Environ. Sci. Technol. 2009 43, 8752–8758
Isotopic Composition of Formaldehyde in Urban Air
PAUL QUAY Department of Oceanography, Box 357940, University of Washington, Seattle, Washington 98195
Received July 16, 2009. Revised manuscript received October 19, 2009. Accepted October 19, 2009.
The isotopic composition of atmospheric formaldehyde was measured in air samples collected in urban Seattle, Washington. A recently developed gas chromatography-isotope ratio mass spectrometry analytical technique was used to extract formaldehyde directly from whole air, separate it from other volatile organic compounds, and measure its 13C/12C and D/H ratio. Measurements of formaldehyde concentration were also made concomitant with isotope ratio. Results of the analysis of nine discrete air samples for δ13C-HCHO have a relatively small range in isotopic composition (-31 to -25‰ versus VPDB [(1.3‰]) over a considerable concentration range (0.8-4.4 ppb [(15%]). In contrast, analyses of 17 air samples for δDHCHO show a large range (-296 to +210‰ versus VSMOW [(50‰]) over the concentrations measured (0.5-2.9 ppb). Observations of δD are weakly anticorrelated with concentration. Isotopic data are interpreted using both source- and sinkbased approaches. Results of δ13C-HCHO are similar to those observed previously for a number of nonmethane hydrocarbons in urban environments and variability can be reconciled with a simple sink-based model. The large variability observed in δDHCHO favors a source-based interpretation with HCHO depleted in deuterium from primary sources of HCHO (i.e., combustion) and HCHO enriched in deuterium from secondary photochemical sources (i.e., hydrocarbon oxidation).
Introduction Formaldehyde (HCHO) is the most abundant carbonyl compound in the atmosphere and among the most abundant volatile organic compounds (VOCs). On the global scale, the majority of HCHO is a product of hydrocarbon oxidation, the largest source of this being CH4 (1). As a partially oxidized product, HCHO is a tracer of VOCs and their reactivity and can provide information on regional and global atmospheric chemistry (2). Improved understanding of HCHO cycling is crucial to our comprehension of the oxidative capacity of the atmosphere. Of particular importance is the oxidation of HCHO by reaction with OH and its photolysis: (i)
* Corresponding author e-mail:
[email protected]. 8752
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(ii)
HCHO + hv (λ < 365 nm) f H2 + CO
(iii)
and the subsequent reactions of the H and HCO radicals generated in (i) through (iii):
ANDREW L. RICE* Department of Physics, Portland State University, Portland, Oregon 97207
HCHO + OH f HCO + H2O
HCHO + hv (λ < 337 nm) f HCO + H
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HCO + O2 f HO2 + CO
(iv)
H + O2 + M f HO2 + M
(v)
to produce the hydroperoxyl radical (HO2) and subsequently the OH radical. Thus HCHO is linked with the HOx cycles as well as those of O3 and NOx. HCHO is a major source of CO and H2 (∼50%), via reactions (ii), (iii), and (iv), species which, in turn, significantly affect the level of OH. HCHO sinks of secondary importance include the reaction with the nitrate radical (NO3) at night and both wet and dry deposition. With a growing number of concentration measurements resulting from a number of new fast response analytical techniques, characterization of HCHO distributions in the remote and polluted atmospheres is improving (3). Despite this growing database atmospheric models generally underestimate observed concentrations, particularly in remote atmospheres, indicating that we lack a fundamental knowledge of processes controlling HCHO distributions (4). Abundances of naturally occurring stable isotopes have provided valuable information on sources and sinks of a number of the more abundant trace gases (i.e., >100 ppb) in the recent past including CO2, CH4, CO, N2O, and H2 (5). The rates of physical, chemical, and biological processes are sensitive to small differences in atomic mass because of the shift in vibrational energy levels which results in a kinetic isotope effect (KIE, expressed as a ratio of the rate constants) in production or destructive processes. KIEs associated with deuterium substitution are particularly strong due to the large relative mass difference and much slower rates of tunneling relative to hydrogen (6). The manifestation of these isotope fractionation effects in natural systems is through isotopic depletion or enrichment of reaction products compared to reactants. By convention, isotopic differences are expressed differentially using the δ notation in parts per thousand [e.g., δ13C(‰) ) [(13C/12C)sam/(13C/12C)std - 1] × 1000 relative to the Pee Dee Belemnite (V-PDB) carbonate standard as established by the International Atomic Energy Agency (IAEA) in Vienna, Austria (7, 8). D/H ratios are similarly expressed using δD reported relative to Standard Mean Ocean Water (VSMOW) standard]. Pioneering work by Rudolph et al. (9) introduced a new application of trace gas stable isotopic analyses for VOCs. Study of one particular class of chemical species, oxygenated VOCs (OVOCs), has yet to take advantage of many of the recent advances in continuous-flow isotope ratio mass spectrometry (IRMS) technology and, as a result, measurements are particularly sparse. Yet many of these species (e.g., HCHO, methanol, acetone) are particularly well suited to isotopic measurement because their atmospheric sources are so varied, i.e., both natural and anthropogenic, both directly emitted and photochemically derived. Measurement of δ13C, δD, and δ18O of HCHO should improve our understanding of HCHO photochemistry and help resolve recent discrepancy between measured and modeled HCHO concentrations. In regions where both natural and anthropogenic and/or both primary and secondary sources of HCHO are present, one might expect isotopic measurements to distinguish between these pro10.1021/es9010916 CCC: $40.75
2009 American Chemical Society
Published on Web 11/05/2009
cesses and potentially yield quantitative estimates of the relative source strengths. Measurements of the isotopic composition of HCHO combined with photolysis isotope effects will add important constraints on the global atmospheric H2 and CO budgets. The isotopic composition of HCHO is the last of the three large known sources of H2 that has yet to be characterized (10). Similarly, isotope budgets of CO could use δ13C-HCHO to integrate the δ13C signature of photochemical CO sources (11). Previously, two dozen measurements of δ13C-HCHO were reported with δ13C-HCHO values ranging from -17 to -40‰ from remote, rural, or mixed atmospheres (12-15). However, one significant drawback in these studies is the lack of temporal resolution in isotope measurements as a result of long collection periods (typically ∼24 h) to generate sufficient sample for their isotopic measurement techniques. Given the short lifetime of HCHO, hourly concentration and isotopic resolution would be useful to distinguish changes in sources. Additional shortcomings of the analytical methods used previously are problems associated with collection efficiencies and corrections for non-HCHO carbon (12-15). A new continuous-flow IRMS method, which provided measurements in work presented here, uses grab samples of air orders of magnitude smaller in size that can provide high temporal resolution and thus the potential to examine relationships among HCHO concentration, isotope ratio, and source variability (16). Of equal significance, the method provides the first measurements of δD-HCHO. Here, we present a handful of measurements of the carbon and hydrogen isotopic composition of atmospheric HCHO from samples collected in urban Seattle, Washington. We then provide two competing interpretations of the resulting data and attempt to reconcile the results with our understanding of HCHO sources and sinks and with prior results.
Methods Air samples were collected from the fourth floor deck of the Ocean Science Building at the University of Washington (Seattle, WA 47.6° N, 122.4° W) through a 6.35 mm (o.d.) × 6 m inlet tube of PFA Teflon tubing. Time of sampling was limited to daylight hours, 8 a.m. to 5 p.m. (local time). Collections were made into an evacuated (95%). Though no correlation of δD-HCHO with time of day was apparent in these data, all but one positive δDHCHO value is from the March-May sampling period (Table S1). There are no published results with which to compare our results (see Discussion).
Discussion Here, we present interpretations of observed variability in coupled HCHO concentration and isotopic results in terms of HCHO sources and sink effects independently. We then discuss our conclusions based on these two constructs and endeavor to understand these and prior results. Source Interpretation. Levels of HCHO in urban atmospheres are strongly influenced by direct sources, the most dominant of these is thought to result from incomplete combustion of fossil fuels. Photochemical sources of HCHO in urban areas are from oxidation of natural and anthropogenic VOCs. Known δ13C and δD signatures of important HCHO sources are shown in Table 1. 8754
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Values of δ13C reported here are comparable to δ13C values (average -27 ( 2‰) of nonmethane hydrocarbons (NMHCs) from vehicle sources and the δ13C of alkanes observed in polluted urban ambient air in Japan (∼ -27‰) (21, 22). Based on two samples of elevated HCHO concentration (5 and 13 ppb) collected in a parking garage, we estimate the δ13CHCHO produced by the internal combustion engine to be between -26 and -28‰. These values are indistinguishable from δ13C-HCHO values determined in urban air (-29 ( 2‰), δ13C of NHMCs from automotive sources, and the δ13C of crude oil (21). Natural photochemical HCHO sources are primarily biogenic VOCs. In ambient air, isoprene has been observed to vary considerably (-16 to -29‰) with extent of photochemical oxidation due to KIEs in reactions with OH and O3 (23). Lowest values approach the δ13C of isoprene emitted from plants (-27.7‰ and -29.2‰) (23, 24). CH4 oxidation is also known to produce a background of HCHO, the mass weighted isotopic source (δ13C) of which is ∼ -52‰, but is not a significant source fraction in urban air (17). KIEs associated with reactions of a number of hydrocarbons with OH and O3 range from k12C/k13C ≈ 1.002 to 1.02, with a preponderance of values on the low end (25, 26). Assuming this first step determines the KIE during oxidation to HCHO, which may be a reasonable first order assumption due to the very short lifetime of intermediates, HCHO produced in hydrocarbon oxidation will be depleted by ∼2 to 20‰ relative to the δ13C of the hydrocarbon source. Under this assumption, the reaction of OH with propane of a δ13C value -28‰ will yield HCHO with a δ13C ≈ -31‰, for example. Although to date there are no δD measurements of either direct or secondary HCHO sources, we expect HCHO emitted from combustion sources to be depleted in δD based on the depleted δD values of H2 (∼ -180 to -310‰) and CH4 (∼ -80 to -170‰) produced by the internal combustion engine and measurements of δD (∼ -120‰) of crude oil (10, 27-30). Likewise, biomass burning has been determined to produce depleted δD values of H2 and CH4 (∼ -200 to -350‰) (10, 31). Thus, we speculate that the depleted δDHCHO values measured at higher HCHO concentrations show more influence from direct HCHO sources associated with fossil fuel combustion. Photochemically produced HCHO is considered to be enriched in deuterium relative to parent material (10, 32-34). As hydrocarbon oxidation proceeds in the atmosphere, the abstraction of a proton occurs preferentially over a deuteron which leaves partially oxidized material dramatically enriched in deuterium. For example, the preferential abstraction of a proton from CH3D to CH2D will change the molecular D:H ratio from 1:3 to 1:2. Though this effect is partially offset by
the fact that the reaction of CH3D will proceed ∼30% more slowly than that of CH4 (35), the resulting HCHO could plausibly be enriched in deuterium by 2:1 relative to parent CH4 in oxidation (32, 33). Similar enrichments in the oxidation of NMHCs will also occur, due to KIEs in their reactions with OH and O3, which are of order 10-100‰ (36). Though the deuterium content of CH4 is well-known (-90‰), the same is not true of VOCs (17). Sink Interpretation. The isotopic composition of HCHO is significantly altered during HCHO loss via photolysis (ii-iii) and reactions with OH (i) and NO3. Several of the isotope effects in these removal processes were recently measured (Table 1) (32, 37-40). Of particular importance to this work are the carbon and hydrogen photolysis isotope effects which will enrich both H13CHO and HCDO; in the reaction of HCHO with OH the KIE will yield a H13CHO depletion, through a rarely occurring inverse KIE, and a HCDO enrichment. To examine sink effects on the isotopic composition of HCHO in more detail, we construct a simple mathematical framework following the work of Rudolph and Czuba (41). We estimate photochemical isotopic enrichment in an urban setting based on isotope effects during OH oxidation and photolysis (neglecting reactions with NO3 and deposition which are of lesser importance during daytime hours). Since the photochemical lifetime of HCHO is determined by photolysis (ii-iii) and by reaction with OH (i), lifetimes of these isotopologues may be described as: τHCHO )
τH13CHO )
τHCDO )
1 OH JHCHO + kHCHO [OH]
JH13CHO
JHCDO
(vi)
1 OH + kH13CHO [OH]
(vii)
1 OH + kHCDO [OH]
(viii)
The ratio of isotopologues can then be determined from the mean photochemical age (t):
[
[H13CHO]t [H13CHO]0 t∆τ ) exp 2 [HCHO]t [HCHO]0 τHCHO + τHCHO∆τ
[
[HCDO]0 [HCDO]t t∆τ ) exp 2 [HCHO]t [HCHO]0 τHCHO + τHCHO∆τ
]
]
(ix)
(x)
where ∆τ is the difference in lifetime between the two isotopologues and the isotopic ratios at the source (e.g., [HCDO]0/[HCHO]0) are invariant. Within this construct, the isotopic enrichment of HCHO from its initial (emitted) value (∆δ) is a function of the chemical lifetime and the difference in chemical lifetimes of the isotopologues. Likewise, the mean photochemical age of HCHO can be predicted from a measured isotope ratio (∆δ). It is this property that we employ below. As the chemical lifetime of HCHO is short and isotope effects are large, no approximation is made for ∆τ/τ , 1, as previously (41). To estimate photochemical lifetime of HCHO in Seattle, photolysis rates for each day of observation are calculated based on the National Center for Atmospheric Research Tropospheric Ultraviolet and Visible radiation model for the two channels of HCHO photolysis (ii-iii) (available at cprm.acd.ucar.edu/Models/TUV/index.shtml). Levels of OH are taken to be 106 molecules cm-3 for a mean value under moderately polluted conditions (42). Rate constant of HCHO + OH is taken to be 9.0 × 10-12 cm3 molecule-1 s-1 (43). Based on this model, lifetimes for HCHO on days of sample collection are determined to range 5-13 h.
FIGURE 2. Effects of photochemical age on the carbon (a) and hydrogen (b) isotopic composition of atmospheric formaldehyde. Model results (shaded region) show the range of theoretical predictions over sampling periods and demonstrate the sensitivity of isotopic composition to changes in actinic flux. Dashed lines (s s s) represent estimates with mean OH levels increased by 50%; dotted lines ( · · · · · ) represent estimates with mean OH levels decreased by 50%. Observations are plotted as difference data (∆δ) along the model curve generated for the particular day and time of observation, assuming that the most depleted isotopic observations represent a mean chemical age of zero hours. To calculate the lifetimes of H13CHO and HCDO, we use lifetimes of HCHO and isotope effects for reactions i-iii in Table 1. Using resulting isotopologue lifetimes and eqs ix-x for each observational period, we then generate a curve that relates the change in isotopic composition (∆δ) to mean photochemical age (t). Figure 2 demonstrates the impact of photochemical age and chemical lifetime on change in δ13C (a) and δD (b) of HCHO. The range of model results generated for sampling periods (represented by shaded regions, Figure 2) results from a strong sensitivity of isotope ratio to changes in actinic flux which determines photolysis lifetimes. Sensitivity to OH concentration was tested by varying levels of OH by (50% and is small compared with a measurement precision. Increasing the rates of chemical oxidation has the opposite impact on values of δ13C due to the inverse KIE (∼ -50‰), thus acting against the photolysis enrichment (Table 1). The impacts on δD are in the same direction with OH as with photolysis, i.e., increasing chemical oxidation leads to an enrichment in deuterium. We plot observations in Figure 2 expressed as difference values, ∆δ13C and ∆δD. In the absence of data, we assume here that the most depleted observed isotopic values (δ13C and δD) represent a mean chemical age of zero. By overlaying each difference value (∆δ13C and ∆δD) on the modeled curve generated for each observation period, we estimate the mean chemical age of HCHO. Observed δ13C values predict HCHO mean chemical ages which range up to 0.4 h, which are very VOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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plausible in an urban area where sources are numerous (Figure 2a). In contrast, the observed δD-HCHO provides predictions of mean chemical ages which range up to 12 h (Figure 2b). With calculated lifetimes on the order of these estimates (5-13 h), it is difficult to justify observed values enriched by more than 200‰ with this mechanism in an urban area. Further, with all the observations of δ13C-HCHO estimated to be within a mean photochemical age of 1 h, it is difficult to reconcile the two isotopic (δ13C and δD) results following this mechanistic approach solely. The long mean chemical ages needed to explain large deviations in δD-HCHO and the lack of corresponding variability observed in δ13CHCHO favors a source-driven isotopic interpretation for the wide range of observed values of δD-HCHO. Analysis of both isotopic species from the same air sampled would clearly be beneficial in the future. Such double isotopic analysis was not feasible here due to difficulties switching between analytical methods on short time scales, concerns over sample storage, and a limited number of sample canisters (2). Finally, we note the sensitivity of this approach to KIEs, of which there is some disagreement. In particular, if we use the substantially smaller carbon photolysis KIE of k12C/k13C ) 1.025 reported by Stone (37), we calculate longer mean photochemical ages of up to 23 h based on δ13C-HCHO data. For δD-HCHO, if we use the larger value of kH/kD ) 1.96 reported by Rhee et al. (40), we estimate mean photochemical ages that are shorter by ∼20%. Further Discussion. Within the framework of HCHO sources and sinks (Table 1), is it possible to reconcile the handful of measurements of δ13C-HCHO to date? Our measurements (mean -29 ( 2‰) are similar to primary sources of NMHCs from mobile sources, unrefined oil, and isoprene isotopic signatures. With the relatively small carbon kinetic isotope effects in hydrocarbon oxidation, the source of observed HCHO in our results could be indicative of either primary or secondary sources of HCHO in the urban atmosphere. Our values are indistinguishable with δ13C values of Wen et al. (14) from near a petrochemical refinery (-27‰) but are considerably depleted compared with δ13C values near a bus station in the same work (-18‰) and may reflect a difference in direct HCHO hydrocarbon source. Recent measurements of δ13C-HCHO in ambient air in Guangzhou, China by Yu et al. (15) (-40‰) are also quite different from those reported here suggesting that there is more variability in δ13C of HCHO sources than observed in Seattle. Our measurements are also similar to those from Baring Head despite the differences between HCHO sources at these locations (urban vs remote) (12). Chemically aged VOC photochemical sources and the increased importance of δ13CHCHO photochemical enrichment (Figure 2a) in remote atmospheres may play a significant role in the similarity of our results. Similarly, through photolytic enrichment of HCHO and its hydrocarbon sources it is possible to explain enriched values of δ13C-HCHO observed by Johnson and Dawson at Mt. Lemmon (-17.0‰) and Tanner et al. (-22.5‰) in Nova Scotia (12, 13). Both sites were impacted by secondary biogenic hydrocarbon sources. In addition to the enriching effect of chemical age on the δ13C of biogenic VOCs (source value ∼ -28‰), evidence suggests that isoprene can be emitted over a range of δ13C values depending whether the plant is of C3 or C4 character, and due to light and temperature effects (23). Overall the variability observed in the several studies of δ13C-HCHO is suggestive that it may be a useful indicator of HCHO sources or their extent of HCHO oxidation in an airmass (or both). Given the substantial discrepancies between the aforementioned studies more measurements of δ13C-HCHO in urban settings are certainly justified. Measurements of δ13C-HCHO from sources that impact urban air, at this point, would be particularly useful as these would 8756
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potentially provide a method to partition between sources in ambient air. Additionally, measurements of δ13C-HCHO have implications for the CO budget. HCHO serves as an indicator for integrated hydrocarbon sources of CO, including CH4, isoprene and terpenes, and anthropogenic volatile organic compounds. More numerous measurements in remote, rural, and urban settings should identify this isotopic signature and help constrain the isotopic CO budget. The observed δD-HCHO range of ∼500‰ and its inverse correlation with concentration (Figure 1b) potentially can be theoretically explained in terms of HCHO sink effects (Figure 2) or variability in the sources. However, the long HCHO chemical ages necessary to explain the δD-HCHO range based on a sink approach is unrealistic in an urban atmosphere. Rather, the anticipation that primary sources of HCHO (i.e., from incomplete combustion of fossil fuels and industrial sources) are depleted in deuterium while secondary sources are enriched provides a plausible explanation of observations. In this framework, lower HCHO concentrations observed here are impacted primarily by secondary sources and high concentrations are dominated by primary sources. If this is correct, δD-HCHO could provide a powerful new technique to help distinguish between primary and secondary sources of HCHO. The observation of HCHO enriched in deuterium (up to +200‰) helps to reconcile the isotopic budget of H2. Several recent estimates of the isotopic signature (δD) of photochemically derived H2 vary widely but are considerably enriched compared with other H2 sources (+130‰ to 340‰; see review by Ehhalt and Rohrer (34)). Assuming the enriched values observed here are a mix of primary and secondary HCHO sources, it will be possible to balance the H2 isotope budget without a radical departure from generally recognized source and sink strengths. However, recent measurement by Feilberg et al. (32) and Rhee et al. (40) of a large tropospheric HCHO and HCDO photolytic isotope effect to the molecular channel products H2 and HD (JHCHOfH2+CO/ JHCDOfHD+CO ) 1.8-2.0) requires that tropospheric HCHO be enriched by >1000‰ at steady state, though the steady state assumption is likely not valid for HCHO. Even more recent work suggests that the isotope effect in the molecular channel has a strong pressure dependence and is smaller at lower pressures (44). In any case, while observations of δD-HCHO up to +200‰ suggest a photochemical mechanism for enrichment is possible, future measurements of δD-HCHO in the remote troposphere will be needed to confirm this hypothesis. Overall, the δ13C and δD of HCHO results presented here suggest that the isotopic composition will be a useful tracer of sources and sinks of HCHO in the Earth’s atmosphere. The analytical method of isotopic analysis used here has the potential to provide the high temporal and spatial resolution necessary to quantify HCHO sources (16). It could, potentially, also be adapted for measurements of oxygen isotopes, which could provide information on the extent of oxidative processing.
Acknowledgments We gratefully acknowledge discussions and additional help from Richard Gammon, David Wilbur, and Johnny Stutsman. Research support for this work was provided by NSF (ATM0091878) and was made possible by a major research instrumentation award from NSF (OCE-0220868). This publication is partially funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement NA17RJ1232.
Supporting Information Available Details of each measurement (Table S1) including meteorological information accompanying each sample. This
information is available free of charge via the Internet at http://pubs.acs.org.
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