Air Quality Model Evaluation Data for Organics. 3. Peroxyacetyl Nitrate

Environ. Sci. Technol. 1996, 30, 2704-2714

Air Quality Model Evaluation Data for Organics. 3. Peroxyacetyl Nitrate and Peroxypropionyl Nitrate in Los Angeles Air ERIC GROSJEAN,† D A N I E L G R O S J E A N , * ,† MATTHEW P. FRASER,‡ AND GLEN R. CASS‡ DGA, Inc., 4526 Telephone Road, Suite 205, Ventura, California 93003, and Department of Environmental Engineering Science, California Institute of Technology, Pasadena, California 91125

Ambient levels of peroxyacetyl nitrate [PAN, CH3C(O)OONO2] and peroxypropionyl nitrate [PPN, CH3CH2C(O)OONO2] have been measured during the 2-week period Aug 28-Sept 13, 1993, at four locations in the urban Los Angeles area. Highest concentrations recorded were 9.9 ppb for PAN and 1.5 ppb for PPN. Diurnal and spatial variations were consistent with photochemical production during eastward (inland) transport. Results for PAN are compared to those obtained in previous studies of ambient PAN in the urban Los Angeles area. Results for PPN, measured for the first time in the urban Los Angeles area, are compared to those recently obtained at southern California mountain sites and other locations. Ambient levels of PPN were correlated with those of PAN, with least squares linear regressions slopes (PPN vs PAN) of 0.153 ( 0.030 (Long Beach), 0.136 ( 0.026 (Los Angeles), 0.166 ( 0.018 (Azusa), and 0.097 ( 0.014 (Claremont). These ratios are compared to those measured previously at other locations and are briefly discussed with respect to spatial changes in the relative importance of those hydrocarbons that are precursors to PAN and PPN. The amount of PAN lost due to thermal decomposition was calculated and was comparable in magnitude to that measured.

Introduction Peroxyacyl nitrates, RC(O)OONO2, play an important role in atmospheric chemistry. The simplest peroxyacyl nitrate, CH3C(O)OONO2 (peroxyacetyl nitrate, hereafter PAN), was unknown to chemists until it was identified and characterized in urban southern California smog by Stephens and * Corresponding author telephone: (805)644-0125, fax: (805)6440142. † DGA, Inc. ‡ California Institute of Technology.

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co-workers in the late 1950s and early 1960s (1-4). Peroxyacyl nitrates are formed in situ in the atmosphere in reactions involving hydrocarbons and oxides of nitrogen. They have no known direct emission sources and, therefore, are good indicators of photochemical air pollution. They have received attention as eye irritants, mutagens, phytotoxins, and possible agents of skin cancer (4-10). They are also important parameters within computer kinetic models that attempt to simulate ozone formation in the atmosphere (11-13). While a number of computer models calculate ozone values that reasonably match measured ambient concentrations, some model applications yield scattered values for PAN and other photochemical products (14-16). Thus, the ability to successfully compute PAN along with ozone is often regarded as a test of the model’s reliability for applications to oxidant control strategies (12, 13, 17). Peroxyacyl nitrates have also received attention as potential sources of aldehydes and free radicals, as interferents in the measurements of NO2 by chemiluminescencesa method employed by virtually all air quality monitoring networks around the worldsand for their importance in the long-range transport of reactive nitrogen on regional and global scales (18-22). However, in spite of their importance, peroxyacyl nitrates are not included in federal, state, or local routine air quality surveillance networks in the United States or elsewhere. Ambient levels of PAN have been measured many times in southern California and, to a more limited extent, in other urban areas (refs 23 and 24 and references therein). To our knowledge, no measurements of PAN in the urban Los Angeles area have been made since 1987 when PAN was measured at several locations as part of the Southern California Air Quality Study (25). In the past 7 years, new air pollution control measures have been implemented, e.g., California Emission Vehicle and Clean Fuel Regulations (26). There also have been significant changes in the composition of vehicle fuels, e.g., the use of alcohols and ethers and the introduction of reformulated gasolines (27, 28). Peroxypropionyl nitrate [CH3CH2C(O)OONO2, hereafter PPN] has not been measured before in the urban Los Angeles area. Quantitative measurements of PPN and of the corresponding PPN/PAN concentration ratios are of value, for example, as a check on computer kinetic models that are applied to ozone control strategies, to assess air pollution damage to agricultural crops and natural vegetation (PPN is more phytotoxic than PAN; 7), and to examine uncertainties and inconsistences in hydrocarbon emission inventories (since only specific hydrocarbons are precursors to PAN, PPN, or both). Measurements of PAN and PPN and of the PPN/PAN concentration ratio may also be useful to assess the relative contribution of anthropogenic and biogenic hydrocarbons to ozone formation in urban areas. This is because PAN is a product of the oxidation of biogenic hydrocarbons including isoprene (29), terpenes (30), and unsaturated oxygenates (31, 32). There are also biogenic precursors of PPN, e.g., the unsaturated alcohol cis-3-hexen1-ol (31). Spatial variations of urban PAN have been characterized only once before (25); those of PPN have not been studied in urban southern California or elsewhere.

S0013-936X(95)00853-4 CCC: $12.00

 1996 American Chemical Society

TABLE 1

Summary of PAN Calibration Data Claremont prestudy calibrations (8/93) slopea SD on slope (%) correlation coeff poststudy calibrations (9/93) slopea SD on slope (%) correlation coeff av of prestudy and poststudy calibration slopes mean ( SD SD (%)

Long Beach

Azusa

Los Angeles

5.15 0.97 0.994

8.91 0.56 0.995

10.30 0.48 0.960

6.25 0.80 0.999

4.57 0.87 0.993

5.57 0.72 0.993

3.45 1.16 0.990

6.57 0.61 0.987

b

6.41 ( 0.16 2.5

4.86 ( 0.29 6.0

7.24 ( 1.67 23.0

a Linear least squares regression forced through origin of peak height (mm) attenuation setting 6 vs PAN (ppb). PAN concentrations ) 0-12 ppb, n ) 6 (prestudy calibrations) and 0-13.8 ppb, n ) 10 (poststudy calibrations). b Not employed for calculating ambient concentrations, see text.

For the reasons listed above, we have carried out in August-September 1993 field measurements of ambient levels of PAN and PPN at four southern California locations. This study was carried out as part of a larger research effort whose main objective was to acquire a comprehensive data base for organic air pollutants that can be used to test the performance of a photochemical airshed model designed to predict both particulate and gaseous organics concentrations directly from data on emissions from sources (33, 34).

Experimental Methods Electron Capture Gas Chromatography Measurements. PAN and PPN were measured at four urban locations in southern California: Long Beach (monitoring station operated by the South Coast Air Quality Management District (AQMD) at 3648 N. Long Beach Blvd.), downtown Los Angeles (AQMD station, Los Angeles Dept. of Water and Power building 3 at 1630 N. Main St.), Azusa (AQMD station at 803 N. Loren Ave.), and Claremont (instrument trailer operated by Unisearch Associates at the California Air Resources Board monitoring site, a gravel pit adjacent to Foothill Blvd. and the Claremont Colleges). PAN and PPN were measured by electron capture gas chromatography (EC-GC, 4, 35) using SRI 8610 gas chromatographs equipped with Valco 140 BN detectors as described previously (24, 25). The columns used were 80 × 0.3 cm Teflon-lined stainless steel columns packed with 10% Carbowax 400 on Chromosorb P 60/80 mesh, acid washed, and dimethylchlorosilane treated. The column temperature was 36 °C. The detector temperature was 60 °C. The carrier gas was ultra-high-purity nitrogen. The column flow rate was 55 mL min-1. Ambient air was continuously pumped through a sampling line connected to a 6.7-mL stainless steel sampling loop housed in the GC oven and was injected every 30 min using a timer-activated 10-port sampling valve. Retention times of PAN under these conditions were 5.41 ( 0.24 (Long Beach), 4.17 ( 0.09 (Los Angeles), 4.99 ( 0.22 (Azusa), and 5.56 ( 0.32 min (Claremont). Retention times for PPN were 6.97 ( 0.50 (Long Beach), 5.30 ( 0.18 (Los Angeles), 6.47 ( 0.24 (Azusa), and 7.21 ( 0.46 min (Claremont). Retention time ratios (PPN/PAN) were 1.28 ( 0.09 (Long Beach), 1.26 ( 0.05 (Los Angeles), 1.29 ( 0.06 (Azusa), and 1.30 ( 0.09 (Claremont). Alkyl nitrates and several halogenated hydrocarbons are also detected by EC-GC and elute on the columns employed in this study. We have verified in previous work that these compounds do not co-elute or otherwise interfere when measuring PAN and PPN in ambient air (36).

Ambient air was sampled using 6 mm diameter Teflon tubing sampling lines with 25 mm diameter, 1.2 µm pore size Teflon filter inlets. The sampling lines were 7.0, 9.0, 7.3, and 4.8 m long in Azusa, Los Angeles, Long Beach, and Claremont, respectively, and the air sampling flow rates were 280-370 mL min-1. Tests carried out in the field and in the laboratory with sampling line lengths (0.6, 1.8, 7.3, and 9.1 m) and sampling flow rates (129, 190, 270, 370, and 440 mL min-1) that bracketed those employed in the field indicated no measurable differences (less than 2%) in measured PAN concentrations. At the four sampling locations, daily checks were made of the following parameters: carrier gas pressure, detector baseline voltage, detector baseline adjustment setting, detector attenuation and temperature control settings, GC column and sampling loop temperature, GC oven temperature control unit, and recorder-integrator functions. Sampling flow rates and GC column flow rates were verified twice at each location. The inlet Teflon filters were inspected and replaced as appropriate. Field tests carried out to verify that PAN concentration measured with and without Teflon filter inlets (with both new filters and with filters that had been used as inlets for several days) gave identical results. The presence of PAN and PPN in ambient air was verified by inserting a tube heated to 115-180 °C in the sampling line upstream of the EC-GC sampling loop. This resulted in 83-95% decreases in the heights of the peaks that had the correct retention times for PAN and PPN. EC-GC Calibrations. To calibrate the gas chromatographs, PAN and PPN were synthesized in the liquid phase (37-39) by oxidation of the commercially available anhydride (Aldrich, purity 97-99%) to the corresponding peroxycarboxylic acid followed by nitration of the peroxy carboxylic acid with nitric acid: H+

(RCO)2O + 2H2O2 98 H2O + 2RC(O)OOH

(R ) CH3 or C2H5) (1)

H+

RC(O)OOH + HNO3 98 H2O + RC(O)OONO2 (2) Solutions of PAN and PPN in n-dodecane (Aldrich, purity >99%) were stored at -5 °C in the dark. These solutions contained small amounts of alkyl nitrates, which are decomposition products of the peroxyacyl nitrates in the liquid phase, i.e., methyl nitrate from PAN and ethyl nitrate from PPN. Retention times relative to that of PAN were 0.36 for methyl nitrate and 0.48 for ethyl nitrate, i.e., these

VOL. 30, NO. 9, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Frequency distribution of ambient concentrations of peroxyacetyl nitrate and peroxypropionyl nitrate from measurements made at four locations in the urban Los Angeles area, Aug 28-Sept 13, 1993. Concentration intervals are indicated as follows for clarity: 0.25 ) from detection limit to 0.25 ppb; 0.50 ) 0.251-0.50 ppb, and so on.

alkyl nitrates are well resolved from the corresponding peroxyacyl nitrates. Parts per billion (ppb) levels of PAN and PPN in the gas phase were obtained by dilution with purified air of the output of diffusion vials containing aliquots of a solution of peroxyacyl nitrate in n-dodecane and maintained at 2 °C (31, 39). Calibration involved simultaneous measurements with the gas chromatographs and with a Monitor Labs 8840 chemiluminescent NOx analyzer, which employs a surface converter (molybdenum) to convert oxides of nitrogen to NO and responds quantitatively to peroxyacyl nitrates including PAN and PPN (21, 39). The chemiluminescent NOx analyzer was calibrated using the diluted outputs of a certified cylinder of NO in N2 and of a certified NO2 permeation tube maintained at 30.0 ( 0.1 °C. Details of the calibration procedure have been given previously (36). We have verified, by inserting a tube heated to 180 °C upstream of the chemiluminescent NOx analyzer, that the converter efficiency for PAN and for PPN was the same as that for NO2. During the calibrations, the four gas chromatographs sampled purified air containing PAN through a common glass manifold. Sampling line lengths and sampling flow rates were within the range of those shown to have no effect on measured peroxyacyl concentrations (see above). We also verified that loss of PAN in the manifold was negligible by carrying out colocated measurements of PAN with and without the sampling manifold. The four EC-GC instruments were calibrated twice, once prior to the field study and once shortly thereafter. Concentrations of PAN in the calibration experiments were 0-14 ppb and bracketed those measured in ambient air at the four locations. Linear regression of the data yielded the slopes (peak height vs ppb) and other statistical parameters that are listed in Table 1. The calibration data given in Table 1 are consistent with those obtained for PAN and PPN with the same instruments as part of laboratory

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and field studies carried out in the past 5 years (29, 30, 36, 39, 40). Detection limits for field measurements varied slightly from one instrument to the next and were ca. 0.010 ppb for PAN and ca. 0.016 ppb for PPN. For the EC-GC instruments operated in Claremont and in Los Angeles, data from the prestudy and poststudy calibrations were in good agreement with relative standard deviations on the mean of the two calibration slopes of 6.0 and 2.5%, respectively. The agreement between prestudy and poststudy calibrations was not as satisfactory for the instrument operated in Long Beach (23% standard deviation on the mean of the prestudy and poststudy calibration slopes). Poststudy checks of the instrument performance gave no indication of loss in sensitivity, and the mean value of the two calibration slopes was used to calculate ambient concentrations. For the instrument operated in Azusa, it became apparent from the daily checks described earlier and from comparison of the diurnal profiles of PAN at the four monitoring locations that a gradual loss in sensitivity was taking place toward the end of the study. This was verified in the field by carrying out, for several days, colocated measurements of ambient PAN in Azusa with two gas chromatographs. Indeed, the poststudy calibration indicated a 3-fold decrease in sensitivity (Table 1). Accordingly, calculations of ambient PAN in Azusa were made using (a) the calibration slope of the prestudy calibration for all measurements made between Aug 28 and Sept 6, (b) a sliding scale (equal increments from prestudy to poststudy values of the calibration slopes) for all measurements made from Sept 7 to Sept 11, and (c) the chromatograms recorded with the second (colocated) gas chromatograph and that instrument’s calibration slope for the measurements made on Sept 12 and Sept 13.

Results and Discussion Ambient levels of PAN and PPN were measured every 30 min in Long Beach (Aug 29-Sept 13, 1993), Los Angeles

TABLE 2

Highest and 24-h Averaged PAN and PPN Concentrations at Four Urban Los Angeles Locations Long Beach PAN (ppb) date (1993) highest 8/28 8/29 8/30 8/31 9/1 9/2 9/3 9/4 9/5 9/6 9/7 9/8 9/9 9/10 9/11 9/12 9/13 a

0.48 1.73 1.10 2.14 1.28 3.00 4.01 1.35 1.45 2.83 4.07 5.52 5.52 3.38 1.97 0.21

ava 0.34 0.50 0.31 0.63 0.48 0.95 0.92 0.53 0.60 1.28 1.33 1.37 1.94 1.74 0.56 0.15

Los Angeles

PPN (ppb) highest

0.35 0.49 0.35 0.23 0.40 0.75 0.35 0.20 0.40 0.75 0.86 0.81 0.40 0.26 0.29

av

PAN (ppb) highest

0.24 0.30 0.16 0.17 0.19 0.27 0.15 0.14 0.22 0.34 0.31 0.42 0.24 0.14 0.29

0.23 1.68 3.94 4.13 3.12 5.62 3.74 4.45 3.74 6.86 4.52 4.52 0.23

av

0.14 0.44 1.05 1.25 0.96 1.26 0.84 1.06 0.94 1.61 1.12 1.44 0.16

Azusa

PPN (ppb) highest

0.33 0.52 0.59 0.39 0.85 0.46 0.52 0.46 1.04 0.52 0.98 0.07

PAN (ppb)

av

0.18 0.24 0.28 0.23 0.29 0.18 0.16 0.19 0.32 0.23 0.31 0.07

Claremont PPN (ppb)

highest

av

highest

av

4.03 4.17 5.63 4.85 4.95 4.56 4.37 3.88 3.45 3.42 2.35 3.49 4.87 6.09 3.21 4.94 1.85

2.15 1.36 1.92 2.96 2.42 2.04 1.98 1.96 1.47 1.79 0.97 1.98 1.69 2.19 1.85 1.78 0.53

0.44 0.57 0.85 0.89 0.97 0.85 0.81 0.89 0.69 0.65 0.49 0.70 1.17 1.46 0.26 0.43

0.27 0.42 0.50 0.55 0.55 0.45 0.51 0.52 0.43 0.37 0.39 0.51 0.64 0.88 0.19 0.25

PAN (ppb)

PPN (ppb)

highest

av

highest

av

2.37 4.32 5.04 5.97 4.84 5.14 5.14 6.38 9.88 9.47 7.30

0.81 1.79 2.56 2.60 2.07 2.77 2.79 3.26 4.26 4.96 7.30

0.17 0.34 0.47 0.73 0.39 0.39 0.47 0.77 1.20 1.20 0.69

0.17 0.21 0.25 0.26 0.21 0.25 0.28 0.30 0.41 0.53 0.69

Average of concentrations > detection limits of 0.010 ppb for PAN and 0.016 ppb for PPN.

TABLE 3

Highest Ambient Levels of PAN (ppb) in Southern California and Other Urban Areas, 1985-1994a southern California year

urban Los Angeles area

1985

Claremont 20

1986 1987

Glendora 35 Claremont 30c Los Angeles 11c, 13d Anaheim 7c, 19d Long Beach 16c, 15d Rubidoux 14c Burbank13c, 19d Hawthorne 16d Azusa 13c Los Angeles 14 Pasadena 12

1988 1989 1990 1991

other locations

San Nicolas Island 1c

Malibu 7 Ventura 4 Tanbark Flat 16 Palm Springs 3 Perris 7 Tanbark Flat 22 Tanbark Flat 13 Franklin Canyon 7

1992 1993

elsewhere Athens, Greece 4 Rio de Janeiro, Brazil 5 Paris, France 9 Paris, France 20, 34 (47)b

Long Beach 5.5g Los Angeles 6.9g Azusa 6.1g Claremont 9.9g

1994

Munich, Germany 5.6 (49) Munich, Germany 2.0 (49) Atlanta, GA 3 South DeKalb, GA 3 (48)e Montelibretti, Italy 40 (50)f Montelibretti, Italy 16 (50)f

Montelibretti, Italy 7 (50)f

a Adapted from refs 24 and 36 unless otherwise indicated; limited here to the past 10 years, see these references for a review of earlier data. The first value is from on-site measurements in Creteil, a southeast suburb of Paris. The second value is from flask samples collected 20 km downwind. c Smog season (June-Sept). d Late fall (Nov-Dec). e Suburban Atlanta location, Aug 1-31, 1992 (48). f In Tiber Valley, downwind of Rome, Italy. g This study, Aug 28-Sept 13, 1993. b

(Aug 30-Sept 11, 1993), Azusa (Aug 28-Sept 13, 1993), and Claremont (Sept 1-11, 1993), thus yielding some 2600 ambient measurements. PAN and PPN were present at concentrations that were above the detection limits of 0.010 ppb for PAN and 0.016 ppb for PPN in 2372 and 1407 chromatograms, respectively. Individual values are not included here and are summarized in the frequency distributions shown in Figure 1. Highest and 24-h averaged

PAN and PPN concentrations are given in Table 2. Peroxyacetyl Nitrate. The highest PAN concentration recorded was 9.9 ppb, measured in Claremont on Sept 9. Highest concentrations at the other locations were 6.9 ppb in Los Angeles, 6.1 ppb in Azusa, and 5.5 ppb in Long Beach. Location-averaged PAN concentrations, a parameter that may be useful to assess population exposure and damage to vegetation, were 0.92 ( 0.94 ppb (one standard deviation,


9

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the same four locations during the l987 smog season (25). Nighttime and early morning minima reflect among other factors increased emissions of nitric oxide and the subsequent reaction of PAN with NO to yield formaldehyde, NO2, and the HO2 radical:

CH3C(O)OONO2 h CH3CO3 + NO2

(3, -3)

CH3CO3 + NO f CH3 + CO2 + NO2

(4)

CH3 + O2 f CH3O2

(5)

CH3O2 + NO f CH3O + NO2

(6)

CH3O + O2 f HO2 + HCHO

(7)

Mid-day maxima reflect in situ production by photochemical reactions of hydrocarbons that are precursors to the acetyl radical:

HC + O3, OH f f CH3CO

FIGURE 2. Highest PAN concentrations measured from 1967 to 1994 in southern California, at other U.S. locations and at urban locations worldwide.

average of concentrations g detection limit of 0.010 ppb) in Long Beach (n ) 691), 1.06 ( 1.27 ppb in Los Angeles (n ) 544), 1.76 ( 1.33 ppb in Azusa (n ) 703), and 2.99 ( 1.64 ppb in Claremont (n ) 434). Ambient levels of PAN measured during this study are comparable to those measured in other urban areas in recent years but are lower than those recorded in southern California in the past 10 years (Table 3). For example, we have measured PAN levels of up to 30, 19, 16, and 13 ppb in Claremont, Anaheim, Long Beach, and Los Angeles, respectively, in 1987 (25) and levels of up to 16, 22, and 13 ppb in 1989, 1990, and 1991, respectively, at a mountain forest location (Tanbark Flat) north of Azusa (36, 39). Shown in Figure 2 is a time series plot of the highest PAN concentrations measured in urban southern California from 1967 to 1993 (data for other urban areas within the United States and for other urban areas worldwide are included in Figure 2 for comparison). The time series plot shows a trend toward lower PAN maxima in southern California and other U.S. urban locations in recent years. The measurements of high PAN concentrations made in early years were carried out by EC-GC in a manner very similar to that employed in this study, and it is unlikely that these early measurements could have suffered from a systematic positive bias. The apparent decrease in peak levels of PAN may reflect, besides year-to-year differences in meteorology, a possible decrease in urban emissions of reactive organics and/or oxides of nitrogen that are precursors to PAN. Diurnal variations of PAN during the period studied exhibited mid-day maxima and nighttime/early morning minima. Examples of diurnal profiles are given in Figure 3. These diurnal profiles are similar to those recorded at

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(8)

followed by CH3CO + O2 f CH3CO3 and the subsequent competition between reaction -3 and reaction 4. Diurnal profiles such as those shown in Figure 3 indicate that the time of maximum PAN concentration (which at all locations closely matched that for maximum ozone) is shifted toward the afternoon at the more inland locations, i.e., from Long Beach and Los Angeles to Azusa and Claremont. This is consistent, as observed before (25), with eastward (inland) transport of polluted air. Composite diurnal profiles, i.e., diurnal profiles that have been averaged over the entire study period, are shown in Figure 4. Levels of PAN are seen to increase gradually from the source region (Long Beach and Los Angeles) to the inland smog receptor areas (Azusa and Claremont). This trend is consistent with photochemical production of PAN during eastward transport of polluted air (25). At the inland smog receptor location, Claremont, nighttime levels of peroxyacetyl nitrate were higher than those observed at the other three locations, see Figures 3 and 4. This is because less NO is available than is the case at the coastal and central locations, resulting in a less severe nighttime sink for PAN via reaction -3 and reaction 4. Peroxypropionyl Nitrate. The highest concentration of PPN, 1.5 ppb, was recorded in Azusa on Sept 10. Highest levels of PPN recorded at the other locations were 0.9, 1.0 and 1.2 ppb in Long Beach, Los Angeles, and Claremont, respectively. PPN concentrations were typically in the range 0.15-0.30 ppb (ca. 60% of the total number of observations, see the frequency distribution shown in Figure 1). Locationaveraged PPN concentrations were 0.25 ( 0.15 ppb (one standard deviation, average of concentrations g detection limit of 0.016 ppb) in Long Beach (n ) 354), 0.24 ( 0.18 ppb in Los Angeles (n ) 319), 0.47 ( 0.24 ppb in Azusa (n ) 331), and 0.31 ( 0.17 ppb in Claremont (n ) 403). Diurnal variations of PPN concentrations matched closely those of PAN at all four locations (see Figure 3), thus indicating a common origin for both compounds. The time of maximum PPN concentrations is shifted toward the afternoon at the more inland locations, i.e., from Long Beach and Los Angeles to Azusa and Claremont. The composite diurnal profiles of PPN shown in Figure 4 are, as is the case for PAN, consistent with photochemical formation of PPN during eastward (inland) transport. Formation of PPN involves photochemical oxidation of those hydrocarbons

FIGURE 3. Diurnal variations of PAN (a) and PPN (b) Sept 8-9, 1993.

that are precursors to the CH3CH2CO radical:

HC + O3, OH f CH3CH2CO

(9)

followed by CH3CH2CO + O2 f CH3CH2CO3 and the subsequent competition between reaction with NO2 to form PPN and reaction with NO, the latter leading to acetaldehyde + NO2 + HO2 in a reaction sequence (not shown) analogous

to reactions 3-7. As discussed earlier for PAN and for the same reasons, nighttime levels of PPN were higher at the inland locations (Azusa and Claremont) than at the coastal and central locations (Long Beach and Los Angeles). To our knowledge, no previous data are available for the Los Angeles area for comparison with the ambient concentrations of PPN measured during this study. PPN has been recently measured during three consecutive smog


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FIGURE 4. Composite diurnal profiles of PAN (a) and PPN (b) Aug 28-Sept 13, 1993.

seasons at a mountain forest location bordering the Los Angeles urban area, i.e., Tanbark Flat, north of Azusa. The highest levels recorded were 5.1 ppb in 1989, 4.3 ppb in 1990, and 2.7 ppb in 1991 (36, 39). These concentrations are higher than those measured in this study. We have also measured PPN at several southern California locations (36, 41) and in downtown Atlanta, GA (24). The highest levels of PPN ranged from 0.4 to 1.2 ppb (Table 4). No

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other urban data are available for comparison. PPN/PAN Concentration Ratios. A frequency distribution of the PPN/PAN concentration ratios is shown in Figure 5, where ambient measurements made at the four locations are pooled together. As the linear least squares regressions parameters of PPN vs PAN listed in Table 5 indicate, PPN and PAN were highly correlated at each of the four locations. Examples of scatter plots of PPN vs PAN are given in Figure

TABLE 4

Highest PPN Concentrations and PPN/PAN Concentration Ratios location, date (month/day/year) Tanbark Flat, CA, 8/8-10/16/89a,b Palm Springs, CA, 8/23-8/25/89c,d Perris, CA, 8/25-8/27/89c,e Tanbark Flat, CA, 8/3-9/5/90a,b Edison, CA, 7/21-7/26/90a,f Tanbark Flat, CA, 8/5-8/26/91a,b Franklin Canyon, CA, 9/4-9/15/91a,g Atlanta, GA, 7/22-8/26/92h Long Beach, CA, 8/29-9/13/93j Los Angeles, CA, 8/30-9/11/93j Azusa, CA, 8/28-9/11/93j Claremont, CA, 9/1-9/11/93j

highest PPN (ppb)

av PPN/PAN concn ratio (ppb/ppb)

5.1 0.42 0.73 4.3

0.280 ( 0.002 0.125-0.135c 0.135-0.182c 0.182 ( 0.001 0.11 ( 0.03 0.187 ( 0.001 0.140 ( 0.002 0.11 ( 0.01i 0.153 ( 0.030 0.136 ( 0.026 0.166 ( 0.018 0.097 ( 0.014

2.70 1.2 0.37 0.86 1.04 1.46 1.20

a From ref 36. b In San Gabriel Mountains northeast of Los Angeles and north of Azusa. c From ref 41, PPN/PAN ratios measured at the time of maximum PAN. d 120 km east of Los Angeles. e 90 km eastsoutheast of Los Angeles. f Near Bakersfield, CA. g In Santa Monica Mountains west of Los Angeles. h From ref 24. i Probably upper limit for actual ratio since PPN was detected only 15% of the time (24). j This study.

FIGURE 5. Frequency distribution for PPN/PAN ambient concentration ratios from measurements made at four locations in urban Los Angeles, Aug 28-Sept 13, 1993. Increments of concentration ratios are indicated as in Figure 1.

6. PPN/PAN concentration ratios, i.e., the slopes of the linear regressions given in Table 5, ranged from 0.097 ( 0.014 to 0.166 ( 0.08. These ratios are consistent with those derived from earlier work (see Table 4) carried out in Atlanta, GA, (0.11 ( 0.01) and at several southern California locations (from 0.13 in Palm Springs to 0.28 in the San Gabriel Mountains). The PPN/PAN ambient concentration ratios reflect differences in formation and removal rates. From a detailed examination of emission inventory data for hydrocarbons that are precursors to PAN and PPN, we have estimated in previous work that, for the urban Los Angeles area, the PPN/PAN in situ production rate ratio was about 0.56 (36).

Measured ambient concentration ratios, 0.10-0.28, are lower than the calculated production ratio. This is not unexpected since hydrocarbon emission inventories are subject to large uncertainties (42). We have speculated in earlier work (36) that PPN/PAN ratios may vary with air mass transport time according to the reactivity of the hydrocarbon precursors. Thus, PPN/PAN ratios for short transport times are expected to reflect the contribution of the most reactive category of PPN and PAN precursors, i.e., olefins, for which the PPN/PAN production rate ratio estimated from emission inventory data is 0.55. For intermediate transport times, alkenes are nearly consumed, and the PPN/PAN ratio is expected to decrease since it now reflects the contribution of intermediate reactivity hydrocarbons, i.e., aromatics and aldehydes, with estimated PPN/ PAN production rate ratios of 0.16 and 0.12, respectively. For longer transport times, the PPN/PAN ratio may increase again to reflect the now dominant contribution of the less reactive peroxyacyl nitrate precursors, i.e., the alkanes, with estimated PPN/PAN production rate ratio of 0.88. For even longer transport times, e.g. transport of the Los Angeles urban plume to the Grand Canyon area, the PPN/PAN ratio should decrease again since the less reactive hydrocarbons still present, ethane and acetone, are oxidixed to products that include PAN but not PPN (36). The PPN/PAN ambient concentration ratios measured in this study are consistent with the above considerations. The highest PPN/PAN concentration ratios were measured in Long Beach, Los Angeles, and Azusa, where PAN and PPN formation is presumably dominated by photochemical oxidation of alkenes. The lowest PPN/PAN concentration ratio was measured at the inland smog receptor site, Claremont. This ratio may reflect a lower PPN production rate (relative to PAN) together with a higher removal rate for PPN (again relative to PAN). Removal processes for PAN and PPN include thermal decomposition, reaction with OH, and deposition. The deposition velocity of PPN has not been measured; it is probably similar to that of PAN. Thermal decomposition of PPN, for which only limited data are available, appears to be comparable in magnitude to that of PAN, and with approximately the same temperature dependence (40, 43, 44). Reaction with OH is negligibly slow for PAN (45). No data are available for the reaction of OH with PPN. Structure-reactivity considerations suggest that k (OH + PPN) < k (OH + propane) owing to the electron-withdrawing influence of the CO3NO2 group. Using k (OH + propane) ) 1.15 × 10-12 molecule cm-3 s-1 at 298 K (45) as an upper limit (this corresponds to a PPN half-life of about 1 week if [OH] ) 1.0 × 106 molecule cm3) and with an arbitrary transport time of 4-6 h, we estimate that only a few percent of the PPN formed upwind would have been consumed by reaction with OH at the time the air mass reaches Claremont. Thus, the lower PPN/PAN ratios observed in Claremont may reflect lower formation rates (from intermediate reactivity hydrocarbons) rather than higher removal by reaction with OH. Diurnal variations of the PPN/PAN ratios are shown in Figure 7 in the form of composite profiles for each location. At all locations, the PPN/PAN ratios are fairly constant during daylight hours and the late evening. Higher PPN/ PAN ratios are observed at night at the urban locations but not at the downwind locations. Higher PPN/PAN ratios at night may reflect preferential formation of PPN, preferential removal of PAN (e.g., faster deposition relative to that of PPN), or diurnal variations in the emissions of PAN and


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TABLE 5

Summary of Statistical Parameters for PPN/PAN Concentrations Ratios location

slope

SD on slope (%)

av PAN (ppb)

av PPN (ppb)

R

n

Long Beach Los Angeles Azusa Claremont

0.153 ( 0.030 0.136 ( 0.026 0.166 ( 0.018 0.097 ( 0.014

19.6 19.1 10.8 14.4

1.41 ( 1.08 1.65 ( 1.38 2.82 ( 1.07 3.15 ( 1.58

0.25 ( 0.15 0.24 ( 0.18 0.47 ( 0.24 0.31 ( 0.17

0.793 0.897 0.786 0.925

354 319 331 403

a Slope ( one SD from least squares linear regression, not forced through origin, of PPN (ppb) vs PAN (ppb) for data from all chromatographs in which PPN was detected (PPN g 0.016 ppb). Linear regressions not forced through the origin gave near-zero intercepts of 92 ( 8 ppt in Long Beach (one standard deviation), R ) 0.793; 48 ( 7 ppt in Los Angeles, R ) 0.897; -24 ( 23 ppt in Azusa, R ) 0.786; and -4 ( 7 ppt in Claremont, R ) 0.925, (1 ppt ) 10-3 ppb).

FIGURE 6. Scatter plots of PPN versus PAN for Los Angeles and Claremont. Linear regression parameters are given in Table 5.

PPN precursors. The higher PPN/PAN concentration ratios at night may simply reflect data scatter since both PAN and PPN are low and often close to detection at night at the urban locations. In contrast, no scatter or trend toward higher PPN/PAN ratios was observed at the downwind locations, where nighttime levels of PAN and PPN were higher than those measured at the urban locations. Further analysis of the diurnal variations in the PPN/PAN concentration ratio will be carried out when other air quality data acquired as part of this study are analyzed in more detail. Magnitude of PAN Removal by Thermal Decomposition. The results obtained in this study provide an opportunity to calculate the extent of thermal decomposition, which is a major removal process for peroxyacyl nitrates in the atmosphere. From reactions 3, -3, and 4 above, loss of PAN due to thermal decomposition is given by (40, 46):

-d (ln [PAN]/dt) ) (k3k4[NO])/(k4[NO] + k-3[NO2]) (10) 2712

9


where k3 (in units of s-1) ) 2.25 × 1016 e(-13,573/T) and k4/k-3 ) 1.95, independent of temperature (46). The loss of PAN due to thermal decomposition was calculated according to eq 10 using the ambient NO and NO2 concentrations measured in this study and the corresponding ambient temperature data. The amount of PAN removed by thermal decomposition hourly is compared to the measured PAN concentration at each hour in Figure 8 for the four locations and for the severe smog episode of Sept 8-9, 1993. The data shown in Figure 8 indicate, as expected, strong diurnal variations in the extent of PAN thermal decomposition with minima at night, maxima in the afternoon (at the time of maximum temperature), and in several instances secondary maxima in the morning hours (reflecting the increase in NO concentrations associated with commuter vehicle traffic). The data shown in Figure 8 also indicate that a substantial fraction of the PAN formed in photochemical reactions is removed by thermal decomposition within the urban Los Angeles area. As reactions 3-7 indicate, thermal decomposition of PAN leads to formaldehyde and free radicals.

FIGURE 7. Composite diurnal profiles of the PPN/PAN ambient concentration ratio at four locations in the urban Los Angeles area, Aug 28-Sept 13, 1993.

FIGURE 8. Diurnal variations in PAN concentrations at four locations in the urban Los Angeles area during the Sept 8-9, 1993, smog episode. Solid bars: measured PAN concentrations. Open bars: PAN concentrations calculated to have been lost during that hour due to thermal decomposition.

Acknowledgments This research was supported by the Electric Power Research Institute under Agreement RP 3189-03, by the California Institute of Technology Center for Air Quality Analysis, and by internal R&D funds, DGA, Inc., Ventura, CA. We thank

the 13 California Institute of Technology graduate students and staff who participated in air monitoring site operation for their assistance; Fabrice Grosjean (California State University, Northridge) for assistance in field operations and in data reduction; William Bope (South Coast Air Quality


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Management District, Diamond Bar) for logisitical support in Azusa, Long Beach, and Los Angeles, and Lowell Ashbaugh (California Air Resources Board) and Gervaise MacKay (Unisearch Associates) for logistical support in Claremont. Denise Velez prepared the draft and final versions of the manuscript.

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Received for review November 13, 1995. Revised manuscript received April 16, 1996. Accepted April 26, 1996.X ES9508535 X

Abstract published in Advance ACS Abstracts, June 15, 1996.

Air Quality Model Evaluation Data for Organics. 3. Peroxyacetyl Nitrate

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