Environ. Sci. Technol. 1993, 27, 1905-1910
Measurements of Peroxyacetyl Nitrate at a Remote Site in the Southwestern United States: Tropospheric Implications Jeffrey S. Gaffney,' Nancy A. Marley
Environmental Research Division, Argonne National Laboratory, Argonne, Illinois 60439 Eric W. Prestbo
Chemistry Department, University of Washington, Seattle, Seattle, Washington 98195
Peroxyacetyl nitrate (PAN) measurements were taken a t a remote site in the southwestern United States from October 1987through January 1989by using an automated gas chromatograph with electron capture detection. The monthly mean PAN concentrations were found to correlate well with temperature, as PAN levels were higher in the colder periods, consistent with PAN'S thermochemical stability. PAN values ranged from below detection limits of 0.030-1.91 ppbv (parts per billion by volume) during this period. During the same period ozone levels were 8-77 ppbv. Monthly mean PAN levels were observed to be inversely correlated with monthly mean ozone levels. These data are discussed with regard to the known thermochemical reactions of PAN and the regional tropospheric implications for the formation of peroxyacetic acid and organic peroxides. In addition, these data are evaluated with regard to PAN contributions as a trace greenhouse gas.
Introduction Peroxyacetyl nitrate (PAN) has been observed to be an important photochemically derived pollutant in urban atmospheres and has also been found to be ubiquitous in the remote troposphere ( 1 , 2 ) .The chemical and physical properties of peroxyacyl nitrates were reviewed recently (1). PAN is a potent lachrymator and phytotoxin as well as a greenhouse gas ( I , 3, 4 ) . Laboratory investigations have indicated that thermochemical decomposition pathways are the predominant loss mechanisms for PAN ( I , 2, 5, 6), because its reaction rate with OH (7) and its photolysis are slow (8). PAN has a low water solubility and therefore, a low wet deposition rate (9). The principal pathway for the thermal decomposition of PAN is the reaction to form peroxyacetyl radical and nitrogen dioxide (1, 2, 4 ) :
-
CH,COO,NO, CH,COO, + NO, (1) The reverse reaction regenerates PAN when nitrogen dioxide is present. The equilibrium is upset, and PAN is lost when either the peroxyacetyl radical or the nitrogen dioxide levels are reduced. Nitrogen dioxide can be reduced by photolysis or by reaction with ozone to form nitrate radical (at night) or OH to form nitric acid. The loss of PAN via the peroxyacetyl radical involves two importantprocesses, the reactions with NO and with HO2:
-
+ NO + 0, CH,CO02 + HO,
CH,COO,
CH,O,
+ CO, + NO,
(2)
+
CH,COO,H 0, (3) Reaction 3 also leads to the formation of ozone and acetic acid approximately 20% of the time (10).
* Author to whom correspondence should be addressed. 0013-936X/93/0927-1905$04.00/0
0 1993 American Chemical Society
Although PAN has been known to be formed in urban photochemical smog in Los Angeles since the 19509,more recent measurements of PAN in remote atmospheres have shown it to be a ubiquitous pollutant in the troposphere. This is probably because of the stability of PAN a t low temperature, its low reactivity with OH radical, its photochemical stability, and its low aqueous solubility ( I 1). All of these factors will cause PAN to have a longer atmospheric lifetime when compared to other important NO, species such as nitrogen dioxide or nitric acid (11). The tropospheric lifetime of PAN is significant, because PAN can act as an important storage medium for NO2 and as a transport agent of this species into remote atmospheres (1, 2, 11). A number of studies have examined the correlation of PAN with ozone and hydrogen peroxide, since all are photochemically produced oxidants (12-1 9). Most of these studies were undertaken where urban or regional pollution levels of NO, controlled the chemistry and were conducted predominantly, if not exclusively, during the daylight hours. The studies typically were for short time periods (less than 1 month) and, therefore, did not allow the seasonal effects of temperature on the PAN and ozone levels to be examined. For many of these case studies, ozone was found to be positively correlated with PAN, presumably because of their photochemical sources. Most of these studies were carried out in regions where nitrogen oxides were not limiting because they were in the low ppbv range (12-15). Recent work has reported positive correlation for PAN and ozone for data taken a t Point Arena, CA (161,in April and May of 1985 and for data taken over Alaska and Greenland during July and August of 1988 (191,while negative and positive correlations have been reported in Canadian studies (12). In an attempt to examine the relationships between PAN and ozone under conditions where nitric oxide and nitrogen dioxide were limited and where the effects of seasonal temperature variations could be examined, a study was initiated to monitor PAN, ozone, and NO, a t a remote site in New Mexico for approximately 2 years. Concurrent measurements of PAN, temperature, relative humidity, ozone, and NO, were taken from October 1987through January 1989 at Frijoles Mesa, NM. The results of this study are reported here.
Experimental Section The field site chosen for this study was located at technical area 33 (TA-33) a t Los Alamos National Laboratory, near Bandelier National Monument in Los Alamos County, northern New Mexico (35" 53.0' N; 106" 19.4' W). The Frijoles Mesa site is at an elevation of approximately 6400 f t and is quite remote. The population of the county is approximately 20 000. The nearest urban centers are Environ. Sci. Technol., Voi. 27, No. 9, 1993 1905
Santa Fe (population 50000) and Albuquerque, NM (population 450 000) located in the Rio Grande Valley at distances of approximately 60 and 130 miles, respectively. Air samples were collected at the top of a 20-ft tower by pumping air through 1/4-in. Teflon tubing into a sampling manifold inside an instrumented, portable trailer located next to the tower. Meteorological measurements including wind speed, wind direction, temperature, relative humidity, and precipitation (Weathermeasure, Model 1018) were taken next to the sample inlet on the tower. All of the meteorological data were transmitted to an IBM PC-AT computer for data storage via a data logger (Fluke Instruments, Model 8840A). Ozone measurements were taken by using ultraviolet absorption (Dasibi Model AH8000), while NO, measurements were taken by using an ozone chemiluminescent detection system (Monitor Labs Model 8840). Measurements were taken in real time and stored as 10-min averages by using the data logger and computer system. Data were recorded on paper tape as well as on magnetic disks for backup and further data processing and analysis. During most of the study period, the levels of NO, observed were below the detection limits of the instrumentation (approximately 2 ppbv NO,). Peroxyacetyl nitrate was analyzed by using an automated gas chromatograph equipped with an electron capture detector. A Teflon column (1/8in. diameter X 40 in. long) packed with 10 % Carbowax on Supelcoport (601 80 mesh) was used for the analyses. Ultrahigh-purity nitrogen was used as the carrier gas at a flow rate of 25 mL min-1. PAN retention times were typically 4.5 min under these conditions. An inert automated sampling valve (Rheodyne; Teflon lined) was used to collect PAN samples (5-mLsampling loop) every 30 min in conjunction with an electronic timer circuit. The PANalyzer was calibrated by using PAN samples synthesized from the strong-acid nitration of peracetic acid, stored in tridecane ( 4 ) ,and placed in diffusion tubes. Standard gas dilution and calibration techniques were used to produce PAN standards, which were simultaneously analyzed with the gas chromatograph and a nitrogen oxide chemiluminescent analyzer. Below 2 ppbv, PAN calibrations were confirmed to be linear by using base hydrolysis of the PAN and ion chromatography. As well, a luminol system was used to confirm the linearity of the PANalyzer below 2 ppbv. PAN calibrations varied by 15% during this field study. Standard computer spreadsheets (Lotus 123R3) were used for the data analysis, for statistical linear least-squares regression analysis of the data, and for frequency distribution calculations. For these correlations and for the frequency distribution analyses, monthly averages were used, except for periods where data pairs were unavailable because of instrument breakdown or power loss. During this sampling period, emissions from a number of forest fire events apd from long-range transport from southern California were found to be impacting the Frijoles Mesa sampling site. These events were not included in the regional background analysis but were examined in a separate evaluation (20), which used back-trajectory determinations to demonstrate the formation and longrange transport of PAN. To evaluate the levels at the nearest major urban center, a duplicate PANalyzer was taken to Albuquerque, NM, and samples were analyzed for PAN during February 1988. The maximum levels of PAN measured at this closest urban site were 1-2 ppbv. Since this site is over 100 miles from the Frijoles Mesa 1906
Envlron. Sci. Technol., Vol. 27, No. 9, 1993
1.2
I
ot
5
10
15
20
i5
30
1.2
> a
I1
0.6
I
JULIAN DATE
Figure 1. PAN Concentrations monitored at Frijoles Mesa, NM, for (A) January 1989 and (B)August 1988. 70 (A)
60
@ 20 10
0 70
60 50
&
-
5
10
2;15
220
15
20
25
30
230
235
240
(B)
40-
3020 -
l o0
1
25 ?.
_j
JULIAN DATE
Figure 2. Ozone concentrations measured at Frijoles Mesa, NM, for (A) January 1989 and (8)August 1988.
sampling station and the only other nearby urban center is Santa Fe (population 50 000), the impact from local urban plumes is believed to be negligible for the collected data set.
Results and Discussion One of the principal focuses of this field study was to examine the temperature dependence of PAN concentrations and the relationship between PAN and ozone concentrations a t a remote continental site. Typical monthly data for PAN and ozone concentrations measured at the Frijoles Mesa site are shown in Figures 1and 2 for a winter month (January 1989) and a summer month (August 1988). Figure 1shows that the overall background concentrations for PAN are higher in the winter (Figure 1A) as compared to the summer months (Figure 1B) and that the diurnal variability has a shorter period in the summer than in the winter. This is consistent with PAN’S greater stability during the colder periods. In contrast, Figure 2 shows that the ozone levels were higher during the summer months. The length of the diurnal pattern for ozone is more similar in summer and winter than for PAN, again indication that thermal loss is not as important for ozone as for PAN. The amplitude of the diurnal variability for ozone is greater during the
cn
I
20
L
1 . Y _ _ -
O v1 W v1
>
Ln W
t; 3 W
0
20
-
30
0 1
L
r
-
05
(6)
-i
-
CL
-
TEMPERATURE
CONCENTRATION RANGES (PPBV)
TEMPERATURE
("C)
Flgure 3. Frequency distribution for (A) PAN analyses, (B) ozone analyses, and (C)temperature measurements at Frijoles Mesa during January 1989.
summer than the winter. The nighttime loss of ozone during the summer months is enhanced due to reactions with monoterpene and isoprene hydrocarbon emissions from the abundant conifer and hardwood forests in the Jemez Mountains directly above the Frijoles Mesa site. Measurements of monoterpene hydrocarbons near this site typically were found to be in the range of 20-30 ppbv with levels as high as 140-150 ppbv total monoterpene observed during high-temperature periods (21). During the wintertime, the natural hydrocarbon emissions are significantly reduced (less than a few ppbv total monoterpene), and the reaction of ozone with the monoterpenes is suppressed. This reaction of the terpenes with ozone during the summertime is probably the reason for the very similar diurnal variability for ozone observed at the Frijoles Mesa site in summer and winter. The monoterpene reactions would compensate for increased ozone production during the summer because of the longer daylight periods. Modeling studies that have examined the potential effects of monoterpenes on ozone in a coniferous forest have indicated that these reactions can be comparable to dry depositional sinks under stable nocturnal conditions (22). A frequency distribution analysis of the PAN, ozone, and temperature data for these two months (Figures 3 and 4) clearly shows that higher PAN concentrations are seen more often during the winter (Figure 3A) than the summer (Figure 4A), while ozone concentrations (Figures 3B and 4B) show the reverse trend at this remote site. These results can be compared to previously published frequency distributions for PAN analyses at Brookhaven National Laboratory on Long Island, NY (I). This site, approximately 60 miles from New York City, has moderate air pollution. Comparisons at Brookhaven for two weeks in March 1985to two weeks in May 1985 indicated a wider frequency distribution of PAN values in the warmer period when PAN was above the detection limit of 0.070 ppbv.
("C)
Flgure 4. Frequency distribution for (A) PAN analyses, (B) ozone analyses, and (C)temperature measurementsat Frijoles Mesa during August 1988.
Table I. Monthly Temperature Averages ("C) for Frijoles Mesa from October 1987 to January 1989
month
av
minimum
maximum
no. of analyses
October November December
13.0 2.9 -2.5
0.9 -9.4 -15.4
24.9 14.4 13.4
2.5 4.9 12.0 15.3 19.6 21.3 19.6 16.3 12.9 5.0 -0.2 -1.2
-6.9 -8.6 0.2 2.9 6.0 12.7 10.8 6.4 2.1 -9.0 -15.6 -17.3
15.4 22.6 21.9 28.2 31.6 32.1 30.3 29.2 23.8 20.0
4445 3777 3672 0 2511 3919 1012 1373 2567 2900 2674 2091 2182 2706 1884 1990
January0
February March
April May June July August September October November December 11.8 January 9.7 a Data missing due to power failure.
Although PAN concentrations were slightly increased during May at Brookhaveh, the number of measurements below the detection limit (0.070ppbv) was also significantly increased when compared to the March data (6% in March; 29% in May). This observation is probably due to the rapid loss of PAN upon thermal decomposition to the peroxyacetyl radical via reaction 2 at Brookhaven, because the NO levels were above 5 ppbv. For the Frijoles Mesa data, the frequency distributions more clearly indicate increased PAN stability during colder periods because the levels of NO were much lower at the remote site, and the lower detection limits (0.030 ppbv) allowed lower PAN concentrations to be determined. Both PAN and ozone, which are formed during the daylight hours show the expected diurnal variability, Le., higher levels during the photochemically active periods. However, close examination of the data indicate that they Envlron. Scl. Technol.. Vol. 27, No. 9, 1993 1907
Table IT. Monthly PAN Averages (ppbv) for Frijoles Mesa from October 1987 to January 1989
month
av
minimum
maximum
no. of analyses
October November December January February March April May June July August September October November December January
0.27 0.38 0.36 0.26 0.28 0.29 0.18 0.22 0.21 0.16 0.17 0.21 0.28 0.28 0.32 0.31
