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Ultrafine nitrate particle events in Baltimore observed by real-time single particle mass spectrometry
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Atmospheric Environment 38 (2004) 3215–3223
Ultrafine nitrate particle events in Baltimore observed by real-time single particle mass spectrometry Michael P. Tolockaa, Derek A. Lakea, Murray V. Johnstona,b,*, Anthony S. Wexlera a
b
Department of Chemistry and Biochemistry, 107 Brown Lab, University of Delaware, Newark, DE 19716, USA Departments of Mechanical and Aeronautical Engineering, Civil and Environmental Engineering, and Land, Air and Water Resources, University of California, Davis, CA 95616, USA
Abstract Ambient particles in Baltimore, Maryland were characterized from April through November 2002 using the real-time single particle mass spectrometer, RSMS III. When particles containing nitrate were examined, two types of ultrafine particle events were revealed: a large burst of nominally ‘‘pure’’ nitrate particles in the 50–90 nm size range, and a smaller (and less frequent) burst of ‘‘pure’’ particles in the 50–90 nm size range that grew to 110–220 nm with time. Coincident with both of these events was an increase in the number of mixed composition particles containing nitrate, suggesting that they were formed by condensation of ammonium nitrate onto pre-existing particles. Meteorological variables, particle number concentrations and continuous nitrate mass measurements were compared to the single particle data. Number and mass concentrations estimated from RSMS III correlated well with similar measurements with other techniques. Ultrafine nitrate particle events were observed during periods of low temperature and high relative humidity as expected from ammonium nitrate equilibrium considerations. During these events, the partitioning of ammonium nitrate to the particle phase strongly influenced the particle number concentration as well as the chemical composition. r 2004 Elsevier Ltd. All rights reserved. Keywords: Ambient aerosol nitrate mass spectrometry
1. Introduction Inorganic ions can comprise a majority of the fine particle mass with the predominant species being ammonium, sulfate and nitrate (Tolocka et al., 2001). The optical, deliquescent and reactive properties are dependent upon how much of each species is present in the aerosol (Hand et al., 2002; Jang et al., 2002; Lightstone et al., 2000). These properties determine the ability of the aerosol to scatter light, act as cloud condensation nuclei (Cruz and Pandis, 1998), and behave as catalysts for organic reactions. Because of *Corresponding author. Tel.: +1-302-831-8014; fax: +1-302831-6335. E-mail address: [email protected] (M.V. Johnston).
their importance in the cycling of ions in the atmosphere, interactions of precursor gases and particles have been extensively modeled (Ansari and Pandis, 1999; Campbell et al., 2002; Clegg et al., 1998a,b; Moya et al., 2002). Two fine particle modes are observed for inorganic components in urban air (John et al., 1990; Wall et al., 1988). The smaller size mode around 100–200 nm in diameter results from condensation of a variety of gasphase species, including ammonium nitrate, onto preexisting particles of both primary and secondary origin. The chemistry of ammonium nitrate in this mode has been well studied (Cheung et al., 2000; Jacob, 2000; Orel and Seinfeld, 1977; Pun and Seigneur, 2001; Wexler and Seinfeld, 1990). The larger size mode around 700 nm in diameter is both a product of in situ droplet phase
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.03.011
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sulfate production and growth of smaller particles through condensation (Kerminen and Wexler, 1995). Recently, ultrafine particle events have been observed in Atlanta, Houston, Pittsburgh, and St. Louis as part of the US EPA Supersite Programs. These events are characterized by elevated concentrations of particles in the 3–45 nm size range, and are distinguished by their diurnal and seasonal pattern (Woo et al., 2001). While diurnal patterns of nitrate have been observed in Atlanta (Jimenez et al., 2003; Lee et al., 2003), the role that nitrate plays in ultrafine particle events is unknown. Current regulations for particulate matter are based on total mass in a specified size range, and there is uncertainty associated with the physiological response to acute or chronic exposure. Moreover, specific physicochemical properties of the ambient aerosol associated with adverse health effects are not fully understood (Pope, 2001; Pope et al., 2002). Growing evidence suggests that number concentration of aerosol (Laden et al., 2000; Oberdorster et al., 1990; Pekkanen et al., 1997; Peters et al., 1997) rather than a total mass measurement may be responsible for these outcomes. What’s more, partitioning of semi-volatile material such as ammonium nitrate into the gas phase may increase exposure levels (Volckens and Leith, 2003). Understanding how the number concentrations of particles change as a function of anthropogenic emissions, meteorology, time and location may help characterize the sources of these emissions and the health effects they induce (Yakovleva et al., 1999). As part of the Baltimore Supersite program, we have obtained size-resolved single particle mass spectra between 48 and 1250 nm in diameter with the real-time single particle mass spectrometer, RSMS III (Lake et al., 2003; Tolocka et al., 2004a). From April through November 2002 over 365,000 particles were analyzed, many of which contained nitrate. These data coupled with other meteorological and aerosol measurements reveal rather frequent ammonium nitrate particle events in this urban environment.
2. Experimental Single particle measurements were performed with RSMS III, a laser ablation time-of-flight mass spectrometer capable of simultaneous positive/negative ion detection. The instrument was housed in a trailer at the measurement site. Outside air was sampled into the mass spectrometer inlet through one of nine orifices used to select the particle size. The nine orifices corresponded to nine specific particle sizes between 45 and 1250 nm in diameter. Single particle spectra were acquired at 2 h intervals on a continuous basis. During each sampling interval, the instrument cycled through each of the nine orifices (particle size set points). Single particle spectra
were acquired at each size for 10 min or until the spectra from 30 particles were collected, whichever came first. Further information on the mass spectrometer design and performance can be found elsewhere (Lake et al., 2003). The measurement site was located in southeast Baltimore, Maryland (Antony Chen et al., 2002; Landis et al., 2001; Suarez and Ondov, 2002). Numerous PM10 sources were located within 10 miles of the site. Immediately to the west was a bus depot; to the east was a medical campus. Highways and street traffic surrounded the site. In this work, positive ion mass spectra were evaluated because they contain the m=z 30 (NO+) peak, a strong indicator ion for nitrate (Kane and Johnston, 2000). Each (positive ion) single particle spectrum consisted of 5000 points (ion signal intensity) along a time axis. The raw spectra were baseline corrected, mass calibrated, and numerically integrated between 7 0.5 Da from each integer mass unit. The integrated spectra were normalized so that the dot product with itself yielded one. Particle mass spectra were selected on the basis of the presence of a signal above a near-baseline threshold at m=z ¼ 30: Selecting particles based upon this threshold excluded particles that contained insignificant nitrate as well as those for which the verification of nitrate was dubious. The Art2-a neural network algorithm assigned the nitrate containing spectra to composition classes using a vigilance parameter of 0.6 and a learning parameter of 0.05 (Phares et al., 2001; Tolocka et al., 2004a). These classes were then segregated into two basic types: nominally ‘‘pure’’ nitrate particles and internally mixed composition particles. Nominally ‘‘pure’’ nitrate particles were defined as the sum of classes having an integrated m=z 30 signal above 75% of the total ion intensity in the positive ion spectrum. The 75% criterion was selected because of background signal at m=z 12 which could represent a significant fraction of the total ion intensity for small particles. Because carbonaceous particles have a m=z 12 signal intensity much greater than background, the 75% criterion was preferable to a higher percentage criterion in which the signal intensity at m=z 12 was excluded. While chemical composition can sometimes be determined from relative signal intensities in single particle mass spectra (Fergenson et al., 2001; Ge et al., 1998; Guazzotti et al., 2001; Mansoori et al., 1994; Tolocka et al., 2004b) matrix effects and the possibility of incomplete particle vaporization usually make quantification difficult (Carson et al., 1997; Reilly et al., 2000). Therefore, the goal of the particle classification step was not to quantitatively determine the amount of nitrate in each particle, but to distinguish the number of particles that are predominantly composed of nitrate from those that contain significant
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nitrate but are predominantly composed of other species.
3. Results and discussion Examples of nominally ‘‘pure’’ and internally mixed nitrate particle spectra are shown in Fig. 1. Fig. 1a is the Art2-a vector spectrum of a particle class composed of nominally ‘‘pure’’ nitrate particles, i.e. the signal intensity at m=z 30 (NO+) is greater than 75% of the total positive ion signal. In this figure, the signal intensity at m=z 12 (C+) is predominantly due to background. The counter ion in these particles is most likely ammonium. Ammonium ions are rarely observed in ultrafine single particle spectra owing to a very low relative sensitivity factor (Gross et al., 2000; Kane and Johnston, 2000) which contrasts alkali metal and other common cations that are detected with high sensitivity. During the entire measurement period, approximately 35% of the particles containing a detectable amount of nitrate were classified as nominally ‘‘pure’’ according to the 75% of total ion current definition. Fig. 1b is the Art2-a vector spectrum of an internally mixed particle class that contains nitrate. Prominent ions in the spectra of this class include m=z 12 and 24 (carbon), 39 (potassium) and 28 (organics and possibly oxygenated organics). The mass spectra of other internally mixed nitrate particles include these same ions but with different relative intensities and in some cases transition or heavy metals. Internally mixed nitrate particles (according to the 75% of total ion current definition) constitute approximately 65% of all particles containing a detectable amount of nitrate. 2.0
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Number concentrations of nominally ‘‘pure’’ and internally mixed nitrate particles were calculated from the hit rate and the number of particles collected in a particular time span, as described previously (Lake et al., 2003; Tolocka et al., 2004a). Shown in Fig. 2 are the average size distributions of ‘‘pure’’ and mixed nitrate particles found in Baltimore. Note that the maximum in the ‘‘pure’’ particle distribution is likely smaller than 48 nm, the smallest particle size considered in this study. A mode around 100 nm is suggested in the ‘‘pure’’ particle distribution as has been found in previous studies (Kerminen and Wexler, 1995). Also shown in Fig. 2 is the size distribution of the mixed nitrate particles. The mixed nitrate particle distribution is somewhat different from the ‘‘pure’’ particle distribution, showing a clear maximum at 140 nm that is five times the concentration of ‘‘pure’’ particles at this size. Fig. 3 shows plots of the mean diurnal pattern of nominally ‘‘pure’’ nitrate particles as a function of particle size. Typically, the highest nitrate concentrations are observed in the early morning hours before sunrise. Concentrations then decrease during the daytime. A small increase is observed in the middle of the day, and finally larger increases occur during the evening hours. A remarkable aspect of Fig. 3 is that the plots for each particle size appear to correlate with each other (R2 ¼ 0:71–0.90). This observation suggests that formation and growth of ‘‘pure’’ nitrate particles is dominated by diurnal emissions and meteorology whose effects are independent of particle size. Similar diurnal variations have been observed in Atlanta (Jimenez et al., 2003; Lee et al., 2003). While the averages in Fig. 3 give a qualitative picture of nitrate particle formation, more detail can be
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different plots are shown for 21–22 May 2002 in this figure: Number concentration of all particles vs. time (Fig. 4a), number concentration of nominally ‘‘pure’’ nitrate particles vs. time and particle size (Fig. 4b) and fraction of all particles that contain a detectable amount of nitrate vs. time and particle size (Fig. 4c). Nitrate particle events are clearly evident from the plot in Fig. 4b. The two nighttime (type 1) events are characterized by a rapid increase in the nominally ‘‘pure’’ nitrate particle concentration between 50 and 90 nm diameter followed by a rapid decrease. The daytime (type 2) event in Fig. 4b is less frequently observed and is characterized by an increase in the number of ‘‘pure’’ nitrate particles between 50 and 90 nm diameter followed by growth into larger sizes. During the growth period, the total concentration of ‘‘pure’’ nitrate particles usually decreases and the peak of the distribution shifts to 100–200 nm diameter. Many ultrafine nitrate particle events similar to those in Fig. 4b were observed and are summarized in Fig. 5. Type 1 events were identified on the basis of an abrupt increase in the concentration of nominally ‘‘pure’’ nitrate particles in the smallest two size bins (48 and 90 nm) by a factor of 10 over the baseline concentration
obtained by examining the diurnal variations individually. When this is done, numerous ultrafine nitrate particle events are revealed. Fig. 4 illustrates the two types of particle events that were observed. Three
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during these time periods, even though the number concentration goes up. This indicates that the concentration increase of nominally ‘‘pure’’ nitrate particles during a nighttime burst is not solely due to meteorology, but also to nitrate chemistry. What causes these events? Fig. 6 shows the correlation between single particle data and other measurements at the Baltimore Supersite for 3–7 May 2002, another time period during which nitrate particle events were observed in rapid succession. Fig. 6a shows the number concentration of nominally ‘‘pure’’ nitrate particles (summed over all sizes) as a function of time. Ultrafine particle events are observed daily during this time period (Fig. 5a). The stars in Fig. 6a indicate ultrafine particle events identified by the criteria set forth above. Note that Fig. 6a is a plot of total particle concentration rather than dN/d(log dp), so identification of particle events is not as straightforward as in Fig. 4b. Figs. 6b and c show the corresponding particulate nitrate mass concentrations determined by RSMS III
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levels in each bin. Type 2 events were identified on the basis of an increase in the concentration of nominally ‘‘pure’’ nitrate particles in the smallest four size bins (48, 90, 114 and 143 nm) by a factor of 10 over the baseline concentration levels. For type 2 events, the 10 increase for the smaller particle sizes occurred earlier in time than for the larger particle sizes. Most events of both type occurred during the spring and fall months. Several events were observed in August, but none in June or July. It should be noted that the instrument did not operate during an 11 day period in mid–June; however, the instrument did operate during the entire month of July. Type 1 events occurred at night or in the early morning hours, while type 2 events usually occurred during the day. Further insight into these events is given in the remaining plots in Fig. 4. Fig. 4a shows plots of total particle number concentration vs. time measured with a scanning mobility particle sizer and RSMS III. The RSMS III curve was generated from the procedure and average particle detection efficiencies in Lake et al. (2002). While the two plots are generally similar (R2 ¼ 0:74), quantitative differences arise because the RSMS III detection efficiencies are dependent upon particle size and composition (Kane and Johnston, 2000). Therefore, average detection efficiencies will not exactly reproduce the true number concentration when the particle composition changes on a short time scale. Figs. 4a and b show that bursts in the concentration of nominally ‘‘pure’’ nitrate particles coincide with short term increases in the total number concentration, although not all increases in the total number concentration are due to nitrate particles. Fig. 4c shows how nitrate is distributed among all particles during these events. This figure is a plot of the fraction of all particles that contain nitrate (number of ‘‘pure’’ and mixed particles divided by total number of particles) vs. time. A striking feature of this plot is that virtually all particles above about 300 nm in diameter contain at least some nitrate all of the time. Smaller particles are less mixed in composition and may or may not contain nitrate. However, when an ultrafine particle event occurs, the majority of particles at all sizes, both large and small, are found to contain nitrate. This observation suggests that when large numbers of ‘‘pure’’ nitrate particles are formed, nitrate also condenses onto most pre-existing particles. An interesting contrast to these data is found in Lake et al. (2002) for carbon (m=z 24) particle concentrations over the same time period. During this time period, the carbon particle concentrations increase at nighttime due to a decrease in mixing layer height. However, the magnitude of the increase of nominally ‘‘pure’’ nitrate particles is much greater than that for carbonaceous particles. In fact, the fraction of all particles in the atmosphere containing carbon actually goes down
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Fig. 6. Particle measurements between 3 and 7 May 2002: (a) number concentration of nominally ‘‘pure’’ nitrate particles summed over all sizes (stars indicate ultrafine particle events by the criteria set forth in the text); (b) nitrate mass calculated from the number concentration and size distribution of nominally ‘‘pure’’ nitrate particles determined by RSMS III; (c) continuous nitrate mass with a flash vaporization technique; (d) relative humidity; (e) temperature.
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(Fig. 6b) and a continuous nitrate monitor (Fig. 6c). The particulate nitrate concentration by RSMS III was obtained from the size distribution of nominally ‘‘pure’’ nitrate particles assuming a pure ammonium nitrate composition and then scaled to equal the continuous nitrate concentration at the highest value. The correlation between ‘‘pure’’ nitrate number concentration in Fig. 6a and continuous nitrate mass in Fig. 6c is surprisingly good (R2 ¼ 0:71), as is the correlation between nitrate mass from ‘‘pure’’ particles in Fig. 6b and continuous nitrate mass (R2 ¼ 0:80). The absolute value of the nitrate mass concentration obtained from RSMS III before scaling differs by a factor of 8 from the continuous nitrate measurement owing to the assumption made concerning the composition of ‘‘pure’’ nitrate particles, to the neglect of nitrate in mixed composition particles, and to the lack of temperature control in the sampling inlet. Figs. 6d and e show how relative humidity and temperature vary during this time period. Generally, ‘‘pure’’ nitrate particle concentrations are greatest when the temperature is low and the relative humidity is high, as expected from ammonium nitrate equilibrium considerations. While the correlation of the nominally ‘‘pure’’ nitrate particle concentration in Fig. 6 with temperature (R2 ¼ 0:40) is poorer than that with humidity (R2 ¼ 0:72), temperature ultimately controls the relative humidity and thus the partitioning of ammonium nitrate between the gas and condensed phases. Note that gas–particle partitioning should be independent of relative humidity when ammonium nitrate is a solid, i.e. below the deliquescence relative humidity (ca. 60–70% depending upon temperature), assuming that metastable aqueous particles below the deliquescence relative humidity are not significant. The first event in Fig. 6 occurred near the deliquescence relative humidity. All others during this time were substantially above. A detailed understanding of ultrafine nitrate particle events will require knowledge of gas and particle phase concentrations as well as temperature and relative humidity.
4. Conclusions Ultrafine nitrate particle events at the Baltimore Supersite were observed with the real-time single particle mass spectrometer RSMS-III. Two types of events were observed. The more frequent one consisted of a fast, intense eruption of small particles while the less frequent one exhibited a fast, moderate production of particles followed by growth to larger sizes over the period of a few hours. Condensation onto existing particles occurred during both of these events and the fraction of all particles at all sizes that contained a detectable amount of nitrate approached 100%. Total particle number
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concentration and nitrate mass measurements from RSMS III correlated well with similar measurements from other techniques, although scaling factors were required. The RSMS III measurements show that when the weather is suitably cold and humid, thermodynamics promotes partitioning of ammonium nitrate to the particle phase, which then dominates the particle number concentration as well as the chemical composition.
Acknowledgements Although the research described in this article has been funded by the United States Environmental Protection Agency through Grant number R82806301, it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. DAL acknowledges support of an EPA-STAR graduate fellowship under grant number U915771. The authors acknowledge the assistance of John Ondov, David Harrison, and Marc Parlange in various aspects of this project.
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Lightstone, J.M., Onasch, T.B., Imre, D., Oatis, S., 2000. Deliquescence, efflorescence, and water activity in ammonium nitrate and mixed ammonium nitrate/succinic acid microparticles. Journal of Physical Chemistry A 104 (41), 9337–9346. Mansoori, B.A., Johnston, M.V., Wexler, A.S., 1994. Quantitation of ionic species in single microdroplets by online laser desorption/ionization. Analytical Chemistry 66 (21), 3681–3687. Moya, M., Pandis, S.N., Jacobson, M.Z., 2002. Is the size distribution of urban aerosols determined by thermodynamic equilibrium?: an application to Southern California. Atmospheric Environment 36 (14), 2349–2365. Oberdorster, G., Ferin, J., Penney, D.P., Soderholm, S.C., Gelein, R., Piper, H.C., 1990. Increased pulmonary toxicity of ultrafine particles? Journal of Aerosol Science 17, 361–364. Orel, A.E., Seinfeld, J.H., 1977. Nitrate formation in atmospheric aerosols. Environmental Science and Technology 11 (10), 1000–1007. Pekkanen, J., Timonen, K.L., Ruuskanen, J., Reponen, A., Mirme, A., 1997. Effects of ultrafine and fine particles in urban air on peak expiratory. Flow among Children with Asthmatic Symptoms. Environmental Research 74, 24–33. Peters, A., Wichmann, H.E., Tuch, T., Heinrich, J., Heyder, J., 1997. Respiratory effects are associated with the number of ultrafine particles. American Journal of Respiratory and Critical Care Medicine 155, 1376–1383. Phares, D.J., Rhoads, K.P., Wexler, A.S., Kane, D.B., Johnston, M.V., 2001. Application of the ART-2a algorithm to laser ablation aerosol mass spectrometry of particle standards. Analytical Chemistry 73, 2338–2344. Pope, C.A., 2001. What do epidemiological findings tell us about health effects of environmental aerosols? Journal of Aerosol Medicine 13, 335–354. Pope, C.A., Burnett, R.T., Thun, M.J., Calle, E.E., Krewski, D., Ito, K., Thurston, G.D., 2002. Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution. JAMA 287 (9), 1132–1141. Pun, B.K., Seigneur, C., 2001. Sensitivity of particulate matter nitrate formation to precursor emissions in the California San Joaquin valley. Environmental Science and Technology 35 (14), 2979–2987. Reilly, P.T.A., Lazar, A.C., Gieray, R.A., Whitten, W.B., Ramsey, J.M., 2000. The elucidation of charge-transferinduced matrix effects in environmental aerosols via realtime aerosol mass spectral analysis of individual airborne particles. Aerosol Science and Technology 33, 135–152. Suarez, A.E., Ondov, J.M., 2002. Ambient aerosol concentrations of elements resolved by size and by source: contributions of some cytokine-active metals from coal- and oil-fired power plants. Energy and Fuels 16 (3), 562–568. Tolocka, M.P., Solomon, P.A., Mitchell, W., Gemmill, D., Wiener, R.W., Homolya, J., Natarajan, S., Vanderpool, R.W., 2001. East vs. West in the US: chemical characteristics of PM2.5 during the winter of 1999. Aerosol Science and Technology 34 (1), 88–96. Tolocka, M.P., Lake, D.L., Wexler, A.S., Johnston, M.V., 2004a. Number concentrations of fine and ultrafine particles containing metals. Atmospheric Environment, this issue, doi:10.1016/j.atmosenv.2004.03.010.
ARTICLE IN PRESS M.P. Tolocka et al. / Atmospheric Environment 38 (2004) 3215–3223 Tolocka, M.P., Saul, T.D., Johnston, M.V., 2004b. Reactive uptake of nitric acid into aqueous sodium chloride droplets using real-time single-particle mass spectrometry. Journal of Physical Chemistry A 108, 2659–2665. Volckens, J., Leith, D., 2003. Partitioning theory for respiratory deposition of semivolatile aerosols. Annals of Occupational Hygiene 47 (2), 157–164. Wall, S.M., John, W., Ondo, J.L., 1988. Measurement of aerosol size distributions for nitrate and major ionic species. Atmospheric Environment 22 (8), 1649–1656.
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Wexler, A.S., Seinfeld, J.H., 1990. The distribution of ammonium salts among a size and composition dispersed aerosol. Atmospheric Environment 24A, 1231–1246. Woo, K.S., Chen, D.R., Pui, D.Y.H., McMurry, P.H., 2001. Measurement of Atlanta aerosol size distributions: observations of ultrafine particle events. Aerosol Science and Technology 34 (1), 75–87. Yakovleva, E., Hopke, P.K., Wallace, L., 1999. Receptor modeling assessment of particle total exposure assessment methodology data. Environmental Science and Technology 33 (20), 3645–3652.
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Ultrafine nitrate particle events in Baltimore observed by real-time single particle mass spectrometry Michael P. Tolockaa, Derek A. Lakea, Murray V. Johnstona,b,*, Anthony S. Wexlera a
b
Department of Chemistry and Biochemistry, 107 Brown Lab, University of Delaware, Newark, DE 19716, USA Departments of Mechanical and Aeronautical Engineering, Civil and Environmental Engineering, and Land, Air and Water Resources, University of California, Davis, CA 95616, USA
Abstract Ambient particles in Baltimore, Maryland were characterized from April through November 2002 using the real-time single particle mass spectrometer, RSMS III. When particles containing nitrate were examined, two types of ultrafine particle events were revealed: a large burst of nominally ‘‘pure’’ nitrate particles in the 50–90 nm size range, and a smaller (and less frequent) burst of ‘‘pure’’ particles in the 50–90 nm size range that grew to 110–220 nm with time. Coincident with both of these events was an increase in the number of mixed composition particles containing nitrate, suggesting that they were formed by condensation of ammonium nitrate onto pre-existing particles. Meteorological variables, particle number concentrations and continuous nitrate mass measurements were compared to the single particle data. Number and mass concentrations estimated from RSMS III correlated well with similar measurements with other techniques. Ultrafine nitrate particle events were observed during periods of low temperature and high relative humidity as expected from ammonium nitrate equilibrium considerations. During these events, the partitioning of ammonium nitrate to the particle phase strongly influenced the particle number concentration as well as the chemical composition. r 2004 Elsevier Ltd. All rights reserved. Keywords: Ambient aerosol nitrate mass spectrometry
1. Introduction Inorganic ions can comprise a majority of the fine particle mass with the predominant species being ammonium, sulfate and nitrate (Tolocka et al., 2001). The optical, deliquescent and reactive properties are dependent upon how much of each species is present in the aerosol (Hand et al., 2002; Jang et al., 2002; Lightstone et al., 2000). These properties determine the ability of the aerosol to scatter light, act as cloud condensation nuclei (Cruz and Pandis, 1998), and behave as catalysts for organic reactions. Because of *Corresponding author. Tel.: +1-302-831-8014; fax: +1-302831-6335. E-mail address: [email protected] (M.V. Johnston).
their importance in the cycling of ions in the atmosphere, interactions of precursor gases and particles have been extensively modeled (Ansari and Pandis, 1999; Campbell et al., 2002; Clegg et al., 1998a,b; Moya et al., 2002). Two fine particle modes are observed for inorganic components in urban air (John et al., 1990; Wall et al., 1988). The smaller size mode around 100–200 nm in diameter results from condensation of a variety of gasphase species, including ammonium nitrate, onto preexisting particles of both primary and secondary origin. The chemistry of ammonium nitrate in this mode has been well studied (Cheung et al., 2000; Jacob, 2000; Orel and Seinfeld, 1977; Pun and Seigneur, 2001; Wexler and Seinfeld, 1990). The larger size mode around 700 nm in diameter is both a product of in situ droplet phase
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.03.011
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sulfate production and growth of smaller particles through condensation (Kerminen and Wexler, 1995). Recently, ultrafine particle events have been observed in Atlanta, Houston, Pittsburgh, and St. Louis as part of the US EPA Supersite Programs. These events are characterized by elevated concentrations of particles in the 3–45 nm size range, and are distinguished by their diurnal and seasonal pattern (Woo et al., 2001). While diurnal patterns of nitrate have been observed in Atlanta (Jimenez et al., 2003; Lee et al., 2003), the role that nitrate plays in ultrafine particle events is unknown. Current regulations for particulate matter are based on total mass in a specified size range, and there is uncertainty associated with the physiological response to acute or chronic exposure. Moreover, specific physicochemical properties of the ambient aerosol associated with adverse health effects are not fully understood (Pope, 2001; Pope et al., 2002). Growing evidence suggests that number concentration of aerosol (Laden et al., 2000; Oberdorster et al., 1990; Pekkanen et al., 1997; Peters et al., 1997) rather than a total mass measurement may be responsible for these outcomes. What’s more, partitioning of semi-volatile material such as ammonium nitrate into the gas phase may increase exposure levels (Volckens and Leith, 2003). Understanding how the number concentrations of particles change as a function of anthropogenic emissions, meteorology, time and location may help characterize the sources of these emissions and the health effects they induce (Yakovleva et al., 1999). As part of the Baltimore Supersite program, we have obtained size-resolved single particle mass spectra between 48 and 1250 nm in diameter with the real-time single particle mass spectrometer, RSMS III (Lake et al., 2003; Tolocka et al., 2004a). From April through November 2002 over 365,000 particles were analyzed, many of which contained nitrate. These data coupled with other meteorological and aerosol measurements reveal rather frequent ammonium nitrate particle events in this urban environment.
2. Experimental Single particle measurements were performed with RSMS III, a laser ablation time-of-flight mass spectrometer capable of simultaneous positive/negative ion detection. The instrument was housed in a trailer at the measurement site. Outside air was sampled into the mass spectrometer inlet through one of nine orifices used to select the particle size. The nine orifices corresponded to nine specific particle sizes between 45 and 1250 nm in diameter. Single particle spectra were acquired at 2 h intervals on a continuous basis. During each sampling interval, the instrument cycled through each of the nine orifices (particle size set points). Single particle spectra
were acquired at each size for 10 min or until the spectra from 30 particles were collected, whichever came first. Further information on the mass spectrometer design and performance can be found elsewhere (Lake et al., 2003). The measurement site was located in southeast Baltimore, Maryland (Antony Chen et al., 2002; Landis et al., 2001; Suarez and Ondov, 2002). Numerous PM10 sources were located within 10 miles of the site. Immediately to the west was a bus depot; to the east was a medical campus. Highways and street traffic surrounded the site. In this work, positive ion mass spectra were evaluated because they contain the m=z 30 (NO+) peak, a strong indicator ion for nitrate (Kane and Johnston, 2000). Each (positive ion) single particle spectrum consisted of 5000 points (ion signal intensity) along a time axis. The raw spectra were baseline corrected, mass calibrated, and numerically integrated between 7 0.5 Da from each integer mass unit. The integrated spectra were normalized so that the dot product with itself yielded one. Particle mass spectra were selected on the basis of the presence of a signal above a near-baseline threshold at m=z ¼ 30: Selecting particles based upon this threshold excluded particles that contained insignificant nitrate as well as those for which the verification of nitrate was dubious. The Art2-a neural network algorithm assigned the nitrate containing spectra to composition classes using a vigilance parameter of 0.6 and a learning parameter of 0.05 (Phares et al., 2001; Tolocka et al., 2004a). These classes were then segregated into two basic types: nominally ‘‘pure’’ nitrate particles and internally mixed composition particles. Nominally ‘‘pure’’ nitrate particles were defined as the sum of classes having an integrated m=z 30 signal above 75% of the total ion intensity in the positive ion spectrum. The 75% criterion was selected because of background signal at m=z 12 which could represent a significant fraction of the total ion intensity for small particles. Because carbonaceous particles have a m=z 12 signal intensity much greater than background, the 75% criterion was preferable to a higher percentage criterion in which the signal intensity at m=z 12 was excluded. While chemical composition can sometimes be determined from relative signal intensities in single particle mass spectra (Fergenson et al., 2001; Ge et al., 1998; Guazzotti et al., 2001; Mansoori et al., 1994; Tolocka et al., 2004b) matrix effects and the possibility of incomplete particle vaporization usually make quantification difficult (Carson et al., 1997; Reilly et al., 2000). Therefore, the goal of the particle classification step was not to quantitatively determine the amount of nitrate in each particle, but to distinguish the number of particles that are predominantly composed of nitrate from those that contain significant
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nitrate but are predominantly composed of other species.
3. Results and discussion Examples of nominally ‘‘pure’’ and internally mixed nitrate particle spectra are shown in Fig. 1. Fig. 1a is the Art2-a vector spectrum of a particle class composed of nominally ‘‘pure’’ nitrate particles, i.e. the signal intensity at m=z 30 (NO+) is greater than 75% of the total positive ion signal. In this figure, the signal intensity at m=z 12 (C+) is predominantly due to background. The counter ion in these particles is most likely ammonium. Ammonium ions are rarely observed in ultrafine single particle spectra owing to a very low relative sensitivity factor (Gross et al., 2000; Kane and Johnston, 2000) which contrasts alkali metal and other common cations that are detected with high sensitivity. During the entire measurement period, approximately 35% of the particles containing a detectable amount of nitrate were classified as nominally ‘‘pure’’ according to the 75% of total ion current definition. Fig. 1b is the Art2-a vector spectrum of an internally mixed particle class that contains nitrate. Prominent ions in the spectra of this class include m=z 12 and 24 (carbon), 39 (potassium) and 28 (organics and possibly oxygenated organics). The mass spectra of other internally mixed nitrate particles include these same ions but with different relative intensities and in some cases transition or heavy metals. Internally mixed nitrate particles (according to the 75% of total ion current definition) constitute approximately 65% of all particles containing a detectable amount of nitrate. 2.0
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Number concentrations of nominally ‘‘pure’’ and internally mixed nitrate particles were calculated from the hit rate and the number of particles collected in a particular time span, as described previously (Lake et al., 2003; Tolocka et al., 2004a). Shown in Fig. 2 are the average size distributions of ‘‘pure’’ and mixed nitrate particles found in Baltimore. Note that the maximum in the ‘‘pure’’ particle distribution is likely smaller than 48 nm, the smallest particle size considered in this study. A mode around 100 nm is suggested in the ‘‘pure’’ particle distribution as has been found in previous studies (Kerminen and Wexler, 1995). Also shown in Fig. 2 is the size distribution of the mixed nitrate particles. The mixed nitrate particle distribution is somewhat different from the ‘‘pure’’ particle distribution, showing a clear maximum at 140 nm that is five times the concentration of ‘‘pure’’ particles at this size. Fig. 3 shows plots of the mean diurnal pattern of nominally ‘‘pure’’ nitrate particles as a function of particle size. Typically, the highest nitrate concentrations are observed in the early morning hours before sunrise. Concentrations then decrease during the daytime. A small increase is observed in the middle of the day, and finally larger increases occur during the evening hours. A remarkable aspect of Fig. 3 is that the plots for each particle size appear to correlate with each other (R2 ¼ 0:71–0.90). This observation suggests that formation and growth of ‘‘pure’’ nitrate particles is dominated by diurnal emissions and meteorology whose effects are independent of particle size. Similar diurnal variations have been observed in Atlanta (Jimenez et al., 2003; Lee et al., 2003). While the averages in Fig. 3 give a qualitative picture of nitrate particle formation, more detail can be
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different plots are shown for 21–22 May 2002 in this figure: Number concentration of all particles vs. time (Fig. 4a), number concentration of nominally ‘‘pure’’ nitrate particles vs. time and particle size (Fig. 4b) and fraction of all particles that contain a detectable amount of nitrate vs. time and particle size (Fig. 4c). Nitrate particle events are clearly evident from the plot in Fig. 4b. The two nighttime (type 1) events are characterized by a rapid increase in the nominally ‘‘pure’’ nitrate particle concentration between 50 and 90 nm diameter followed by a rapid decrease. The daytime (type 2) event in Fig. 4b is less frequently observed and is characterized by an increase in the number of ‘‘pure’’ nitrate particles between 50 and 90 nm diameter followed by growth into larger sizes. During the growth period, the total concentration of ‘‘pure’’ nitrate particles usually decreases and the peak of the distribution shifts to 100–200 nm diameter. Many ultrafine nitrate particle events similar to those in Fig. 4b were observed and are summarized in Fig. 5. Type 1 events were identified on the basis of an abrupt increase in the concentration of nominally ‘‘pure’’ nitrate particles in the smallest two size bins (48 and 90 nm) by a factor of 10 over the baseline concentration
obtained by examining the diurnal variations individually. When this is done, numerous ultrafine nitrate particle events are revealed. Fig. 4 illustrates the two types of particle events that were observed. Three
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during these time periods, even though the number concentration goes up. This indicates that the concentration increase of nominally ‘‘pure’’ nitrate particles during a nighttime burst is not solely due to meteorology, but also to nitrate chemistry. What causes these events? Fig. 6 shows the correlation between single particle data and other measurements at the Baltimore Supersite for 3–7 May 2002, another time period during which nitrate particle events were observed in rapid succession. Fig. 6a shows the number concentration of nominally ‘‘pure’’ nitrate particles (summed over all sizes) as a function of time. Ultrafine particle events are observed daily during this time period (Fig. 5a). The stars in Fig. 6a indicate ultrafine particle events identified by the criteria set forth above. Note that Fig. 6a is a plot of total particle concentration rather than dN/d(log dp), so identification of particle events is not as straightforward as in Fig. 4b. Figs. 6b and c show the corresponding particulate nitrate mass concentrations determined by RSMS III
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levels in each bin. Type 2 events were identified on the basis of an increase in the concentration of nominally ‘‘pure’’ nitrate particles in the smallest four size bins (48, 90, 114 and 143 nm) by a factor of 10 over the baseline concentration levels. For type 2 events, the 10 increase for the smaller particle sizes occurred earlier in time than for the larger particle sizes. Most events of both type occurred during the spring and fall months. Several events were observed in August, but none in June or July. It should be noted that the instrument did not operate during an 11 day period in mid–June; however, the instrument did operate during the entire month of July. Type 1 events occurred at night or in the early morning hours, while type 2 events usually occurred during the day. Further insight into these events is given in the remaining plots in Fig. 4. Fig. 4a shows plots of total particle number concentration vs. time measured with a scanning mobility particle sizer and RSMS III. The RSMS III curve was generated from the procedure and average particle detection efficiencies in Lake et al. (2002). While the two plots are generally similar (R2 ¼ 0:74), quantitative differences arise because the RSMS III detection efficiencies are dependent upon particle size and composition (Kane and Johnston, 2000). Therefore, average detection efficiencies will not exactly reproduce the true number concentration when the particle composition changes on a short time scale. Figs. 4a and b show that bursts in the concentration of nominally ‘‘pure’’ nitrate particles coincide with short term increases in the total number concentration, although not all increases in the total number concentration are due to nitrate particles. Fig. 4c shows how nitrate is distributed among all particles during these events. This figure is a plot of the fraction of all particles that contain nitrate (number of ‘‘pure’’ and mixed particles divided by total number of particles) vs. time. A striking feature of this plot is that virtually all particles above about 300 nm in diameter contain at least some nitrate all of the time. Smaller particles are less mixed in composition and may or may not contain nitrate. However, when an ultrafine particle event occurs, the majority of particles at all sizes, both large and small, are found to contain nitrate. This observation suggests that when large numbers of ‘‘pure’’ nitrate particles are formed, nitrate also condenses onto most pre-existing particles. An interesting contrast to these data is found in Lake et al. (2002) for carbon (m=z 24) particle concentrations over the same time period. During this time period, the carbon particle concentrations increase at nighttime due to a decrease in mixing layer height. However, the magnitude of the increase of nominally ‘‘pure’’ nitrate particles is much greater than that for carbonaceous particles. In fact, the fraction of all particles in the atmosphere containing carbon actually goes down
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Fig. 6. Particle measurements between 3 and 7 May 2002: (a) number concentration of nominally ‘‘pure’’ nitrate particles summed over all sizes (stars indicate ultrafine particle events by the criteria set forth in the text); (b) nitrate mass calculated from the number concentration and size distribution of nominally ‘‘pure’’ nitrate particles determined by RSMS III; (c) continuous nitrate mass with a flash vaporization technique; (d) relative humidity; (e) temperature.
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(Fig. 6b) and a continuous nitrate monitor (Fig. 6c). The particulate nitrate concentration by RSMS III was obtained from the size distribution of nominally ‘‘pure’’ nitrate particles assuming a pure ammonium nitrate composition and then scaled to equal the continuous nitrate concentration at the highest value. The correlation between ‘‘pure’’ nitrate number concentration in Fig. 6a and continuous nitrate mass in Fig. 6c is surprisingly good (R2 ¼ 0:71), as is the correlation between nitrate mass from ‘‘pure’’ particles in Fig. 6b and continuous nitrate mass (R2 ¼ 0:80). The absolute value of the nitrate mass concentration obtained from RSMS III before scaling differs by a factor of 8 from the continuous nitrate measurement owing to the assumption made concerning the composition of ‘‘pure’’ nitrate particles, to the neglect of nitrate in mixed composition particles, and to the lack of temperature control in the sampling inlet. Figs. 6d and e show how relative humidity and temperature vary during this time period. Generally, ‘‘pure’’ nitrate particle concentrations are greatest when the temperature is low and the relative humidity is high, as expected from ammonium nitrate equilibrium considerations. While the correlation of the nominally ‘‘pure’’ nitrate particle concentration in Fig. 6 with temperature (R2 ¼ 0:40) is poorer than that with humidity (R2 ¼ 0:72), temperature ultimately controls the relative humidity and thus the partitioning of ammonium nitrate between the gas and condensed phases. Note that gas–particle partitioning should be independent of relative humidity when ammonium nitrate is a solid, i.e. below the deliquescence relative humidity (ca. 60–70% depending upon temperature), assuming that metastable aqueous particles below the deliquescence relative humidity are not significant. The first event in Fig. 6 occurred near the deliquescence relative humidity. All others during this time were substantially above. A detailed understanding of ultrafine nitrate particle events will require knowledge of gas and particle phase concentrations as well as temperature and relative humidity.
4. Conclusions Ultrafine nitrate particle events at the Baltimore Supersite were observed with the real-time single particle mass spectrometer RSMS-III. Two types of events were observed. The more frequent one consisted of a fast, intense eruption of small particles while the less frequent one exhibited a fast, moderate production of particles followed by growth to larger sizes over the period of a few hours. Condensation onto existing particles occurred during both of these events and the fraction of all particles at all sizes that contained a detectable amount of nitrate approached 100%. Total particle number
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concentration and nitrate mass measurements from RSMS III correlated well with similar measurements from other techniques, although scaling factors were required. The RSMS III measurements show that when the weather is suitably cold and humid, thermodynamics promotes partitioning of ammonium nitrate to the particle phase, which then dominates the particle number concentration as well as the chemical composition.
Acknowledgements Although the research described in this article has been funded by the United States Environmental Protection Agency through Grant number R82806301, it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. DAL acknowledges support of an EPA-STAR graduate fellowship under grant number U915771. The authors acknowledge the assistance of John Ondov, David Harrison, and Marc Parlange in various aspects of this project.
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