Environ. Sci. Technol. 2010, 44, 4070–4075
Probing the Source of Hydrogen Peroxide Associated with Coarse Mode Aerosol Particles in Southern California YING WANG, CHUAUTEMOC ARELLANES,† DANIEL B. CURTIS,‡ AND SUZANNE E. PAULSON* Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California 90095
Received November 10, 2009. Revised manuscript received April 12, 2010. Accepted April 14, 2010.
Coarse mode aerosols were collected at three sites in the Los Angeles area, two in Riverside, CA, one upwind and the other downwind of a major freeway, and also on the campus of the University of California, Los Angeles (UCLA). Coarse mode aerosol mass, H2O2, and H2O2 normalized to aerosol mass averaged 46 ( 22 µg/m3, 17 ( 8 ng/m3, and 0.48 ( 0.32 ng/ µg at the upwind Riverside site and 97 ( 27 µg/m3, 34 ( 14 ng/ m3, and 0.37 ( 0.18 ng/µg at the downwind Riverside site, respectively. H2O2, which appears to be generated by the particles (Arellanes, C.; Paulson, S. E.; Fine, P. M.; Sioutas, C. Environ. Sci. Technol. 2006, 40, 4859-4866), was uncorrelated with particle mass, but was strongly correlated with soluble iron, zinc, and copper (r ) 0.47-0.67, p ) 0.00-0.01). H2O2 levels were not affected by the addition of dithiothreitol, a marker for quinone redox activity. H2O2 levels were sensitive to the pH of the particle extraction solutions, increasing as the pH was decreased. The initial rate of H2O2 generation by coarse mode aerosols was 7.8 ((5.7) × 10-8 M min-1, similar to initial rates of hydroxyl radical generation from dissolved Fe2+, Cu2+, and Zn2+ solutions. The results support the notion that the majority of coarse mode H2O2 generation is mediated by a small set of transition metals.
Introduction Coarse mode particles (usually 2.5-10 µm) affect both human health and a variety of atmospheric processes. Health effects studies of ambient particles have traditionally focused on particulate matter (PM) smaller than 10 µm in diameter (PM10) or, more recently, particles smaller than 2.5 µm in diameter (PM2.5); the coarse fraction of PM10 has only recently been investigated separately. Fine particles have frequently been found to be more toxic than coarse particles (see, e.g., ref 2). Several recent studies, however, clearly show that coarse particles exert toxic effects, with potency at least equivalent to fine particles. For example, studies in the Coachella Valley, California (3), Toronto, Canada (4), and four Chinese cities (5) reported stronger associations with coarse than fine particles for respiratory symptoms, asthma, cardiorespiratory disease, and cardiovascular mortality. Several toxicological * Corresponding author phone: (310) 206-4442; fax: (310) 2065219; e-mail
[email protected]. † Now at Environ Corp. ‡ Now at California State University Northridge. 4070
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studies support the epidemiological results, indicating stronger responses to coarse than fine particles by human airway macrophages (see, e.g., ref 6) and higher induced oxidative DNA damage (see, e.g., ref 7). A class of compounds thought to be at least partly responsible for particle-related adverse health effects are the reactive oxygen species (ROS) (see, e.g., ref 8), which include hydrogen peroxide (H2O2), hydroxyl radical (OH), and superoxide (O2•-). ROS plays a prominent role in the pathogenesis of various inflammatory airway disorders (see, e.g., ref 8). H2O2 and its related ROS, OH, and O2•- have long been recognized as key to the aqueous phase reactions that take place in water suspended in the atmosphere and, thus, are central to cloud processing of air (see, e.g., ref 9). Additionally, ROS in aerosols likely contribute to aerosol aging, oxidizing organics within particles. Limited measurements have demonstrated high levels of H2O2 associated with ambient coarse particles. Our earlier measurements (1) reported concentrations of 14 ( 10 ng/m3 (0.58 ( 0.30 ng/µg) and 20 ( 9 ng/m3 (1.05 ( 0.30 ng/µg) in coarse particles (>2.5 µm) at the University of California at Los Angeles (UCLA) and a Los Angeles freeway site, respectively. A number of measurements have been made using dichlorofluorescin, a technique that is calibrated with H2O2, but which may respond to more ROS species than H2O2. Results from this assay reported H2O2 equivalents of 2 ( 1 ng/m3 (0.3 ( 0.1 ng/µg) in coarse particles (3.2-10 µm) in Taipei (10), 26 ng/m3 (2.5-10 µm) in Rubidoux, CA (11), and 5 ng/m3 (2.5-10 µm) in Flushing, NY (12), respectively. Both methods are performed by collecting particles on filters and immersing them in an aqueous extraction solution. Although this creates a much more dilute solution than aerosol liquid water, it is related, to various degrees, to the processes that take place when particles interact with aqueous phases, including during condensational growth in the atmosphere and in lung fluids. Previous work in our laboratory (1, 13) showed that the levels of H2O2 associated with ambient particles (i.e., H2O2 measured in extraction solutions) exceed levels predicted using Henry’s law by 2-3 orders of magnitude. This conclusion was based on simultaneous measurements of gas phase H2O2, H2O2 in particle extraction solutions, and aerosol liquid water content estimated from measurements (1). Because it is unlikely that ambient particles have a liquid phase that is in disequilibrium with respect to gas-liquid partitioning, the particles most likely are generating the substantial quantities of H2O2 in solution for hours, and in some cases days, after they are collected. Although currently there are no techniques to probe this chemistry in suspended particles, it seems likely that particles actively generate ROS in the atmosphere and continually release H2O2 and other ROS to maintain equilibrium. Because aerosol liquid water content is small, release of H2O2 should not appreciably affect H2O2 in the gas phase. Redox active species such as transition metals and quinoid compounds (see, e.g., refs 14 and 15) are the most likely dominant sources of ROS. H2O2 can be produced (and destroyed) via the redox cycling between HO2/O2- radical and transition metals, which, in ambient particles and cloud drops, may be complexed by organics (16). Additionally, reactions and cycles may involve multiple metals and/or organics. The complex web of reactions that govern the concentrations of ROS is not well understood and is beyond the scope of this paper. Here we probe the species contributing to H2O2 generation by ambient coarse particles. 10.1021/es100593k
2010 American Chemical Society
Published on Web 04/29/2010
Ambient coarse particles (>2.5 µm) were collected at two sites in Riverside, CA, and on the UCLA campus. Particles were analyzed for H2O2 generation, particle mass, and soluble element concentrations. The dependence of H2O2 generation on extraction solution solutes and pH was investigated, and the initial rate and time profile of H2O2 generation were monitored. The effect of aging aerosol samples on H2O2 generation activity was explored. Results from these measurements are discussed in terms of their relationship with potential generation mechanisms.
Experimental Section Aerosol Sampling. Field measurements were made on the campus of the University of California at Riverside (UCR) in 2005 and within the Citrus Research Center and Agricultural Experiment Station (CRCAES), also at UCR, in 2008. CRCAES is upwind of the surrounding freeways, whereas UCR is downwind of the major freeways and the busy UCR campus during daytime. Coarse mode aerosols were collected during daylight hours, from August 2 to 10, 2005, and from June 23 to August 28, 2008. In 2005 sampling was initiated between 7:00 and 8:30 a.m. and continued throughout the day at 60-90 min intervals up to 6:30 p.m. In 2008 samples were collected between the hours of 7:00 a.m. and 1:00 p.m. on Mondays through Thursdays. Hourly concentrations of gaseous pollutants (O3, NO2, SO2, CO) were obtained from the closest complete air quality monitoring station at Rubidoux, CA, 8.6 km northwest of the Riverside sampling sites. The hourly data from Rubidoux were averaged to match the corresponding aerosol sampling time intervals. Additional sampling was conducted on the UCLA campus (1) intermittently between November 2004 and July 2009, also during daylight hours. Sampling times ranged from 2 to 6 h. For most samples, parallel sampling trains, consisting of virtual impactors (VI) followed by stainless steel filter holders and flow meters, were used to collect pairs of fine and coarse mode samples, on 47 mm Teflon filters (2 µm pores, Pall Corp.). The total flow through the VIs was 57.5 L min-1. One of each filter pair was analyzed for H2O2 and the other for aerosol mass and elements. Field blanks were created by loading a fresh filter into a filter holder and turning on the pump for 30 s. Blanks were analyzed in the same manner as sample filters for all analyses. The data are reported as average ( standard deviation. H2O2 Measurement. Generally within 30 min of completing sampling, particles were extracted into the aqueous phase by immersing filters into a known volume (4 mL for whole filters) of extraction solution for 2 h, unless otherwise specified. Extraction solutions had various compositions. Many solutions contained 0.1 mM Na2EDTA, and all were made with 18 MΩ water. The “standard” extraction solution was adjusted to pH 3.5 with 0.1 N H2SO4, contained EDTA, and is referred to as ES 3.5. Extraction solutions at other pH values were adjusted using 0.1 M NaOH or 0.1 N H2SO4. Dithiothreitol (DTT) solutions contained 1 × 10-6 M DTT. H2O2 in extraction solutions was measured by HPLCfluorescence, as described in detail elsewhere (1, 17). Normally, peroxides are separated on a C-18 reversedphase column (Alltech) on the basis of their polarity, and the retention time of H2O2 is around 4.5 min. Because earlier measurements established that organic peroxides were very low or absent from ambient aerosols (13), the C-18 column was removed in this study; thus, H2O2 was eluted at 0.5 min. The eluent is mixed with fluorescent reagent containing horseradish peroxidase enzyme and p-hydroxyphenylacetic acid (PHOPAA). The enzyme catalyzes a stoichiometric reaction between hydroperoxides and PHOPAA, resulting in quantitative conversion of H2O2
FIGURE 1. Temporal variations of H2O2 generation by coarse mode aerosols at UCR (upper panel) and CRCAES (lower panel) in Riverside, CA. to PHOPAA dimer. The PHOPAA dimer is then detected via fluorometry. Calibrations were performed three times a week with titrated H2O2 standard solutions. The response was linear in the concentration range from 10-8 to 10-6 M. H2O2 concentrations in blanks were about 16 ( 10% of that in ambient samples. Particle Mass and Element Measurements. Mass concentrations were determined using a microbalance (1 µg precision, ME 5, Sartorius) in a room with controlled temperature (22-24 °C) and humidity (40-45%). In most cases, filters were allowed to equilibrate in the room for at least 24 h before weighing. The concentrations of 14 elements (Al, Ca, Mg, K, Na, Si, Fe, Cu, Pb, Mn, Ni, Se, Zn, V) were determined by inductively coupled plasma atomic emission spectroscopy (Perkin-Elmer, TJA Radial Iris 1000). Filters were extracted under similar conditions as the H2O2 analysis, in 4 mL acidic solution (pH 3.5, HNO3) for 2 h. The resulting extracts were further acidified with 1 mL of 25% HNO3 to match the pH of the ICP-AES matrix (5% HNO3). A multiple elements stock solution, containing 10 or 20 ppm of each element, was prepared from individual element standards (CPI). Element standards (0.01-1.2 ppm) were prepared by serial dilution of the stock solution. Element concentrations in blanks ranged from 0.4 to 35% of those in ambient samples. Supporting Measurements. To elucidate the temporal characteristics of H2O2 generation, a 100 µL aliquot was taken from the extraction solution and analyzed every 2-30 min during the first 2 h of the extraction process and every 8-24 h thereafter. The effect of sample aging was investigated by collecting sample pairs on filters that were then cut in half using a Ni-plated scalpel. Immediately after a sampling, one of the filter halves was analyzed for H2O2 and used as the initial reference activity. The remaining filters were allowed to age while stored in Petri dishes in the dark and analyzed after 48, 96, or 136 h.
Results Ambient Aerosols. Figure 1 shows temporal variations of H2O2 generation by coarse particles at CRCAES and UCR, respectively. H2O2 generation at UCR was about double that at CRCAES, at 34.1 ( 13.7 and 17.0 ( 7.6 ng/m3, respectively, but mass was also about double, at 96.7 ( 27.1 and 45.9 ( 21.7 µg/m3, respectively. Normalized to particle mass, H2O2 generation was not statistically different at the two sites VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary Statistics of the Levels of Soluble Elements and H2O2 (nmol/m3) in Coarse Mode Aerosols at the UCR and CRCAES Sites UCR, 2005 mean H2O2 K Ca Na Mg Al Si Fe Cu Zn Mn V Ni Se Pb transition metalsa a
SD
CRCAES, 2008 median
N
mean
SD
median
N
0.50 2.12 11.18 11.64 6.08 3.28 1.10 1.58 0.42 0.67 0.20 0.01 0.01 0.03 0.01 2.89
0.22 0.75 3.94 7.57 2.38 0.83 0.35 0.56 0.69 0.93 0.08 0.01 0.01 0.02 0.00 1.67
0.44 2.21 11.14 9.64 5.76 3.14 1.04 1.44 0.22 0.42 0.19 0.01 0.01 0.03 0.01 2.40
31 27 27 27 27 27 27 27 27 27 27 26 26 24 22 27
1.00
0.40
0.97
35
159.74
40.14
170.24
15
2.11 1.44 0.84
1.78 1.31 0.43
1.61 1.26 0.96
34 22 13
0.13
0.10
0.12
18
0.06 3.33
0.04 3.09
0.06 2.31
15 35
Transition metals are estimated from the sum of Fe, Cu, Zn, Mn, V, and Ni.
(independent sample t test, significance (two-tailed) > 0.05), at 0.37 ( 0.18 and 0.48 ( 0.32 ng/µg at UCR and CRCAES, respectively. Also, aerosol H2O2 generation (in ng/m3) and particle mass were not correlated at either site (r ) -0.02 to 0.28). Measurements to date by our group as well as others suggest that the ability of coarse mode particles to generate H2O2 (and possibly other ROS) is location specific (Table S1, Supporting Information). Our measurements of H2O2 associated with particles at Riverside (17-34 ng/m3) are in good agreement with the dichlorofluorescin result for coarse mode particles at Rubidoux, CA (26 ng/m3) (11). The Riverside results are well above the dichlorofluorescin results at Taipei (∼2 ng/m3) (10) and Flushing, NY (5 ng/m3) (12) and moderately above the Los Angeles measurements (14-20 ng/m3) (1). When normalized to particle mass, however, the Riverside values are comparable to the measurements made in Los Angeles (1) and Taipei (10). Correlations with Gas Phase Pollutants and Particulate Elements. Coarse mode H2O2 was positively correlated with NO2 at UCR (p < 0.01) and with CO at CRCAES (p ) 0.01) and negatively correlated with O3 at both sites (p < 0.05). Each of these variables is strongly correlated with one another and with time of day. In contrast, Venkatachari et al. (11) found no correlation between coarse mode H2O2 (dichlorofluorescin method) and O3 at the Rubidoux site. Significant positive correlations between coarse mode H2O2 and CO and/ or NO2 could possibly be due to a combustion source for one or more of the redox active metals, Fe, Cu, and Zn, which are strongly correlated with H2O2 (below). Total soluble transition metals (Fe, Cu, Zn, Mn, V, Ni) account for only ∼0.3% of the aerosol mass, averaging ∼3 nmol/m3. This is about a factor of 3 higher than that of H2O2 (∼0.8 nmol/m3) (Table 1). For calculations of correlation between H2O2 generation and soluble elements, outliers, defined as values that fell away from the median by a distance of >3 times of interquartile range (i.e., the 25th-75th percentile distance), were excluded. For the UCR and CRCAES data sets combined, one each of the Fe, Cu, and Ni, and three each of the Zn and Na data points were excluded. At both the CRCAES and UCR sites, Fe, Zn, and Cu correlated with H2O2 generation, and at CRCAES, Al and Si did as well (Table 2; Figure S1, Supporting Information), possibly due to their strong correlations with Fe (below), rather than a direct contribution to H2O2 generation. Al and Si were not measured at UCR. That other elements, including Ca, V, Pb, Na, Ni, K, Mg, Mn, and Se, were not significantly 4072
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TABLE 2. Correlations between H2O2 Generation by Coarse Mode Aerosols and Elements for Which There Was a Significant or Very Significant Correlation UCR, 2005
Fe Zn Cu Al Si
CRCAES, 2008
ra
p
N
ra
p
N
0.66** 0.51 0.40
0.00 0.08 0.06
33 13 22
0.67** 0.60** 0.47* 0.44* 0.43*
0.00 0.00 0.01 0.02 0.02
27 24 26 27 27
a *, correlation is significant at the 0.05 level (two-tailed); **, correlation is significant at the 0.01 level (two-tailed).
correlated with H2O2 perhaps due either to their lack of redox activity or to their low concentrations in particles (not shown). Transition metal-mediated pathways to generate H2O2 have been reported previously. For example, aqueous free iron, Fe(II), may first be oxidized by oxygen to generate O2•[Fe(II) + O2 f Fe(III) + O2•-] (18), which can further react with Fe(II) to generate H2O2 [Fe(II) + O2•- + 2H+ f Fe(III) + H2O2] (19). Similar reactions can occur with Cu, Cr, V, and Ni, albeit with considerably varying efficiency (see, e.g., ref 20), and, additionally, metals may be complexed, further altering the reaction profiles. Although not measured here, soluble Fe(II) has been found to comprise 40-80% of total soluble Fe [Fe(II) + Fe(III)] in aqueous solutions of ambient particles at pH 2.5-4.3 (21), suggesting Fe(II) concentrations may range from 0.8 to 1.7 and from 0.6 to 1.3 nmol/m3 at UCR and CRCAES, respectively. These estimated Fe(II) concentrations are similar to those reported previously (22) and may have been sufficient in our coarse particles to generate a significant portion of the observed H2O2 (1.0 and 0.5 nmol/m3, respectively, Table 1). There are colinearities among Fe, Zn, Cu, Al, and Si. Fe correlates with all of the other elements (r ) 0.69-0.86), and the pairs Cu and Zn and Al and Si are intercorrelated (r ) 0.51 and 0.95, respectively). Those correlations are possibly due to their common sources [e.g., crustal material for Al, Si, Fe, and industrial or vehicle exhaust emissions for Cu, Zn, and Fe (23)] and hence confound our ability to discriminate and independently quantify the impact of individual elements on H2O2 generation. Although no studies have considered specifically the relationship between soluble metals and dark generation of
FIGURE 2. Variations of H2O2 generation by coarse mode aerosols in extraction solutions of different pH values. Results are presented relative to H2O2 generation in the pH 3.5 extraction solution. Data for pH 3.5 are set to 100% and are included as a visual guide. The data point for pH 1.5 is an average of three measurements, hence the lack of a box. Data points are colored for samples collected in different periods. The box plot represents the 10, 25, 50, 75, and 90% quantiles, respectively. H2O2, a number of studies have related ROS generation with transition metals in particles. One study measured OH generation by ambient particles in the presence of H2O2 (7) and showed that water-soluble Cu, but none of the other metals (Fe, Cr, V, Ni), was significantly correlated with OH generation, and in coarse but not fine particles. A handful of other studies have associated total ROS generation (24, 25) and OH generation in the presence of H2O2 (7, 26) with specific metals for various ambient or combustion source samples. Soluble Fe and Cu (24, 26, 27) and total Zn (24, 25) were found to be most responsible for the measured redox activity of particles in the above studies. Effects of Extraction Solution Composition on H2O2 Generation. pH, ionic strength, and the presence of chelators in the extraction solution are expected to affect the solubility, speciation, and reaction rates of transition metals (28, 29). EDTA added to the extraction solution has no effect at pH 3.5; at higher pH values, EDTA can increase the observed H2O2 (Figure S2, Supporting Information). This appears to be consistent with a role for Fe(II): at low pH, Fe(II) may be mostly in the free form (30), so EDTA does not affect H2O2 generation. At high pH and EDTA/Fe(II) ratios, EDTA may complex Fe(II) in various forms (30). The complexation may increase H2O2 generation by limiting iron precipitation and accelerating the rate of Fe(II) oxidation [Fe(II) + O2 f Fe(III) + O2•-, Fe(II) + O2•- + 2H+ f Fe(III) + H2O2] (31) as well as by inhibiting the rate of H2O2 decomposition [Fe(II) + H2O2 f Fe(III) + •OH + OH-] (32). Figure 2 shows the dependence of coarse mode H2O2 generation on the extraction solution pH, with added EDTA. H2O2 generation reached a maximum at the most acidic pH values tested, 1.5-2.5, exceeding the pH 3.5 reference solution by 30%. H2O2 generation monotonically decreases to ∼30% of pH 3.5 levels at pH 6.5 and remains fairly constant as the pH is increased further, to pH 7.5. In the absence of EDTA, the shape of the curve remains, but the decrease at higher pH is somewhat larger (Figure S2, Supporting Information). The substantial sample-to-sample variability in Figure 2 is likely due largely to the real variations in particle composition. The solubilities of Fe, Cu, and Zn as well as the fractions of Fe(II) and Cu(I) in total Fe and Cu have been shown to increase with decreasing pH (33). Perhaps also significant is the pKa of the acid/base pair HO2•/O2•- of 4.8 (34). At pH
