Environ. Sci. Technol. 1991, 25, 710-715
Ambient Levels of Formaldehyde, Acetaldehyde, and Formic Acid in Southern California: Results of a One-Year Base-Line Study Daniel Grosjean DGA, Inc., 4526 Telephone Road, Suite 205, Ventura, California 93003
Ambient levels of formaldehyde, acetaldehyde (six locations), and formic acid (one location) have been measured over a l-year period (September 1988-September 1989) in southern California. Samples were collected every sixth day by using DNPH-coated CIScartridges and were analyzed by liquid chromatography with UV detection. Method evaluation and validation studies included Sampling efficiency, elution recovery, effect of humidity, sampling parameters, and analytical performance. The 24-h ambient levels of formaldehyde, acetaldehyde, and formic acid reached up to 29, 13, and 8 ppb, respectively, with location-averaged values of 5.0-6.1, 2.9-4.8, and 2.8 ppb, respectively. Concentration ratios were 0.48-0.80 for acetaldehyde/formaldehyde and 0.53 for formic acid/ formaldehyde. Seasonal variations are briefly discussed. The data obtained may serve as a “base line” to evaluate the possible impact of methanol fuel on ambient levels of aldehydes and of formic acid in southern California.
Introduction Ambient formaldehyde can contribute significantly to the overall risk associated with population exposure to toxic air pollutants in urban areas. As an indoor pollutant, formaldehyde is currently regulated for its carcinogenic properties (1). As an outdoor pollutant, formaldehyde is directly emitted by mobile and stationary sources. Formaldehyde is also formed in the atmosphere by photochemical reactions involving virtually all classes of hydrocarbon pollutants. During smog episodes, in situ production of formaldehyde may be larger than direct emissions, e.g., 500 vs 3G70 tons per day in southern California ( 2 ) . Concurrently, formaldehyde is removed from the atmosphere by photolysis, by reaction with the hydroxyl radical and the nitrate radical, and by wet and dry deposition ( 3 ) . Ambient levels of formaldehyde in southern California have been documented several times as part of intensive, short-term studies ( 2 , 4-18). While these short-term studies have provided useful information regarding ambient levels of formaldehyde and their diurnal and spatial variations, a long-term monitoring program to characterize population exposure has never been carried out. The need for such a long-term monitoring effort appears even more critical in view of the projected large-scale introduction of methanol as an alternative fuel (19). The current interest in methanol stems from the realization that current emission control technologies are near their practical limits and that substituting methanol for conventional, more reactive fuels may lead to progress toward meeting the federal ozone air quality standard (19). However, the use of methanol may result in higher emissions of formaldehyde and higher ambient levels as well (19, 20). Therefore, it is important to obtain “base-line” data for ambient formaldehyde prior to the introduction of methanol as a fuel. Accordingly, we have undertaken a 1-year monitoring survey of ambient levels of formaldehyde at six locations in southern California. Also included in this article are long-term monitoring data for acetaldehyde and formic acid. Formic acid is the metabolite involved in methanol 710
Environ. Sci. Technol., Vol. 25, No. 4, 1991
toxicity. Methanol fuel use may impact formaldehyde and acetaldehyde in opposite directions, i.e., a possible increase for formaldehyde (direct emissions and in situ formation from emitted methanol) and a possible decrease for acetaldehyde (replacement, by methanol, of fuel hydrocarbons that are precursors to acetaldehyde in vehicle exhaust and in photochemical smog reactions). Thus, acquiring “base-line” data for acetaldehyde and formic acid is also relevant to the overall assessment of methanol as an alternative fuel.
Experimental Methods Sampling Protocol. Ambient air samples were collected a t six South Coast Air Quality Management District (AQMD) monitoring stations: Anaheim, Azusa, Burbank, Hawthorne, Upland, and West Los Angeles. A 24-h sample was collected every sixth day a t each location from September 12, 1988, to September 25, 1989, using portable sampling units, which included a calibrated flowmeter, a vacuum gauge, a timer, a sampling pump, and associated tubing and electrical connections. Small CI8 cartridges (Sep-Pak, Millipore Corp.) coated with 2,4-dinitrophenylhydrazine(DNPH) were employed to collect formaldehyde, acetaldehyde, and other carbonyls (21, 22). Each cartridge was sealed with Teflon tape, wrapped in aluminum foil, and stored in a glass vial sealed with a Teflon-lined screw cap. The sampling configuration initially involved a 25-mm-diameter Teflon filter housed in an open-face plastic (Delrin) filter holder a t the inlet of a 10-15 ft, 1 / 4 in. diameter Teflon sampling line, with the cartridge located next to the sampling unit. The Teflon filter inlet was subsequently removed and replaced by the cartridge itself, which was oriented downward and protected by aluminum foil to minimize exposure to sunlight and to rain. Sampling flow rates were in the range 70-470 mL/min, with most samples being collected at 200-250 mL/min, corresponding to volumes of air sampled of 0.29-0.46 m3 (mean 0.33 m3). Pressure drops through the cartridges were 9-20 in. HzO, thus requiring sampling flow rate corrections of 3.6 f 1.2%. The cartridges were stored refrigerated in the dark at all times before and after sampling. Ambient formic acid was measured at one location, Upland, according to the same schedule and sampling protocol, but with cartridges coated with KOH (23)instead of DNPH. The formic acid and aldehyde sampling units were colocated and operated on a common timer. Analytical Protocol. Our protocol for carbonyl analysis has been described in detail (22) and only a brief summary is given here. DNPH cartridges were eluted with acetonitrile and were analyzed by liquid chromatography with ultraviolet detection. The DNPH derivatives of formaldehyde and other carbonyls were separated on a Whatman Partisphere 5-pM CIS column, 110 X 4.7 mm, with 55:45 CH3CN-HzO eluent at a flow rate of 1mL/min. The detection wavelength was 360 nm. The liquid chromatograph components included a solvent delivery system equipped with 0.2-pm pore size Teflon filters, a SSI Model 300 pump, a 20-pL Valco injection loop, a Whatman
0013-936X/91/0925-0710$02.50/0
0 1991 American Chemical Society
Table I. Analytical Accuracy: Response to Independently Prepared Hydrazone Standards date (1988-1989) September October November December January February March April May June July August September
formaldehvdea aeak ht. mm att 9. for i p g / k ~carbonyl std 5A std C std B std D mean 22.6 23.6 22.1 20.2 22.0 19.0 20.4 19.0 22.6 23.0 22.2 19.7 21.2
25.1 25.3 25.6 18.7 21.0
20.1 17.7 18.0 17.9 21.5 22.5 23.8 18.7 18.9
23.8 24.4 23.8 19.4 21.0 18.3 19.2 18.5 22.0 22.7 23.0 19.2 20.0
RSD, %
5.2 3.5 7.3 4.1 4.8 3.8 6.2 2.7 2.7 1.1
3.5 2.6 6.0
acetaldehvde” oeak ht. mm att 9. for i pgjmL carbonyl std 5A std C std B std D mean 15.6 16.2 15.4 13.7 15.6 13.0 13.9 13.0 15.6 15.8 16.8 13.6 14.1
16.8 16.4 16.9 13.3 15.2
14.9 13.0 13.4 13.4 15.9 16.5 17.6 14.0 13.7
RSD, 7’0
16.2 16.3 16.1 13.5 15.2 13.0 13.7 13.2 15.8 16.1 17.2 13.8 13.9
3.7 0.6 4.6 1.8 2.6 0 2.2 1.5 1.3 2.5 2.3 1.4 1.4
OConcentrations (pg/mL) of formaldehyde and acetaldehyde standards: 5A, 5.00 and 5.31; C, 2.86 and 3.93; B, 2.86 and 4.71; D, 3.16 and 3.61.
Partisphere CI8 guard cartridge, a Perkin-Elmer LC 75 UV-visible detector, and a Hitachi D2000 recorder. Quantitative analysis involved the use of external hydrazone standards, from which calibration curves, i.e., absorbance (peak height) vs concentration, were constructed. Following llOH cartridge elution with HPLC-grade water, formic acid was measured as formate by liquid chromatography (23) using a Hamilton PRP-X-300 anion size-exclusion column, 0.5 mM aqueous H2S04as eluent, and ultraviolet detection at 210 nm. The eluent flow rate was 1 mL/min and the injection volume was 100 pL. All analyses included solvent and reagent blanks, cartridge laboratory blanks, cartridge field controls, multiple injections of a t least two independently prepared standards, and replicate analyses of 10% of the field samples.
Results and Lliscussion Method Performance for Aldehydes Measurements. Analytical detection limits were 9 and 12 ng/cartridge for formaldehyde and acetaldehyde, respectively (22). The carbonyl “background” content of DNPH cartridges was verified by analyzing cartridges taken at random from each batch prepared. Formaldehyde ranged from 35 to 132 ng/cartridge, corresponding to a small positive bias of 0.08-0.31 ppbl (mean value 0.16 ppb). The cartridge “background” acetaldehyde content corresponded to ambient levels of 0.1-0.35 ppb. Acetone was also present as a background impurity. Other carbonyls were not detected, with detection limits of 20-100 ng/cartridge. A measure of analytical accuracy is given by the results of analyses of several standards independently prepared from hydrazories synthesized and purified in our laboratory. The data in Table I indicate accuracy of 1-770 for formaldehyde and of 1-4% for acetaldehyde in 13 sets of comparisons carried out over a 13-month period, with each comparison involving two or three independently prepared hydrazone standards. Analytical precision was 1-7 70for formaldehyde and 1-9% for acetaldehyde for multiple injections of standards and was typicidly less than 5% for replicate analysis of field samples. The data in Table I indicate good overall stability of the liquid chromatograph, with day-to-day fluctuations of less than 14% in instrument response over a 13-month period during which more than 1000 samples were analyzed for this project and for other studies. The cartridge collection efficiency is >0.96 for all carbonyls (22) and was not verified further in this work. Cartridge elution recovery, established in earlier work to be >0.99 for all carbonyls studied (22),was verified by
carrying out two consecutive elutions of four field samples and was >0.99 for formaldehyde, acetaldehyde, acetone, propanal, n-butanal, methyl ethyl ketone, and benzaldehyde. Long-term stability of cartridge extracts stored refrigerated in glass vials with Teflon-lined caps was examined by reanalyzing two field samples 1 year after the first analysis. The results were within 5% of the initial values for both formaldehyde and acetaldehyde. The chromatograms were also identical for all other carbonyls, with no indication of hydrazone decomposition. Two hydrazone recovery experiments were carried out. The first one involved spiking of three DNPH-coated cartridges with 1 f 0.1 pg of carbonyl hydrazone per cartridge, followed by elution and analysis. The second one involved sampling of ambient air for 24 h at 0.3 L/min on two sets of cartridges, with one set spiked before sampling with 1 f 0.1 pg of carbonyl hydrazone per cartridge. Within experimental precision, recovery of formaldehyde, acetaldehyde, and acetone was quantitative in both experiments. Effect of Humidity on Aldehyde Cartridge Collection Efficiency. The effect of humidity on carbonyl collection by DNPH-coated cartridges has not been studied before and was investigated by collecting air samples with sets of cartridge (upstream)-impinger (downstream) sampling units. Impinger samples are not subject to humidity effects. The DNPH-coated cartridges were identical with those we employed in the field. The impingers contained 1 mL of a mixture containing 0.85 g of DNPH 40 mL of CH&N + 10 mL of H3P04 and 12 mL of a 50:50 CH3CN-H20 mixture. The cartridge-impinger sampling trains were operated at 0.2 L/min for 22.5 h and sampled a photochemical mixture prepared by sunlight irradiation, in a 3.5-m3 chamber constructed from 200A FEP Teflon film, of 0.25 ppm NO and 1 ppm 2-methyl-2-butene in purified air a t ambient humidity. The air humidity was varied by adding dry cylinder air to the photochemical mixture, and three sets of samples were collected at RH = 71,44, and 28%. The results are summarized in Table I1 and indicate good cartridge collection efficiency, i.e., 88-99%, in the range of relative humidity studied. Collection efficiencies for formaldehyde were 96 f 2 ( n = 3), 95 f 3 ( n = 2) and 88 f 1% ( n = 3) a t R H = 71,44, and 28%, respectively. Those for acetaldehyde were 99 f 1 ( n = 3), 99 (n = l),and 92 f 1%( n = 2 ) . Thus, cartridge collection efficiencies are high in the range of humidity relevant to ambient sampling but appear to decrease slightly at low air humidity.
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Environ. Sci. Technol., Vol. 25, No. 4, 1991
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Table 11. Effect of Air Humidity on Cartridge Collection Efficiency formaldehyde
Table 111. Elution Recovery of Formate from Alkaline Cartridges
acetaldehyde
RH
= 71% cartridge (upstreamp impinger (downstreamp cartridge, % of total RH = 44% cartridge' impingeP cartridge, % of total RH = 28% cartridge' impinger' cartridge, % of total ~~~
21.0 20.3 11.7 32.1 24.7 1.0 0.1 0.7 0.4 0.4 95 99 94 99 98
42.3 0.1 99
9.0 0.8 92
43.6 15.4' >74'
sample date (198&1989) 12/17 12/23 12/29 812 Si8 8/14
~~~
11.1 28.6 37.8 14.9 0.5 0.5 98 99
0.90 10.4 6.2 0.13 1.3 0.8 87 89 88
5.1 0.5 91
27.3 7.3b >79'
peak height, mm 1st 2nd 3rd elutn elutn elutn 144 46 120 16 64 50
10 7 8 8 10 22
0 0
10 n
22.0 1.6 93
Units. pg carbonyl per sample; volume of air sampled, 0.270 m3 (22.5 h a t 0.2 L/min): matrix air is the photochemical smog mixture prepared by sunlight irradiation of 2-methyl-2-butene-NO. in purified air a t ambient humidity; mixture contains carbonyls, ozone, PAN, nitric acid, etc. Humidities lower than 71% were obtained by dilution of the mixture with dry zero air. bPossible leak in sampling train, measured value is lower limit for cartridge eol-
34 16 47
9/19 9/25
6 2 16
8
93.5 86.8 93.7 66.6 86.5 69.4 94.1 100 88.6 85.0 85.0 88.9 74.6
l e d o n effieianev.
Effect of S a m p l i n g Line, I n l e t Filters, a n d "Passive" Sampling. The possible removal of aldehydes by the inlet filter and by the Teflon sampling line upstream of the cartridge was examined in the laboratory. Tests were carried out at ambient relative humidity by collecting cartridge samples from a sunlight-irradiated nitric oxide2-methyl-2-butene smog mixture containing ppb levels of formaldehyde, acetaldehyde, acetone, other carbonyls, and other reaction products (ozone, PAN, etc.). The results of these tests indicated no measurable loss of carbonyl in sampling lines ranging from 0.9 to 7.6 m, on upstream 25-mm-diameter Teflon or Nylon filters, or in empty inlet Delrin holders. The negligible retention of aldehydes on inlet filters was verified by analysis of the filters used at all six field locations: the highest filter content was equivalent to 0.10 ppb for formaldehyde and 0.02 ppb for acetaldehyde. Mean values were 0.04 and 0.015 ppb, respectively. Cartridges were installed a t the field sites 0.5-2 days prior to sampling startup and were typically retrieved within 0.5-2 days following sample collection. Thus, passive sampling time was typically 1-4 days, vs 1day for actual (active) sampling. Laboratory studies carried out for 72 b with a 2 m long, in. diameter Teflon line showed that passive uptake of aldehydes was indeed measurable, but was only 0.4-0.9% of the amount collected by active sampling over the same length of time. Thus, the corresponding worst-case positive bias for passive sampling in the field was only 3.6% for formaldehyde, 2.0% for acetaldehyde, and 1.7% for acetone. Method Performance f o r Formic Acid Measurements. Analytical precision was 1.7-7.3% (average 4.1%) for multiple injections of hydrazone standards and was 0.6-6.4% (average 2.4%) for replicate analysis of field samples. The amount of formate in laboratory blanks varied from batch to batch and was equivalent to 0.06-0.77 ppb. The formate content of field controls was the same as that of laboratory blanks. Cartridge elution recovery, which has not been studied in earlier work, was investigated by carrying out two to three consecutive elutions of 13 field samples. The results, listed in Table 111,indicate good recovery in all cases. A recent interlaboratory comparison study indicated good agreement between our method and another method, Fourier transform infrared spectroscopy, in side-by-side measurements of ambient 712 Environ. Sci. Technol.. Vol. 25. No. 4, 1991
,
recovery 1st elutn, 1st + 2nd 90 of elutn, % 1st + 2nd of total 100
100
88
100
89
1 60
73.5 120
g f
5A 9
5
