ARTICLE IN PRESS

Atmospheric Environment 41 (2007) 4258–4269 www.elsevier.com/locate/atmosenv

Atmospheric production of oxalic / and /nitrate in the Tampa Bay airshed: Parallel pathways

P. Kalyani Martinelango, Purnendu K. DasguptaÃ, Rida S. Al-Horr1

Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061, USA

Received 9 February 2006; received in revised form 1 May 2006; accepted 2 May 2006

Abstract

Oxalic acid is the dominant (DCA), and it constitutes up to 50% of total atmospheric DCAs, especially in non-urban and marine atmospheres. A significant amount of particulate H2Ox/oxalate (Ox) occurred in the coarse particle fraction of a dichotomous sampler, the ratio of oxalate concentrations in the PM10 to PM2.5 fractions ranged from 1 to 2, with mean7sd being 1.470.2. These results suggest that oxalate does not solely originate in the gas phase and condense into particles. Gaseous H2Ox concentrations are much lower than particulate Ox concentrations and are well correlated with HNO3, HCHO, and O3, supporting a photochemical origin. Of special relevance to the Bay Region Atmospheric Chemistry Experiment (BRACE) is the extent of nitrogen deposition in the Tampa Bay estuary. Hydroxyl radical is primarily responsible for the conversion of NO2 to HNO3, the latter being much more easily deposited. Hydroxyl radical is also responsible for the aqueous phase formation of oxalic acid from alkenes. Hence, we propose that an estimate of dOH can be obtained from H2Ox/Ox production rate and we accordingly show that the product of total oxalate concentration and NO2 concentration approximately predicts the total nitrate concentration during the same period. r 2007 Elsevier Ltd. All rights reserved.

Keywords: Oxalate; Oxalic acid; BRACE; Nitrogen deposition; Photochemical production

1. Introduction presence of up to 3.5 mM Ox in precipitation and 3 2 nmol m H2Ox/Ox in air, mostly present as The occurrence of oxalic acid (H2Ox) and/or particulate Ox. Nylon backup filters did contain oxalate (Ox) in snow, rain, particulate matter and measurable oxalate indicating the presence of possibly in the gas phase was first demonstrated by gaseous H2Ox but it was not possible to establish Norton et al. (1983). Aside from direct analysis of its presence unequivocally as Ox was shown to rain and snow by ion chromatography (IC), this evaporate from the sampled filters. This early early work used serial filters to demonstrate the seminal work was remarkably thorough; because of the relatively low resolution of IC analysis at that time, the authors conclusively proved the presence à Corresponding author. Tel.: +1 806 742 3064; of Ox in their samples by showing the disappearance fax: +1 806 742 1289. E-mail address: [email protected] (P.K. Dasgupta). of the putative chromatographic peak upon prior 1Present address: Dionex Corporation, 1228 Titan Way, treatment of the sample with Oxalate decarboxylase. Sunnyvale, CA 94086, USA. Earlier, in a characterization of dicarboxylic

1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.05.085 ARTICLE IN PRESS P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269 4259

(DCAs) in Los Angeles aerosol, Grosjean et al. Boring et al. (2002) recently presented a new fully (1978) did not report the presence of Ox. If Norton automated instrument for the measurement of acid et al. can be faulted at all, it is perhaps that they gases using a parallel plate wet denuder (PPWD) placed undue importance on this earlier finding and with an on-line particle collector and analyzer suggested that H2Ox/Ox might only constitute a following it. At a highly industrialized site close to very small fraction of DCAs. the shipping channel in Houston, TX, H2Ox There is a large body of work that has since concentrations ranged from 0.6 to 762 ng m3 and accumulated which unequivocally shows that not the particulate Ox was 1.3–749 ng m3. only H2OX/Ox is the dominant DCA, it frequently In the present work, the gas–aerosol partitioning constitutes 50% of total atmospheric DCAs in of H2Ox–Ox, the equilibrium relationship between mid-ocean rainwater (Sempere and Kawamura, ammonia, H2Ox and particulate 1996). In non-urban rainwater in southern Califor- and the influence of different parameters like nia, Ox routinely exceeded formate and sometimes temperature, ozone, concentration of other gas acetate (Kawamura et al., 1996); in Schenectady and aerosol species on the concentrations of H2Ox aerosol, Ox was reported to frequently exceed the and Ox are discussed. sum of formate and acetate (Khwaja, 1995). Oxalate A central concern in the Bay Region Atmospheric was reported to constitute up to 86% of DCAs in Chemistry Experiment (BRACE) studies was the urban (Nanjing, China) PM10 and up to 65% of the extent of nitrogen deposition in the Tampa Bay PM2.5 fraction (Wang et al., 2002). Oxalate may estuary. The hydroxyl radical (dOH) is primarily well be the most important aerosol species in its role responsible for the conversion of NO2 (a gas with a as cloud condensation nuclei (Yu, 2000). low deposition velocity) to HNO3 (a gas with a high Main sources of oxalic acid are thought to be deposition velocity). The primary agent responsible photochemical oxidation of anthropogenic, biogenic for the formation of oxalic acid is also believed to be and oceanic emissions and/or primary traffic emis- dOH. We are fortunate to have a wealth of near- sions (Kawamura and Kaplan, 1987; Kawamura real time data from continuous measurement of et al., 1996). Very high concentrations of oxalic acid atmospheric gases and particles over a 5-week were detected in the biomass burning plumes, period in 2002 at the Tampa Bay airshed. Here, suggesting that either oxalic acid is directly emitted we intend to show that the production of HNO3/ or formed in the plume from a biogenic precursor nitrates and H2Ox/Ox often follow such parallel (Jaffrezo et al., 1998). Isoprene is the major organic pathways, that the production rate of one is well compound emitted by plants. It has been shown that correlated with that of the other. and methylglyoxal formed by the oxidation of isoprene act as intermediates in the 2. Experimental section in-cloud formation of oxalic acid (Ervens et al., 2004; Lim et al., 2005). Oxalic acid is a likely end 2.1. Field data product of the photochemical oxidation reactions and can accumulate in the atmosphere (Chebbi and Data were obtained from 27/4/2002 to 31/5/2002 Carlier, 1996; Kawamura and Ikushima, 1993). at the BRACE supersite at Sydney, FL (271 580 N, Once formed, it is expected to be very stable and 821 130 W). This site is located at a suburban–rural to exist as fine particles. Hence, the major removal interface, 22 km ENE of downtown Tampa. The mechanism is expected to be wet deposition. exact location of this site on a map appears in a In most studies conducted thus far, oxalic acid published paper (Fig. 3b, Dasgupta et al., 2005). has not been observed in the gas phase in measur- Gas and aerosol samples were collected using a able quantities (Saxena and Hildemann, 1996). This PPWD (Boring et al., 2002) coupled to a mist could largely be due to inadequate sensitivity/ chamber–hydrophobic filter reflux particle collector, methods. Limbeck et al. (2001) used a dual filter PC (Al-Horr et al., 2003) with the sample inlet 1m sampling strategy to determine the gas/aerosol above shelter rooftop and 4.5 m from the ground distribution of oxalic acid and other polar organic level. The particle collection system had an effective compounds. They reported a gas phase concentra- cutoff at 12.5 mm. tion of 23715 ng m3 and aerosol concentration of Soluble gases, including oxalic acid, were 3 68740 ng m . However, the uncertainty of these collected using a PPWD. H2O2 of 0.5 mM serves values was high. as the denuder liquid which captures the gases. ARTICLE IN PRESS 4260 P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269

The denuder liquid containing the soluble gases was Greenbush, NY) that separated the samples into aspirated by a peristaltic pump at a flow rate of PM2.5 and PM102.5 components. More details are 1 ml min1 and sent into the IC. given in Poor et al. (2002). The collection efficiency of oxalic acid by the PPWD was tested in our laboratory (680 mm Hg, 3. Results and discussion 22 1C) by generating oxalic acid gas by soaking a 47 mm Whatman GF/B glass fiber filter in 0.1 M 3.1. Gas– particle partitioning of oxalic acid oxalic acid solution, drying it and drawing air though it at 8 standard liters per minute (SLPM). This was Over the period of the study, gaseous oxalic acid found to generate oxalic acid at a concentration of concentrations ranged from 0.014 to 0.81 mgm3 76876 mgm3. This stream was sampled at 5 SLPM with mean and median values of 0.074 and through two serial PPWDs. The collection efficiency 0.059 mgm3. Particulate Ox was present typically for oxalic acid gas under these conditions was at 4 greater concentration, ranging from 0.025 measured to be 97.670.2%. Considering that this to 5.8 mgm3 with mean and median values of 0.29 concentration is much higher than ambient concen- and 0.21 mgm3. In supplementary information, trations and the fact that at Tampa atmospheric detailed frequency distribution data are presented pressure is sea-level pressure such that sampling at 5 (Figure S1). This is consistent with what has been SLPM actually represents a significantly lower flow observed elsewhere: in Hong Kong, another coastal velocity, we believe that the denuder capture of H2Ox city, only 6–12% of the total oxalate (OxT) was was essentially quantitative. H2Ox (Yao et al., 2002). Clegg et al. (1996) reported For particle collection and analysis, ambient air that OxT in the atmosphere partitions almost was first passed through a PPWD to remove the completely into aqueous aerosols, except under gases. Disodium hydrogen phosphate Na2HPO4 conditions of combined low relative humidity, low (10 mM, adjusted to pH 6) containing 0.5 mM aerosol pH, and temperatures greater than about H2O2 was used as denuder liquid to remove both 15 1C. acidic and basic gases. The effluent stream contain- Oxalate has been reported to be distributed over ing the particles entered the hydrophobic filter- the whole particle size spectrum, having a dominant based particle collection system. Water was pumped accumulation mode (0.1–2 mm), an Aitken mode into the PC through a capillary and the air was (o0.1 mm) and a coarse particle mode (42 mm) drawn through a small aperture surrounding it. The (Fridlind et al., 2000). This suggests that multiple liquid generated a fine mist. The flow was ultimately pathways may exist for the formation of oxalate: drawn through a 0.5 mm pore size PTFE filter. while some of the H2Ox is formed as gaseous H2Ox Water droplets coalesced on the filter forming a film and condenses to fine particles, another portion of in which the aerosols were captured. The droplets oxalate may be formed in the aqueous phase and fell below and were collected at the bottom of the H2Ox may even degas from it given appropriate PC. The liquid was then aspirated by a peristaltic conditions of droplet acidity, temperature and RH. pump and sent to the IC for analysis. In Tampa, NaCl aerosol concentration is large and The IC analysis system consisted of alternating it is present with a mode around 4 mm. This can TAC-LP1 anion preconcentrator columns, provide a significant surface area for condensation/ AG11HC guard and AS11HC separation columns dissolution of H2Ox leading to a prominent coarse and an electrodialytically regenerated suppressor particle mode for Ox. Previously, we had measured (ASRS, operated at 100 mA). The chromatographic H2Ox and PM2.5 Ox in Houston and Philadelphia. system itself consisted of an IS-25 chromatographic In Tampa, a coastal city, the overall mode of pump coupled to an EG-40 electrodialytic eluent ambient aerosol size is shifted up due to the major generator (15.5 mM KOH, 1.5 mL min1, LC-30 presence of sea-salt particles, this is not the case in oven at 29 1C), and a conductivity detector (CD, the other two cities and a comparison of PM12.5 Ox model ED40). Chromatography was conducted in Tampa with PM2.5 Ox in Houston and Philadel- either on a 10- or a 15-min cycle. All chromato- phia is not inappropriate. Median concentrations of graphic equipment and columns were from Dionex Ox (H, P, T: 41, 369 and 214 ng m3) and especially 3 Corp. H2Ox (H, P, T: 66, 67, 59 ng m ) are not The dichotomous filter sampler was a Partisols- dramatically different in the three cities but the Plus Model 2025 (Rupprecht and Patashnik, East highest concentration of Ox is significantly higher in ARTICLE IN PRESS P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269 4261

3 Tampa (5.9 mgm ), compared to those in Houston We have recently measured the gaseous H2Ox (0.75 mgm3) or Philadelphia (1.9 mgm3). We concentrations in equilibrium with solid oxalic acid believe this may be related to actual production of as a function of temperature. These results will be Ox in the solution phase associated with sea-salt published elsewhere. It is sufficient to note here that p aerosol from isoprene and/or ethene and acetylene H2Ox is 3–5 orders of magnitude lower than (Lim et al., 2005; Warneck, 2003). Warneck (2003) predicted by the equilibrium above. Limbeck and has previously suggested that ethene and acetylene Puxbaum (2000) have shown that the atmospheric may serve as precursors for the intermediates phase distribution of DCAs is not necessarily , , that are directly related to the vapor pressure; rather, other oxidized to form oxalic acid in the aqueous phase physical and chemical processes such as adsorption and Crahan et al. (2004) have observed evidence of onto available particle surfaces, absorption into a in-cloud production of oxalate. Very recently Yu liquid phase, etc. greatly modify the observed et al. (2005) have also argued that cloud processing results. Indeed, some gas to particle conversion must be involved in the formation of Ox and occurs even when pH2Ox is significantly below the Warneck (2005) have explored in detail the conver- predicted equilibrium pressure at that temperature. sion of ethene and acetylene in the marine atmo- The concentration maxima for H2Ox occur sphere in a box model. during the time of the day when the temperature Other evidence also indicates that oxalate in the and light intensity are at their maximum (Fig. 1). Tampa Bay airshed does not solely form in the gas This is consistent with a photochemically mediated phase and then condense to fine particles. We path for the formation of oxalate, whether via analyzed 19 randomly selected PM102.5 and PM2.5 homogeneous gas phase or heterogeneous solution filter pairs from a dichotomous sampler from the phase pathways. Oxalate peaks after gaseous H2Ox period spanning June 2002–May 2003. The data when the temperature starts to decrease; as con- were reconstituted to PM10 and PM2.5. The ratio of densation is favored as the temperature decreases. PM10/PM2.5 for sulfate which forms dominantly in Regardless of day or night, H2Ox concentration the fine particles was 1.170.1 while nitrate which exhibits a good positive correlation with tempera- forms nearly exclusively in coarse particles was ture. Fig. 2 shows hourly average data, covering 14.1717.0. This ratio for oxalate was 1.470.2. More- 5–95% of the total concentration span sorted in 451 over, for individual filter samples, the PM10/PM2.5 wind direction bins (the 315–3601 bin does not ratio for nitrate and oxalate was reasonably well contain enough data to plot) and sorted in daytime correlated (r2 ¼ 0:49). (10 AM–6 PM) and nighttime (8 PM–6 AM) values. The large concentration of NaCl present in Both the nighttime and the daytime concentrations Tampa can result in a substantial liquid water of gaseous H2Ox appear to be driven by tempera- content associated with the aerosol phase at high ture and there is no obvious dependence on wind RH values. The sampling period, however, was direction. mostly an exceptionally dry period for the region As shown in Fig. 3, the maxima in oxalate that year; only 4 days had any rainfall during these 5 concentrations always coincide with the minima in weeks. With respect to in-cloud formation of RH. Since the latter is also related to temperature, oxalate, it is interesting to note that Ox was the we suspect that this is more directly related to highest on 18th and 19th May, the 2 days on which temperature than to RH; however, at this point, no the rainfall was the highest (1.5 and 0.9 cm, definitive conclusions can be drawn, especially as respectively). temperature and RH changes are diurnal and thus also associated with change in air mass origin. 3.2. Diurnal variation 3.3. Is oxalate present as ammonium oxalate? As has been previously observed in many other locations, in Tampa, daytime concentrations of both Lefer and Talbot (2001) studied the similarity of Ox and H2Ox were almost always higher than the the size distributions of oxalate and ammonium and corresponding nighttime concentrations. Only on suggested that ammonium oxalate aerosol may be occasion, when the daytime concentrations were very directly formed from the gaseous precursors: low for reasons that are presently unclear to us, did the ammonia and oxalic acid. If ammonium oxalate is nighttime concentrations exceed those during the day. indeed the form in which oxalate is present in the ARTICLE IN PRESS 4262 P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269

0.6 36 1000 Oxalate (μg/m3) Oxalic acid (μg/m3) Solar radiation (W/m2) 800 Temperature (°C) 32 ) ) 2 3 0.4 600 μ g/m

28 Temperature

400 0.2 Solar radiation (W/m Concentration ( 24 200

0 20 0 0:00 6:00 12:00 18:00 0:00 Time

Fig. 1. Diurnal variation (7/5/2002) of particulate oxalate and gaseous oxalic acid with light intensity and temperature.

0.14

0.12

Day, r2 = 0.54

Night, r2 = 0.39 3 0.1 g/m μ Ox, 2 0.08 H Wind Direction 270-315 deg 0.06 225-270 deg 180-225 deg 135-180 deg 90-135 deg 0.04 45-90 deg 0-45 deg

10 20 30 Temperature, degrees C

Fig. 2. Hourly average H2Ox concentrations sorted in 451 wind direction bins (insufficient data in some bins) as a function of temperature. aerosol phase, the temperature-dependent dissocia- not of course possible to definitively determine in tion of ammonium oxalate (rather than H2Ox what form oxalate exists in the Tampa aerosol directly) may well predict the observed temperature without single particle analysis. However, we note + dependence behavior of the ambient aerosol. It is that particulate NH4 and Ox are generally well ARTICLE IN PRESS P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269 4263

1

80 ) 3

40 Oxalate ( μ g/m Relative Humidity (%)

0

0.1

5/8/02 5/10/02 5/12/02 5/14/02 Date

Fig. 3. Maxima in oxalate concentrations always corresponded with minima in relative humidity.

Combined r2 = 0.469 0.8 Nighttime data, r2 = 0.686 Daytime data, r2 = 0.253

0.6 ) 3 ( μ g/m 2- 4 0.4 O 2 C

0.2

0

01234 + μ 3 NH4 ( g/m )

Fig. 4. Relation between particulate oxalate and ammonium (4/5/2002–13/5/2002).

correlated, especially in the nighttime (the data for a correlation between gaseous ammonia and H2Ox is 10-day period are shown in Fig. 4, r2 ¼ 0:686) and poor and is of the same order both during day and less so during the daytime (r2 ¼ 0:253). The overall night (r2 ¼ 0:247 for day and nighttime data ARTICLE IN PRESS 4264 P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269 combined). It does not therefore appear that the peroxides during olefin oxidation is predicted by the data conform to ammonia and oxalic acid primarily Warneck (2005) model. Similar photochemical originating from the dissociation of ammonium origins of HNO3/nitrate and H2Ox/Ox are sug- oxalate. Other auxiliary experiments not discussed gested by other, significant portions of the data. For here indicate that ammonium in the Tampa aerosol the 10-day period beginning on 4/5/2002, for is primarily in PM2.5 whereas as previously dis- example, gaseous HNO3 and H2Ox concentrations cussed, oxalate is dominantly in the PM102.5 during the daytime were well correlated (Fig. 7, fraction. r2 ¼ 0:642). In the liquid phase, oxalate is dominantly ionized 3.4. Evidence of photochemical production and remains as such irrespective of the change in other parameters such as light intensity, oxidant Formaldehyde is also a product of atmospheric concentration, etc. Thus, the correlations between photooxidation. Formaldehyde was measured at the particulate oxalate and HCHO, O3, or MHP are site at the same time (Li et al., 2005; Dasgupta et al., significantly weaker (r2 ¼ 0:310, 0.361, and 0.404, 2005) During 6 May to 11 May the wind field was respectively) during these periods. For particulate stable and referring to these data, gaseous oxalic nitrate and Ox, we sorted the hourly averaged data acid concentrations were very well correlated with into night and day bins and also calculated the wind that of HCHO (Fig. 5a, r2 ¼ 0:784). The correlation direction (and standard deviation) during this between H2Ox concentrations and O3 is almost as period. These data are plotted in Fig. 8. In all good (Fig. 5b, r2 ¼ 0:700). Hydrogen peroxide and cases, when the windfield is stable (this is more methyl hydroperoxide were similarly measured at typical in the nighttime than during the day), there the site (Li et al., 2003). It is interesting that H2Ox is superb correlation between the two species. 2 and H2O2 exhibited no correlation (r o0.0001) Moreover, on all but one of the nighttime data whereas methyl hydroperoxide, which requires an sets, the airmass flows out to sea, originating in a active hydrocarbon intermediate to be formed, is 90–1201 direction. The sampling site is 30 km due reasonably well correlated with H2Ox (Fig. 6, west of Old Tampa Bay and 60 km west from the r2 ¼ 0:609). Significant production of alkyl hydro- Gulf of Mexico. On the west of the sampling site,

ab 0.04 2 r = 0.700 r2 = 0.784

0.03

0.02 Oxalic acid (ppb)

0.01

02460 20406080100 Formaldehyde (ppb) Ozone, ppb

Fig. 5. Evidence of photochemical production. Relation between gaseous oxalic acid and (a) formaldehyde (b) ozone (6/5/2002–11/5/ 2002). ARTICLE IN PRESS P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269 4265

0.04

0.03

Oxalic acid (ppb) 0.02 r2 = 0.607

0.01

031 2 Methyl hydroperxide (ppb)

Fig. 6. Evidence of photochemical production. Relation between gaseous oxalic acid and methyl hydroperoxide (6/5/2002–11/5/2002).

0.6

0.4 (ppb) 3 HNO

0.2

daytime data, r2 = 0.642 nighttime data, r2 = 0.425 0 0.01 0.02 0.03 0.04

H2Ox (ppb)

Fig. 7. Relation between gaseous oxalic and nitric acids (4/5/2002–13/5/2002). ARTICLE IN PRESS 4266 P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269

4

3 ) 3

2 ( μ g/m - 3 Date Wind Dir r2 Slope NO May 4-5 196+/-120 0.71 17.8 May 5-6 89+/-10 0.98 7.1 May 7 129+/-62 0.86 6.4 May 7-8 1 111+/-16 0.98 6.6 May 8 142+/-102 0.71 3.0 May 8-9 119+/-11 0.96 7.1 May 9 165+/-55 0.77 6.5 May 9-10 119+/-30 0.99 8.4 May 10 97+/-41 0.69 7.3 May 10-11 110+/-13 0.91 5.6 0

0.2 0.4 0.6 0.8 2- μ 3 C2O4 ( g/m )

Fig. 8. Relation between particulate nitrate and oxalate, night and day sorted (5/5/2002–11/5/2002).

the Atlantic ocean is more than 200 km distant, et al., 2001). The senior author of this manuscript across the width of the peninsula. The recent history was responsible for deploying the first airborne IC of the sampled air mass is thus of inland origin. The for trace gas measurements in the summer of 1988 nitrate–oxalate relationship has a slope 7.071.0 on (Kok et al., 1990). He observed at the time that all five data sets and on the one occasion that the gaseous H2Ox concentration was always higher daytime wind direction falls within these bounds, it above forest canopies and have had reservations has a slope of 7.3. Remarkably, on the one night about auto exhaust being the dominant source of (4–5 May) that the wind direction was very variable oxalic acid since that time. Only very recently have and significantly originated from the direction of the alternative mechanisms and precursors been pro- bay where the power plants are located, nitrate was posed for oxalic acid (Warneck, 2003, 2005; Crahan proportionally much greater than oxalate. et al., 2004; Lim et al., 2005; Yu et al., 2005). In a All of these observations suggest a photochemical major urban area such as Tampa, automotive origin for oxalic acid and further support the exhaust emission could conceivably be the domi- contention that the formation of oxalate is linked nant source for OxT. However, the correlation of with that of nitrate. H2Ox with potential traffic markers such as NO, NO2,NOx, and especially CO were extremely poor 3.5. Is automotive emission a significant source of (r2 values, 0.0025, 0.0189, 0.0183, and 0.0308, oxalic acid? respectively). The correlation with toluene (data courtesy of R. Zika, University of Miami) was The bulk of the extant literature proposes that the investigated and also found to be poor (r2 ¼ primary source of oxalic acid is homogeneous gas 0:0037). Analysis of the oxalic acid/oxalate data as phase oxidation of aromatic hydrocarbons, most a function of wind direction did not indicate that notably toluene, originating from automotive ex- high oxalate concentrations are correlated with haust (Kawamura and Kaplan, 1987; Kawamura incoming airmass from downtown Tampa. These ARTICLE IN PRESS P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269 4267 results do not support the contention that auto- 0.4 motive exhaust emissions are a major source of H2Ox or precursors thereof. The corresponding correlations with Ox with any of the above 0.3 parameters were also very poor. 2 3.6. Oxalic acid formation and its implications in 2 * NO 0.2 r = 0.620 nitrogen processing T

A central concern in the BRACE studies is the Δ Ox extent of nitrogen deposition in the Tampa Bay 0.1 estuary. The location of N deposition can be dependent on the efficiency with which primary emissions of NOx, gases with relatively low deposi- 0 tion velocity, are converted to HNO3, a gas with very high deposition velocity (Pryor et al., 2002), or 0 0.1 0.2 0.3 0.4 0.5 Δ NO3T to particulate nitrate. The conversion of NO2 to HNO is brought about by dOH and the aqueous 3 Fig. 9. Oxalate as OH marker. Relation between the product of phase formation of oxalic acid from alkenes is also nitrogen dioxide and D(total oxalate)daytime and D(total nitrate)daytime thought to be brought about by dOH (Lim et al., for the period 10/5/2002–21/5/2002. 2005; Crahan et al., 2004). Indeed, one end product of oxidation of some of these alkenes is formalde- hyde, with which the excellent correlation of H2Ox concentration parameter being defined as f ðdOHÞ¼ has already been mentioned. We make the following 2 DNO3T=NO2 (r ¼ 0:514). assumptions:

(a) the production of oxalic acid is limited by the 4. Conclusions availability of dOH (since we do not have data on reactive hydrocarbons, we cannot take into Gas and aerosol phase data of several atmo- account any dependence on potential hydro- spheric species collected from a 5 week study in carbon precursors), and Tampa, FL has been used to study the gas–aerosol (b) the loss of OxT and NO3T (particulate nitrate partitioning behavior of H2Ox/Ox and to shed light and gaseous HNO3) from our system in a box into the parameters influencing the formation of occur at comparable rates. oxalic acid. The median Ox concentration is nearly an order of magnitude greater than H2Ox concen- With these assumptions, if we confine ourselves to tration which is consistent with what others have the daytime data when the OxT and NO3T are both observed elsewhere. Significant amounts of oxalate increasing, the incremental increase in OxT between occurred in an aerosol size larger than 2.5 mm. This two successive 15 min periods, DOxT, should be is consistent with appreciable amounts of oxalate related directly to the dOH concentration during being formed in liquid water associated with coarse that period. Considering that NO3T originates from particles. the reaction: The concentration maxima for H2Ox coincide with the maxima in temperature and insolation NO2 þ dOH ! HNO3 ! NO3 : (1) intensity. Gaseous H2Ox concentrations are often If DOxT is multiplied by the prevalent NO2 well correlated with levels of HCHO, HNO3, and O3 concentration, this should approximately predict (r2 ¼ 0:784, 0.642, and 0.704, respectively), all DNO3T during the same period, DNO3T being consistent with a photochemically mediated path defined the same way as DOxT. Although data for the formation of oxalate. The concentrations of + meeting both criteria were no plentiful, such a particulate NH4 and Ox are also often well plot is shown for hourly averaged data in Fig. 9 for correlated, especially in the nighttime (r2 ¼ 0:686) a 12-day period. Particulate oxalate alone is also which suggests that as temperature decreases, H2Ox reasonably correlated with the f(dOH), the dOH may be condensing as ammonium oxalate. ARTICLE IN PRESS 4268 P.K. Martinelango et al. / Atmospheric Environment 41 (2007) 4258–4269

Much of the extant literature suggests that the References primary source of oxalic acid is homogeneous gas phase oxidation of aromatic hydrocarbons originat- Al-Horr, R., Samanta, G., Dasgupta, P.K., 2003. A continuous ing from automotive exhaust followed by condensed analyzer for soluble anionic constituents and ammonium in atmospheric particulate matter. Environmental Science and phase reaction of the intermediates. We did not find Technology 37, 5711–5720. any correlation between H2Ox and/or Ox and traffic Boring, C.B., Al-Horr, R., Genfa, Z., Dasgupta, P.K., Martin, markers such as NO, NO2,NOx, and especially CO. M.W., Smith, W.F., 2002. Field Measurement of acid gases Since the primary focus of BRACE was to study and soluble anions in atmospheric particulate matter using a the extent of nitrogen deposition in the Tampa Bay parallel plate wet denuder and an alternating filter-based automated analysis system. 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