ARTICLE IN PRESS
Atmospheric Environment 41 (2007) 4258–4269 www.elsevier.com/locate/atmosenv
Atmospheric production of oxalic acid/oxalate and nitric acid/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 dicarboxylic acid (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 acids
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 m 3 and accumulated which unequivocally shows that not the particulate Ox was 1.3–749 ng m 3. 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 ammonium oxalate 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. pyruvic acid 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 m 3 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 PM10 2.5 components. More details are 1 ml min 1 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 mgm 3 76876 mgm 3. 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 mgm 3. 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 mgm 3 with mean and median values of 0.29 concentration is much higher than ambient concen- and 0.21 mgm 3. 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 min 1, 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 m 3) 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