Environ. Sci. Technol. 2002, 36, 3512-3518
On-line Analysis of Urban Particulate Matter Focusing on Elevated Wintertime Aerosol Concentrations PHILLIP V. TAN, GREG J. EVANS,* JULIA TSAI, SANDY OWEGA, MICHAEL S. FILA, AND OSCAR MALPICA Department of Chemical Engineering, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada JEFFREY R. BROOK Air Quality Processes Research Division, Meteorological Service of Canada, 4905 Dufferin Street, Toronto, Ontario M3H 5T4, Canada
A 10-day winter sampling campaign was conducted in downtown Toronto for particulate matter (PM) air pollution in the fine (0.52 µm) are shown. b
Combined classes: C2 and CnHm+/organics. c Combined Cl/organics, NaCl-. e Combined classes: Na, NaCl+, NaCl+/metals.
classes: sulfates/nitrates/organics and nitrates/organics. d Combined classes: f Spectra with a combination of metal ion peaks. Metals include: Al, Ca, Fe, K, Mn, Na, Ti, V.
lower PM during this campaign. There was detectable NaCl and K. The ability of LAMS to characterize the NaCl- and K-containing classes was important since these are good indicators of road de-icing and wood combustion or crustal sources, respectively. For the other classes presented on this day, number concentration values were generally equivalent or lower than the time periods with elevated PM. Higher PM. During periods with higher particulate matter levels, Ca, CnHm+/C2/organics, NO/CH4N/organics, and classes that contained K, sulfate, and nitrate had number concentrations well above those for the day with lower PM. These observations were supported in part by filter-based PM2.5 measurements which also showed higher sulfates, nitrates, and organic carbon on these days with higher PM (Table 1). However, the number concentrations of the dominant LAMS classes differed between the higher PM occasions, and for other chemical classes, number concentrations were selectively elevated at different times. (A) PM Event A: December 15th, 10:30h. Investigations of PM Event A revealed that the number concentrations of amine, nitrate, and cation organic particle classes increased up to the time when the APS and TEOM concentrations were at a maximum, while conversely that of the Ca and hydrocarbon classes decreased (Figure 3). The concurrent increase in amines, CH4N/NO, and other organic-containing particles detected in the positive ion mode, with nitrates and mixed nitrates/sulfates in the negative ion mode, suggests the formation of amine nitrates and sulfates on the particles. Nitrates, and to a lesser extent sulfates, have been identified as the counterion for cation amines previously (30). The simultaneous decrease in Ca, CnHm+/organics, H/C2/organics, and CnHm- suggests that these were the particle types that became transformed into amines. Inspection of the individual spectra in the CnHm+/organic and amine/organic classes during PM Event A revealed that the prevalence of ions in these mass spectra changed gradually over time. Tracking the CnHm+/organics class, at
FIGURE 3. Temporal trends of amine-related and Ca/hydrocarbon particulate classes, suggesting particle transformations occurred during PM Event A. It appears that Ca/hydrocarbons were coated (decline in these classes) from partitioning of gaseous amines onto particle surfaces (increase in amines). Footnotes: 1positive ion classes included: 58 Da amine/organics, NO/CH4N/organics, 86 Da amine/organics; negative ion classes included: nitrates, sulfates/ nitrates; 2positive ion classes included: Ca, CnHm+/organics; negative ion classes included: CnHm-, H/C2/organics. Larger diamond and triangle markers are anion particle classes scaled by a factor of 1.5. Trend lines are third-order polynomial fits to the data. ∼3:00h, hydrocarbon type particles existed (Figure 4i) in low number concentrations (Figure 3). Afterward at ∼7:30h, some intermediate organic particle types were observed (Figure 4ii), characterized principally by the addition of m/z peaks of 58 and 86 Da. At this point, these particles were still classified as CnHm+/organics (Figure 3, CnHm and Ca peak). Finally by ∼10:30h, very few of the hydrocarbon type particles remained. Instead, particles with distinct amine spectra were evident (Figure 4iii). The presence of these intermediate particles suggested that the original hydrocarbon particles were transformed into amine-type particles. This suspected evolution in particle surface chemistry was observable by VOL. 36, NO. 16, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Occurrence of Selected Chemical Ion Peaks in Toronto during Wintertime Samplinga cation peak 27Al 138Ba 12C 24(C
2)/
24Mg
36(C
3)
58(C
3H8N)
59(C
3H9N) 48 4)/ Ti
48(C 86(C
5H12N)
40Ca 63Cu 56Fe/56(CaO) 39K 213(K SO ) 3 4 23Na 81(Na
2Cl) 165(Na SO ) 3 4 18(NH ) 4 30(NO)/30(CH 208Pb 64Zn/64(TiO)
4N)
% occurrence 35 1.8 21 24 25 35 34 8.2 24 23 14 23 71 1.4 34 9.6 5.4 37 37 3.4 5.3
anion peak 12C 24(C ) 2 25(C H) 2 36(C ) 3 48(C ) 4 60(C ) 5 35Cl 26(CN)/26(C
2H2)
43(CNOH) 19F 1H 97(HSO
4)
46(NO
2)
62(NO
3)
16O 17(OH) 79(PO
79 3)/ Br
32S 76(SiO
3)
80(SO
3)
96(SO
4)
% occurrence 15 30 23 16 23 13 23 28 13 8.0 36 70 63 58 33 30 7.9 9.8 7.5 20 22
a
Based on a total of 12 300 positive and 8400 negative spectra (>0.52 µm).
FIGURE 4. Winter particle types during PM Event A. (i-iii) show sequence of spectra for single particles transformed from a hydrocarbon type (i), to an intermediate hydrocarbon/amine (ii), to finally an amine particle (iii). (iii-v) show organic-containing particles with common 58(C3H8N)+ and 86(C5H12N)+ amine marker peaks. LAMS because for aerosols in this size range LAMS is primarily a surface analysis method. During PM Event A, the spectra for the organic-containing particles included a m/z ) 58 and/or 59 Da peak with an intense 86 Da peak (Figure 4iii-v). Smaller 100 and 101 Da peaks, along with 18(NH4)+ ion, were also present in the particles. The presence of ammonium, an ion with a low relative sensitivity factor (31), suggested that significant amounts of the ion were present on the particle surfaces, and gaseous NH3 was a likely precursor involved in PM Event A. The ammonium was most likely present as ammonium nitrate and sulfate (as supported in Table 2 by elevated sulfateand nitrate-containing classes). The 58, 59, 86, 100, and 101 Da peaks were attributed to 58(C3H8N)+, 59(C3H9N)+, 86(C H N)+, 100(C H N)+, and 101(C H N)+, respectively, from 5 12 6 14 6 15 a diethylethanoloamine salt, triethylamine or diethylamine based on similar spectra reported in the literature (30, 32). The source of this amine is unclear. Amines have been detected by others (33) at a rural site, and the presence of gaseous amines of agricultural origin has been proposed as a source (30). In the city, the source of the amines could possibly be derived from volatile amines outgassed from municipal sewer wastewater (34). In contrast to the work by Angelino et al. (30), this study, although limited in duration, did not find the frequency of amine marker peaks (m/z ) 58 or 86 Da) or the amines/organics class to be correlated with wind direction over College Street. It is possible that the particulate amines were formed through gas-to-particle partitioning of gaseous amines onto the surface of existing particles. Alternatively, although perhaps less likely, the amines may have been a secondary organic aerosol produced 3516
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through a surface reaction of possibly: (1) NH3 with carbonaceous components on a particle, or (2) gaseous organic NOy with acidic particles. The Ontario Ministry of the Environment measured NOx in Toronto as high as 300 ppb during PM Event A, suggesting ample gaseous NOx for the last reaction possibility. Regardless of their origin, the presence of the amine on the surface would have created elevated R3NH+(aq) within the aqueous films on the particles which would have promoted the accumulation of sulfate and nitrate. Amine-related classes were present in their largest concentration during PM Event A; however, in a summary of various chemical ions collected during the winter campaign, the amine marker peaks of 58(C3H8N)+, 59(C3H9N)+, and 86(C5H12N)+ were also frequently observed (Table 3). Amine classes were also evident on days with lower PM as indicated in Figure 1. Correlation analysis between meteorological, gas-phase, and atmospheric stability variables, with the frequency of occurrence of amine class particles or particle spectra with m/z ) 58 or 86 Da, did not show any significant associations. In work where ambient particles were exposed to amines, results showed that the particles quickly obtained amine marker peaks, demonstrating the ease with which amines can be adsorbed by various particulates (30). (B) PM Event B: December 20th, 02:00h. Table 2 shows that particle classes containing sulfate were increased in PM Event B to several times the levels found in PM Event A. This is also shown in Table 1 for sulfate mass measured on PM2.5 filters. In addition, elevated SO2 concentrations of 26 ppb were observed. The classes that included organics were also elevated during this episode. The individual mass spectra for PM Event B revealed that the organics in the K/organics and CnHm+/organics classes were mainly hydrocarbon fragments. These carbonaceous particulates have been associated with vehicles (35, 36), but the time span of the event occurring after the evening rush hour period, and only on this particular night, suggests it was not due to usual mobile sources. Increased concentrations of amine particles were also observed during PM Event B. The presence of these particles indicated that their source or formation mechanism was not unique to the conditions of PM Event A. However, the amine spectra in PM Event B showed some difference from those
FIGURE 5. Temporal variation in the size distribution of K/organics (light) and sulfates (dark) during PM Event C, showing that submicron particles peaked at about 23:00h. Darker histograms are the anion sulfate class scaled by a factor of 1.5. of PM Event A. In the amine mass spectra from PM Event B, 58(C3H8N)+ and 59(C3H9N)+ ion peaks were intense in the amine group instead of the 86(C5H12N)+ peak that was a base peak in the amine class of PM Event A. Furthermore, there was a noticeable absence of the 100(C6H14N)+ and 101(C6H15N)+ amine markers from most of the amine particles in PM Event B. It is not clear if these more subtle differences indicated that the amine particles in PM Event B were truly different than those of PM Event A. (C) PM Event C: December 20th, 23:00h. PM Event C peaked over only a 4 h time interval. A temporal size distribution of the relative number concentrations for the two most elevated chemical classes, likely counterions for one another, is shown in Figure 5. Both the K/organic and sulfate classes were submicron in size with greater number concentrations for smaller particles. Prior to 22:00h on December 20th, some particles were detected by LAMS in these classes, but between 22:00h and 00:00h of the next day, the relative number concentrations of the K/organics and sulfate classes rose significantly. From 02:00h of December 21st onward, the number concentrations for these classes returned to lower levels. This rapid chemical composition change observed for K/organics and sulfate, by the enhanced temporal resolution of the LAMS, was not evident from bulk 24 h filter analyses (as there was no sulfate spike in Table 1 for PM Event C). The LAMS results suggested that this episode possibly originated from a wood combustion source. This source identification was supported by the strong presence of K-containing particles, a marker found from wood-burning emissions (37). Other organic-containing classes, CnHm+/ organic and NO/CH4N/organic (Table 2), also showed an increase, and the organics within these classes and the K/organics class were identified to be mainly hydrocarbons. These hydrocarbons are associated with primary combustion (36, 37), and their increase was concurrent with an increase in PM2.5 carbon concentrations of EC and OC (Table 1). During PM Event C, the ratio of the number of particles in the K and K/organics classes to those in the Fe class (characterized by dominant Fe, and usually smaller K) showed a substantial K enrichment. An enrichment in the K to Fe mass concentration ratio, above its level for crustal materials, has been used in filter-collected PM analyses to indicate
combustion (15). For LAMS, the ratio of K to Fe classes was 14 at 16:00h, December 20th, a time prior to the PM event. This ratio rose to 108, at the local maximum concentration which occurred at 00:00h on December 21st. K to Fe ratios after the peak, and during other times in this study, including PM Events A and B, were
