerosol are found between clean and polluted time periods. In both cases the organic aerosol fraction accounts for almost half of the total measured mass concentration. [45] Averaged mass size distributions for nitrate, sulfate and total organics are shown for the clean and polluted time intervals independently in Figures 9a and 9b. These distributions show mean modes in the range of d va = 150 to 240 nm for Figure 8. Times series of NOx, CO, Q-AMS sulfate, Q-AMS nitrate, m/z 57, m/z 44 and wind direction. 12 of 17