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Determination of gas-phase nitric acid using a tracer technique
Atmospheric Environment 34 (2000) 2333}2341
Determination of gas-phase nitric acid using a tracer technique C.D. Judd , L. Husain * Wadsworth Center, New York State Department of Health, Empire State Plaza, Albany, NY 12201-0509, USA Department of Environmental Health and Toxicology, School of Public Health, State University of New York, Albany, NY 12201, USA Received 15 December 1998; received in revised form 1 September 1999; accepted 7 October 1999
Abstract A tracer technique has been developed in this laboratory which allows the measurement of aqueous-phase oxidation of SO in clouds. We are now extending this technique to the chemistry of nitrogen species in clouds. Using a conservative tracer such as selenium we can determine the fraction of cloudwater nitrate that is the result of aerosol scavenging. At remote sites this allows the calculation of the nitric acid mixing ratio in the precloud air from measurements made during a cloud event. We have used this technique to determine the pre-cloud nitric acid partial pressure at Whiteface Mountain (WFM) for three cloud events. The calculated concentrations for an event on 2 August 1997 agree with measurements made at a second site below cloud level using "lterpacks. Nitric acid concentrations for cloud events during July 1995, yielded values of (0.1}10.0 ppbv. At highly polluted sites closer to NO sources the technique may provide a method of V determining if there is any aqueous-phase oxidation of N(III) occurring. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Nitric acid; Cloud chemistry; Nitrogen oxides; Acid precipitation
1. Introduction The principle species which contribute to acid precipitation are sulfate and nitrate (e.g. NAPAP, 1991). It is therefore important to understand the relationship between the aerosol, gaseous and aqueous phases of sulfur and nitrogen species in clouds. Sulfur is present in the atmosphere as gaseous sulfur dioxide and in the aerosol phase as sulfate. Cloudwater sulfate is the result of two processes; scavenging of aerosol sulfate, and absorption of SO followed by aqueous-phase oxidation. Aqueous SO may be oxidized by ozone, hydrogen peroxide, or trace metal catalysis, depending on the pH and the concentrations of oxidants in the cloudwater. H O is the principle oxidant at the low pH's ((4.5) typical of cloudwater in the northeastern United States (Seinfeld, 1986).
* Corresponding author. Fax: #1-518-473-2895. E-mail address: [email protected] (L. Husain)
The chemistry of nitrogen oxides in the atmosphere is more complex, and not yet fully understood. These species are important for a number of reasons. While SO emissions in both Europe and North America have been decreasing signi"cantly in recent years, emissions of nitrogen oxides have not, due to the increasing number of motor vehicles and a lack of regulatory pressure on industry (Lammel and Cape, 1996). The relative contribution of nitrogen species to acid precipitation is therefore increasing. Nitrogen oxides are important for a number of other reasons, as well. The photolysis of nitrous acid (HONO) produces the hydroxyl radical (OH), which is important to many processes in tropospheric chemistry. Nitrogen oxides are also factors in global climate forcing in both gas and aerosol phases (Lee et al., 1997). Several di!erent species of oxidized nitrogen may be present in the gas phase, e.g. NO ("NO#NO ), V HONO, HNO , N O . The solubility of these substan ces in water varies greatly, as does their stability in the atmosphere. N O , for example, is in equilibrium with NO and NO . Since NO is rapidly photolyzed during daylight hours, signi"cant concentrations of N O only
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 4 7 3 - 2
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occur at night. Aerosols may contain nitrate and nitrite, which are scavenged into cloud droplets. The major gas-phase contributor to cloudwater nitrate is HNO , which is rapidly and quantitatively absorbed due to its high solubility. N O reacts rapidly with water to form nitrate, but is usually present in much lower concentrations than nitric acid. NO also reacts with water to produce nitrate and nitrite according to the reaction 2NO (g)#H O(l)"NO\#NO\#2H>,
(1)
possibly followed by aqueous-phase oxidation of nitrite to nitrate. The importance of this reaction to acid precipitation has been a subject of debate. Initially it was thought to make an important contribution. Lee and Schwartz (1981a,b) made a theoretical and experimental analysis of this reaction and concluded that the reaction of NO with cloudwater should obey a second-order dependence on NO (g). This second-order dependence and the low Henry's law constant for NO indicated that the reaction should be too slow at ambient concentrations to provide a signi"cant source of cloudwater nitrate. More recent research has indicated that this may not be the case. Mertes and Wahner (1995) and Ponche et al. (1993) have measured the uptake of NO and found the apparent Henry's law constant to be much higher than that recommended by Lee and Schwartz. Bambaur et al. (1994) have studied the reaction of NO with water droplets in a laboratory cloud chamber and found a "rst-order dependence on the gas phase NO concen tration. They also found evidence of enhanced NO uptake and oxidation to nitrate when gaseous nitric acid is also present, as is likely in the atmosphere. The presence of oxidizing agents, such as H O and trace metals, may also enhance this process in cloud droplets. Other researchers have also found a "rst-order dependence on NO (Kle!man et al., 1998). In addition, extrapolation from laboratory measurements to "eld conditions is often not accurate. It is therefore desirable to obtain a direct measurement of nitrate production in cloudwater. There are several di$culties inherent in making in situ measurements of atmospheric reactions. The atmosphere is not a closed system, but rather is undergoing continual mixing and movement. There also may be multiple reaction paths creating the same product. Following the course of a reaction in the atmosphere is complicated by these factors. In order to determine whether a reaction is occurring in cloud droplets, it is necessary to partition the total concentration of the oxidized species in cloudwater into fractions derived from the aerosol and gas phases. Various methods for accomplishing this have been suggested. NH> has been used as an indicator of nucleation scavenging in order to estimate the nonaerosol fraction of both sulfate and nitrate in fog water (Liu et al., 1993; Minami and Ishikaza, 1996).
The use of NH> concentrations to determine nonaerosol sulfate and nitrate is limited by the possibility of artifacts in its collection and measurement. In addition NH> is not a conservative tracer; it reacts with many species present in the atmosphere. The technique developed in this laboratory (Husain, 1989) uses instead a conservative tracer such as selenium for the determination of aerosol and nonaerosol fractions. The technique has been successfully applied to determine the amount of SO oxidation in gas phase (Husain and Dutkiewicz, 1992) and cloud droplets in over 40 cloud systems at Whiteface Mountain, NY (Dutkiewicz et al., 1995; Burkhard et al., 1995; Husain et al., 1991). Selenium was chosen as the tracer for the studies presented here for several reasons. After emission it undergoes rapid gas-to-particle conversion ('90% within 10 km), so that at remote sites it is almost entirely partitioned in the aerosol phase (Kitto, 1987). The concentrations at Whiteface Mountain are usually high enough so that it can be measured with good precision, and there are no local sources. These conditions may not apply to other sites. The tracer technique may also be used with other trace metals, taking into account their chemical and physical properties. Arsenic and antimony have also been demonstrated as suitable tracers at WFM (Burkhard et al., 1995); other elements may prove useful at di!erent sites. The tracer technique can also be applied to nitrogen chemistry (or to any cloudwater species which is derived from both aerosol and gas phases). The concentration of cloudwater nitrate, [NO ] , can be expressed as [NO ] "a/L[NO ] #[NO ] #[NO ] , (2) where [NO ] is the concentration of aerosol nitrate in the ambient air, [NO ] is the aqueous concentration of nitrate in cloudwater due to contributions from gasphase HNO and N O , and [NO ] is the nitrate concentration due to oxidation of NO ; a is the scaveng ing coe$cient for aerosol NO , and L is the cloud liquid water content. The sole source of selenium in cloudwater is from the scavenging of aerosol selenium, which is present from the combustion of fossil fuels. It can thus be represented as [Se] "b/L[Se] , (3) where b is the scavenging coe$cient for selenium. Combining Eqs. (2) and (3) yields [NO ] #[NO ] "([NO /Se] !(a/b)[NO /Se] )[Se] . (4) At sites where the precloud NO and HNO concenV trations are suitable, [NO ] can be calculated from Eq. (4), if the aerosol and cloudwater NO /Se ratios, and the gas-phase HNO concentration are known. Since NO V oxidization occurs relatively rapidly in the gas phase, this
C.D. Judd, L. Husain / Atmospheric Environment 34 (2000) 2333}2341
would be at sites near NO sources. Nitrite has been V observed in both fog-and cloudwater (Cape et al., 1992; Fuzzi et al., 1992; Lammel and Metzig, 1998). The determination of [NO ] is limited by the precision of the [NO Se] measurement. This ratio can be determined with a precision of 7%. Therefore, for a S/N ratio of 2, [NO ] would have to be '14% of the total cloud water nitrate to be detectable. This is a minimum estimate, as the actual detectable level would depend on the relative magnitudes and precisions of the [NO ] and [NO /Se] terms. The contribution from [NO ] would probably have to be &20% of the total cloudwater nitrate to be detectable. The conditions where this might occur would only exist at highly polluted urban sites, where NO concentrations of tens of ppbs can occur. At more remote sites the amount of cloudwater nitrate which results from the absorption of nitric acid is usually much larger than that which could arise from other gasphase sources, or nitrite oxidation. Under these conditions [NO ] can be neglected and Eq. (4) reduces to [NO ] "([NO /Se] !(a/b)[NO /Se] )[Se] . (5) Here [NO ] is the aqueous nitrate concentration, which originated from gaseous nitric acid. For highly soluble species such as nitric acid, the partial pressure in the pre-cloud air can be obtained from, P "LRT[NO ] . (6) &,- This allows the calculation of the pre-cloud nitric acid partial pressure from aerosol and cloudwater data alone P "LRT([NO /Se] !(a/b)[NO /Se] )[Se] . &,- (7) The scavenging e$ciencies of aerosol particles should be largely dependent on their size and water solubilities. We have shown previously that the scavenging e$ciencies of SO and Se are similar enough to set the (a/b) ratio to unity (Husain et al., 1991). Since continental nitrate and sulfate size distributions are similar and they are both very soluble, it is reasonable to use (a/b) equal to unity for nitrate chemistry also. An advantage to this technique is that the aerosol samples may be collected from the interstitial air at the same site as the cloudwater sampling, or at a nearby site below the cloud. Although there are several direct methods for measuring gaseous nitric acid, none of them may be applicable during particular cloud study. Nitric acid is highly soluble and is rapidly adsorbed by cloud droplets; numerical simulation indicates that this occurs for both in-cloud and below-cloud air (Levine and Schwartz, 1982). In order to measure gas phase nitric acid concentrations directly during cloud events, it is necessary to "nd a suitable second site, which is su$ciently removed from the cloud, yet is close enough to ensure
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that the same air mass is being sampled. This may not be possible for all cloud chemistry studies. Even at locations with multiple sampling sites, there may be occasions when they are not measuring the same air mass due to meteorological conditions. This is the case in over 30% of the cloud events that we have sampled at WFM. Gas-phase concentrations measured at the cloud sampling site before arrival of the cloud are not necessarily representative of the cloud air parcel. Cloud events often last many hours and may have rapidly changing pollutant concentrations. Since the tracer technique allows the determination of the pre-cloud gas-phase HNO concen tration from measurements at a single site during a cloud event, it can extend the possibilities for studies of nitrogen chemistry in clouds. At Whiteface Mountain, HNO is usually the domi nant source of cloudwater nitrate. Nitric acid concentrations at WFM have been previously measured by Kelly (1989). He reported a summer average of 0.87 ppbv with values ranging up to &3 ppbv. The sampling period in that study was 24 h, so it is likely that higher concentrations occurred for shorter time periods. There is little data on other oxides of nitrogen at WFM. The Atmospheric Sciences Research Center at the State University of Albany has recently begun monitoring NO at WFM using a chemi-luminescence detector; concentrations are generally around 0.1 ppbv with occasional values of 0.3}0.4 ppbv (U. Roychowdhury, personal communication, 1997). Using values of 0.4;10\ for L (typical of WFM), 0.4 ppbv NO and 153C, and assuming that 10% of the NO reacts with water and that all the nitrite thus produced is oxidized, the calculated maximum value of [NO ] is &4 lM. The actual contribution is probably smaller due to the kinetics of the reaction and the low e!ective Henry's law constant of HONO at the pHs typical of Whiteface Mountain. The cloudwater NO concentrations measured are usually '100 lM. For most cloud events at Whiteface Mountain the contribution from in situ oxidation is negligible, and Eq. (7) can be used to calculate the pre-cloud nitric acid concentration. In this paper we have used the tracer technique in this manner to calculate the HNO concentrations at WFM for three cloud events occurring in the summers of 1997 and 1995. Under certain meteorological conditions, NO V may be transported rapidly enough from a source to WFM to allow measurement of any aqueous oxidation which is occurring. At more polluted sites nearer to NO V sources, the conditions necessary for determining the contribution from aqueous-phase oxidation might occur more often.
2. Experimental Whiteface Mountain is a high-elevation rural site located in the Adirondack Mountains of New York (443
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23 26 N, 733 5 34 W 1483 amsl.). The summit is above treeline and is exposed to air#ow from all directions; it is frequently immersed in clouds during the summer. The nearest urban centers are Montreal, QueH bec (130 km to the north), Albany, NY (180 km to the south), and Syracuse, NY (220 km to the southwest). Although the site is quite removed from major sources of pollutants, it is often in the path of polluted air masses originating in the Ohio Valley, and less frequently from Sudbury, Ontario. The air quality therefore varies from pristine to quite polluted. Sampling procedures and analytical methodology have been described previously (Husain et al., 1990,1991). Cloudwater samples were collected at the summit (1.5 km) of Whiteface Mountain (Summit) using a passive collector (Mohnen and Kadlecek, 1989). Cloudwater samples were collected continuously every 15 min at
Summit, but the sampling was discontinued while rain was falling. A few cloudwater samples were collected for longer periods (30 min}1 h) due to low collection rates. Cloud liquid water content was monitored using a forward light scattering instrument (Gerber PVM-100). Aerosols were collected on Whatman 41 "lters using high-volume samplers at both Summit (in-cloud air) and a second research station (Lodge) located at 0.6 km (precloud air). The aerosol sampling period was 2 h. At the Lodge site two samplers were operated in parallel, with their start times staggered by 1 h in order to provide an e!ective time resolution of 1 h. One of the Lodge samplers was synchronized with the Summit sampler to the extent that it was possible. A portion of each Whatman "lter was analyzed for SO\, NO\, NO\, and Cl\ by ion chromatography and for trace elements by neutron activation analysis (Dutkiewicz et al., 1987). Cloudwater
Table 1 Measured nitrate and selenium concentrations at Whiteface Mountain, NY Aerosol Sampling interval
2 Aug 1997 8:00}9:00 9:00}10:00 10:00}11:00 11:00}12:00 12:00}13:00 18}19 July 1995 23:00}0:00 0:00}1:00 1:00}2:00 2:00}3:00 3:00}4:00 4:00}5:00 5:00}6:00 6:00}7:00 7:00}8:00 8:00}9:00 9:00}10:00 10:00}11:00 23 July 1995 6:00}7:00 7:00}8:00 8:00}9:00 9:00}10:00 10:00}11:00 11:00}12:00 12:00}13:00 13:00}14:00 14:00}15:00 15:00}16:00 16:00}17:00
NO $S (nmol/m)
Cloudwater Se$S (pmol/m)
NO /Se$s
NO $s (lmol/l)
Se$s (nmol/l)
NO /Se$s
2.6$3.0 2.6$3.0 3.2$2.8 2.6$2.7 1.0$2.8
28.5$2.0 28.5$2.0 26.7$1.9 24.0$1.7 23.6$1.7
91$105 91$105 120$105 110$113 44$119
316$19 365$22 340$20 370$22 381$23
76.3$4.6 46.2$2.8 48.1$2.9 74.6$4.5 95.9$5.8
4142$353 7900$674 7069$601 4960$422 3973$338
2.7$0.5 (1.8 (1.8 2.6$0.5 2.4$0.5 2.1$0.5 2.0$0.5 (1.8 2.4$0.5 2.6$0.5 2.9$0.6 2.9$0.6
2.0$0.1 2.0$0.1 2.8$0.2 3.9$0.3 3.8$0.3 4.1$0.3 2.9$0.2 2.7$0.2 2.0$0.1 2.7$0.2 2.4$0.2 1.8$0.1
1350$259 (900 (643 667$138 632$141 512$128 690$179 (669 1200$257 963$198 1208$270 1667$346
48$2.9 26$1.6 12$0.7 14$0.8 20$1.2 19$1.2 10$0.6 6.0$0.4 6.8$0.4 9.4$0.6 22$1.3 24$1.5
6.7$0.4 4.5$0.3 2.8$0.3 3.1$0.2 5.1$0.3 6.0$0.4 4.4$0.3 3.2$0.2 3.3$0.2 3.5$0.2 4.5$0.3 5.6$0.3
7164$609 5778$524 4286$523 4516$389 3922$330 3167$291 2273$206 1875$171 2061$174 2686$230 4889$436 4286$353
2.3$0.5 (1.8 (1.8 (1.8 8.5$0.8 (1.8 4.1$0.6 3.8$0.6 2.3$0.5 1.9$0.5 (1.8
27.4$1.9 28.0$2.0 34.4$2.4 31.6$2.2 30.2$2.1 25.3$1.8 30.2$2.1 26.5$1.9 22.2$1.6 19.7$1.4 21.0$1.5
84$19 (64 (52 (57 281$33 (71 136$22 143$25 104$24 96$26 (86
525$32 669$40 452$27 351$21 434$26 411$25 413$25 226$14 229$14 87$5 181$11
159$9.5 262$15.7 94.8$5.7 60.8$3.6 77.8$4.7 68$4.1 71$4.3 48.8$2.9 54.1$3.2 33.2$2.0 52.5$3.2
3302$280 2553$216 4738$405 57733$487 5578$475 6044$513 5817$496 4631$390 4233$357 2620$222 3448$295
C.D. Judd, L. Husain / Atmospheric Environment 34 (2000) 2333}2341
samples were analyzed for ions in the same manner and for trace elements using hydride-generator atomic absorption spectroscopy. During some of the cloud events in 1997 a "lterpack system was also used to measure the gas-phase nitric acid concentration at the Lodge site. There were four "lters in series; a PTFE "lter to collect particulates, two nylon "lters to collect HNO , and a cel lulose "lter impregnated with KOH/glycerin to collect SO . The sampling period for the "lterpack system was 2 h. In previous studies at this site we have used several indicators to determine whether the two sites are sampling the same air mass. We have determined that ozone, total sulfur concentrations ("aerosol SO #SO at Lodge and aerosol SO #SO #cloudwater SO at Summit), and aerosol SO /Se and SO /As ratios are useful criteria for determining whether or not the two sites are coupled (Husain et al., 2000). During the 1997 "eld campaign our group measured SO at both sites using Thermo Environmental (TECO) model 43S pulsed #uorescence analyzers (detection limit 0.06 ppbv, precision $1%). Hourly ozone data was supplied by the New York State Department of Environmental Conservation (Delaware, 1998).
3. Results and discussion We report here the results of applying the tracer technique to three cloud events at WFM. The pre-cloud HNO partial pressure was determined for these events using Eq. (7). In addition to the cloudwater and aerosol measurements made during an event on 2 Aug 1997, the ambient gas-phase nitric acid was also measured directly at the Lodge site using nylon "lters. There were no nitric acid measurements made at WFM in the summer of 1995, but extensive cloudwater and aerosol measurements were made during the month of July. We have applied the tracer technique to calculate the ambient pre-cloud nitric acid concentrations for several clouds observed during this period, and report here the results of two events. Table 1 shows the measured aerosol and cloudwater nitrate and selenium concentrations for the three cloud events. The cloudwater values shown are averages of the samples collected during each aerosol sampling period The aerosol values shown are from the samples collected at the Lodge site. These samples were used because the aerosol loading at this site is higher, providing a lower experimental uncertainty, and the use of two samplers with overlapping sampling periods allows an e!ective time resolution of 1 h, versus 2 h at the Summit site. The nitrate concentration was below the detection limit for several of the samples. In these cases the detection limit was used to calculate the NO /Se ratio. The use of this ratio then provides an upper limit to [NO ] , in which
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Table 2 Calculated Values Sampling interval (NO ) $s (NO ) $s (l mol/l) (lmole/l) 2 Aug 1997 8:00}9:00 9:00}10:00 10:00}11:00 11:00}12:00 12:00}13:00 18}19 July 1995 23:00}0:00 0:00}1:00 1:00}2:00 2:00}3:00 3:00}4:00 4:00}5:00 5:00}6:00 6:00}7:00 7:00}8:00 8:00}9:00 9:00}10:00 10:00}11:00 23 July 1995 6:00}7:00 7:00}8:00 8:00}9:00 9:00}10:00 10:00}11:00 11:00}12:00 12:00}13:00 13:00}14:00 14:00}15:00 15:00}16:00 16:00}17:00
7.0$16.6 4.2$10.1 5.8$10.6 8.2$17.8 4.2$23.6
309$22.9 361$20.8 334$20.0 362$25.7 377$30.3
HNO (g)$s (ppb)
2.16$5.1 5.22$12.5 3.76$6.9 2.53$5.5 1.51$8.4
9.0$2.0 4.1 1.8 2.1$0.4 3.2$0.7 3.1$0.8 3.1$0.8 2.1 4.0$0.9 3.4$0.7 5.4$1.2 9.3$2.1
39.2$2.3 22.2 10.0 119.9$0.6 17.2$0.9 16.3$0.9 6.5$0.9 3.9 2.8$0.9 6.0$0.8 16.5$1.3 14.9$2.2
0.62$0.1 0.4 0.2 0.19$0.0 0.27$0.1 0.35$0.1 0.15$0.0 0.1 0.04$0.0 0.07$0.0 0.16$0.0 0.04$0.0
13.3$3.2 17.0 4.9 3.5 21.9$3.0 4.8 9.6$1.8 7.0$1.3 5.6$1.3 3.2$0.9 4.5
511$13.5 651.9 447.0 347.5 412$11.2 406 403$10.5 219$5.8 224$5.9 84$2.4 177
4.95$1.2 8.00 7.39 4.02 5.15$0.7 4.80 6.20$1.2 2.69$0.05 3.32$0.8 1.15$0.3 3.68
Values with no uncentainties shown were calculated from the detection limit as described in the text.
case Eq. (7) yields a minimum value for P . Table 2 &,- shows the calculated values for [NO ] and [NO ] based on the aerosol and cloudwater nitrate and selenium concentrations for the three clouds. 3.1. 2 August 1997 In the morning, a cold front was located in southeastern Canada, stretching from the border of Maine to northern Minnesota. The front advanced slowly during the day, reaching WFM at about 24:00. A cloud formed at the summit in advance of the front at 5:00 Cloudwater samples were collected from 8:00 to 14:00. A hindcast trajectory generated using the HYSPLIT 3 model (Draxler, 1992) shows that the air mass sampled during this period arrived from due west, passing
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Fig. 1. Whiteface Mountain cloudwater nitrate on 2 August 1997. (a) Total cloudwater nitrate, (NO ) , and calculated aerosol fraction (NO ) . The aerosol fraction has been scaled by a factor of 10. The di!erence between the two values corresponds to the gas phase contribution, (NO ) . (b) Nitric acid partial pressure, P , measured at the Lodge site, and cal&,- culated using Eq. (7).
Fig. 2. Whiteface Mountain cloudwater nitrate on 18}19 July, 1995. (a) Total cloudwater nitrate, (NO ) , and calculated aerosol fraction (NO ) . (b) Calculated nitric acid partial pres sure, P . &,-
3.2. 18}19 July 1995 through industrialized regions in Michigan and southern Ontario. The cloudwater nitrate and sulfate concentrations were high, indicating a polluted air mass. Aerosol SO /Se, NO /Se, and Cl/Se ratios, gas phase SO and O measurements, and total sulfur concentrations all indicate that the Lodge and Summit sites were sampling the same air mass. Fig. 1a shows the total cloudwater nitrate and the fraction derived from aerosol nitrate. The aerosol fraction has been scaled by a factor of 10 to show more detail. In this event the aerosol fraction was (5% of the total cloudwater nitrate. Fig. 1b shows the measured gaseous HNO concentration at the Lodge site and the concentration calculated from Eq. (7), averaged over the "lterpack sampling times. There is good agreement between the two techniques. Due to unusually dry weather and instrument problems, all the data necessary to compare the techniques were collected only for this one event in the 1997 "eld campaign. The agreement must therefore be regarded as preliminary at this time. We intend to make further comparisons in future "eld campaigns in order to further characterize and develop the technique.
A cold front passed through upstate New York in the afternoon of 18 July 1995. A cloud formed that night at about 23:00, and cloudwater samples were collected from 23:15 until 10:45 on 19 July 1995. The winds were WNW during this period at 10}15 m/s from 23:00 until 7:00, and then decreased to 5 m/s. The temperature was 9}113C at the summit. The HYSPLIT back trajectory indicates that the air mass arriving at WFM at the beginning of the event had passed over Lake Huron, and for the remainder of the event it had originated in central Ontario. The pollutant levels were low during this event, consistent with the trajectory, which originates in Canada and does not pass through any major industrialized areas. The cloudwater nitrate concentration started out at 60 lmol/l, and decreased to 6 lmol/l at about 6:00. It remained at this level with some variation until 7:30 and then increased again to a "nal value of 35 lmol/l at the end of the event. Fig. 2a shows the total cloudwater nitrate and the aerosol component. In this event the aerosol nitrate fraction ranged from 15 to 60% of the total, with a mean of 27%. Fig. 2b shows the calculated pre-cloud nitric acid partial pressure during this cloud.
C.D. Judd, L. Husain / Atmospheric Environment 34 (2000) 2333}2341
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Fig. 3. Trajectories for air parcels arriving at Whiteface Mountain summit on 23 July, 1995. The trajectories were calculated at the 850 millibar level using the HYSPLIT 3 program.
The initial nitric acid concentration was 0.9 ppbv, followed by a relatively steady decrease throughout the period to near zero at the end of the cloud. The time resolution in the "gure is to some degree arti"cial. Each point represents a calculation using a single-cloudwater sample. Since the aerosol sampling period was longer, there are four points calculated for each aerosol value. An aerosol sampling method with better time resolution, such as the one described by Buhr et al. (1995), would improve the actual time resolution. Comparison of Fig. 2a and b shows that the initial decrease in the cloudwater nitrate is due to a decrease in the gas-phase nitric acid concentration, while the "nal increase is mostly due to the changing liquid water content of the cloud. 3.3. 23 July 1995 The meteorological conditions on 23 July 1995 were quite complex. In the morning a weak cold front reached from an area of low pressure in southern Ontario to Missouri. A second cold front was located further north in Ontario and QueH bec, and moved south rapidly during the day, passing through New York State. A cloud formed at &6:00 ahead of the front, and cloudwater samples were collected at 15 min intervals until 17:45. The winds were from WSW during the cloud, shifting to the northwest at the end of the event. The HYSPLIT back trajectories are unusual. During the earlier part of the event the air mass originated in the Ohio valley, traveled eastward to the Philadelphia area, then turned sharply northward, passing near Binghamton, NY and proceeding northward to WFM. The trajectories for later in the event are similar in shape, but shifted progressively further eastward (Fig. 3). This was one of the most polluted air masses sampled at WFM; both sulfate and nitrate concentrations were very high. Fig. 4a shows the
Fig. 4. Whiteface Mountain cloudwater nitrate on 23 July, 1995. As in Fig. 2. The aerosol fraction has been scaled by a factor of 10.
total cloudwater nitrate concentrations and the aerosol fraction, which is scaled by a factor of 10 to show more detail. As in the event of 2 August 1997, the contribution from the aerosol component was negligible. Fig. 4b shows the calculated nitric acid concentration. At the beginning of the event the concentration was &5 ppbv. It increased rapidly to 10 ppbv, and then decreased during the rest of the cloud to a "nal value of 1 ppbv. In the above events the aerosol component of cloudwater varied from (5 to 60%. The principal gas-phase contribution was from HNO . Whiteface Mountain is far removed from any NO sources, so that the majority V of nitrogen oxides in the air reaching the site have been oxidized. In addition, the aerosol nitrate component is typically quite small. The amount of ammonia or other neutralizing species present is usually insu$cient to react with all the sulfate and nitrate, so the majority of the nitrate remains in the gas-phase. Under these conditions the application of the technique to determine the precloud nitric acid concentration is straightforward. At sites closer to emission sources the distribution of nitrogen oxides will be di!erent, and the tracer technique may be useful in determining the extent of nitrogen processing in cloud or fog droplets.
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4. Conclusions We have extended the tracer technique developed in this laboratory (Husain, 1989) to nitrogen chemistry in clouds. When gas-phase nitric acid is the dominant source of cloudwater nitrate, the technique can be used to calculate nitric acid mixing ratio in the precloud air. We have applied the technique to previously collected samples of aerosols and cloudwater from Whiteface Mountain, NY, in order to calculate the ambient nitric acid concentrations in the pre-cloud air for three cloud events. The calculated pre-cloud nitric acid partial pressure agrees with the measured value for a cloud occurring on 2 August 1997. We will be making more measurements to further test and develop the technique in future campaigns. Pre-cloud nitric acid concentrations were calculated for two clouds which occurred during July, 1995. The calculated HNO partial pressures had a range of 0}10 ppbv. Gas-phase HNO was the dominant source of cloudwater nitrate for all of these events. At other sites di!erent forms of nitrate may make a larger contribution. It would be desirable to test the technique at sites where the aerosol contribution to cloudwater nitrate is larger than at WFM. It would also be interesting to apply the technique to urban fog samples, where the NO concenV trations are much higher, and there may be measurable processing of NO species in fog droplets. V Acknowledgements The authors are grateful to the Atmospheric Sciences Research Center, State University of New York, for the use of their facilities for sample collection, and to Douglas Wolfe and Paul Casson for assistance in the operation of the sites. We also thank Adil Khan and Martin Buchter for assistance in sample collection, analytical measurements and data reduction. This research was funded by USEPA through grant R82342201.
References Bambauer, A., Brantner, B., Paige, M., Novakov, T., 1994. Laboratory study of NO reaction with dispersed and bulk liquid water. Atmospheric Environment 28, 3225}3232. Buhr, S.M., Buhr, M.P., Fehsenfeld, F.C., Holloway, J.S., Karst, M., Uwe, N., Richard, B., Parrish, D.D., Sievers, R.E., 1995. Development of a semi-continuous method for the measurement of nitric acid vapor and particulate nitrate and sulfate. Atmospheric Environment 29 (19), 2609}2624. Burkhard, E.G., Ghauri, B.M., Dutkiewicz, V.A., Husain, L., 1995. A multielement tracer technique for the determination of SO oxidation in clouds. Journal of Geophysical Research 100 (D12), 26051}26059.
Cape, J.N., Hargreaves, K.J., Stoveton-West, R., Fowler, D., Colvite, R.N., Choularton, T.W., Gallagher, M.W., 1992. Nitrite in orographic cloud as an indicator of nitrous acid in air. Atmospheric Environment 26A, 2301}2307. Draxler, R., 1992. Hybrid single particle lagrangian integrated trajectories (HYSPLIT): version NOAA Technical Memo ERLARL-195. Air Resources Laboratory, Silver Springs, MD. Delaware, W., 1998. Personal communication. Dutkiewicz, V.A., Burkhard, E.G., Husain, L., 1995. Availability of H O for oxidation of SO in clouds in the northeastern United States. Atmospheric Environment 29, 3281}3292. Dutkiewicz, V.A., Parekh, P.P., Husain, L., 1987. An evaluation of regional elemental signatures in the northeastern United States. Atmospheric Environment 21, 1033}1044. Fuzzi, S., Facchini, M.C., Orsi, G., et al., 1992. The Po Valley fog experiment 1989: an overview. Tellus 44B, 448}468. Husain, L., 1989. A tracer technique for determining in-cloud formation of SO\. Geophysical Research Letters 16, 57}60. Husain, L., Dutkiewicz, V.A., 1990. A long-term (1975}1988) study of atmospheric SO\ regional contributions and concentration trends. Atmospheric Environment 24A, 1175}1187. Husain, L., Dutkiewicz, V.A., 1992. Elemental Tracers for the Study of Homogeneous Gas Phase Oxidation of SO in the Atmosphere. Journal of Geophysical Research 97 (D13), 14635}14643. Husain, L., Dutkiewicz, V.A., Hussain, M.M., Khwaja, H.A., Burkhard, E.G., Mehmood, G., Parekh, P.P., Canelli, E., 1991. A study of heterogeneous oxidation in summer clouds. Journal of Geophysical Research 96 (D10), 18789}18805. Husain, L., Rattigan, O.V., Dutkiewicz, V.A., Das, Mita, Judd, C.D., Khan, A.R., Richter, R., Bala, R., Walcek, C.J., 2000. Case studies of the H O #SO reaction in clouds. Journal of Geophysical Research, in press. Kelly, T.J., McLaren, S.E., Kadlecek, J.A., 1989. Seasonal variations in atmospheric SO and NO in the Adirondacks. V W Atmospheric Environment 23, 1315}1332. Kitto, M.E., 1987. Receptor modeling of atmospheric particles and acidic gases. Ph.D. Thesis, Univ of MD, College Park, MD. Kle!man, J., Becker, H., Wiesen, P., 1998. Heterogeneous NO conversion processes on acid surfaces: possible atmo spheric implications. Atmospheric Environment 32 (16), 2721}2729. Lammel, G., Cape, J.N., 1996. Nitrous acid and nitrite in the atmosphere. Chemical Society Reviews, 361}369. Lammel, G., Metzig, G., 1998. On the occurrence of nitrite in urban fogwater. Chemosphere 37 (8), 1603}1614. Lee, D.S., Kohler, I., Grobler, E., Rohrer, F., Sausen, R., Gallardo-Klenner, L., Olivier, J.G.J., Dentener, F.J., Bouwman, A.F., 1997. Estimations of global NO emissions and their V uncertainties. Atmospheric Environment 31, 1735}1749. Lee, Y.-N., Schwartz, S.E., 1981a. Reaction kinetics of nitrogen dioxide at low partial pressure. Journal of Physical Chemistry 85, 840}848. Lee, Y.-N., Schwartz, S.E., 1981b. Evaluation of the rate of uptake of nitrogen dioxide by atmospheric and surface liquid water. Journal of Geophysical Research 86, 11971}11982.
C.D. Judd, L. Husain / Atmospheric Environment 34 (2000) 2333}2341 Levine, S.Z., Schwartz, S.E., 1982. In-cloud and below-cloud scavenging of nitric acid vapor. Atmospheric Environment 16, 1725}1734. Liu, P.S.K., Leaitch, W.R., MacDonald, A.M., Isaac, G.A., Strapp, J.W., Wiebe, H.A., 1 Sulfate production in summer clouds over Ontario, Canada. Tellus 45B, 368}389. Mertes, S., Wahner, A., 1995. Uptake of nitrogen dioxide and nitrous acid on aqueous surfaces. Journal of Physical Chemistry 99, 14000}14006. Minami, Y., Ishizaka, Y., 1996. Evaluation of chemical composition in fog water near the summit of a high mountain in Japan. Atmospheric Environment 30 (19), 3363}3376.
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Mohnen, V.A., Kadlacek, J.A., 1989. Cloud chemistry research at Whiteface Mountain. Tellus 41b, 79}91. NAPAP, 1991. National Acid Precipitation Assessment Program - State of Science and State of Technology Irving, P.M. (Ed)., National Acid Precipitation Assessment Program, Washington, DC 20503. Ponche, J.L., George, Ch., Mirabel, Ph., 1993. Mass transfer at the air/water interface: mass accommodation coe$cients of SO , HNO , NO , and NH . Journal of Atmospheric Chemistry 16, 1}21. Seinfeld, J.H., 1986. Atmospheric Chemistry and Physics of Air Pollution. Wiley, New York.
Determination of gas-phase nitric acid using a tracer technique C.D. Judd , L. Husain * Wadsworth Center, New York State Department of Health, Empire State Plaza, Albany, NY 12201-0509, USA Department of Environmental Health and Toxicology, School of Public Health, State University of New York, Albany, NY 12201, USA Received 15 December 1998; received in revised form 1 September 1999; accepted 7 October 1999
Abstract A tracer technique has been developed in this laboratory which allows the measurement of aqueous-phase oxidation of SO in clouds. We are now extending this technique to the chemistry of nitrogen species in clouds. Using a conservative tracer such as selenium we can determine the fraction of cloudwater nitrate that is the result of aerosol scavenging. At remote sites this allows the calculation of the nitric acid mixing ratio in the precloud air from measurements made during a cloud event. We have used this technique to determine the pre-cloud nitric acid partial pressure at Whiteface Mountain (WFM) for three cloud events. The calculated concentrations for an event on 2 August 1997 agree with measurements made at a second site below cloud level using "lterpacks. Nitric acid concentrations for cloud events during July 1995, yielded values of (0.1}10.0 ppbv. At highly polluted sites closer to NO sources the technique may provide a method of V determining if there is any aqueous-phase oxidation of N(III) occurring. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Nitric acid; Cloud chemistry; Nitrogen oxides; Acid precipitation
1. Introduction The principle species which contribute to acid precipitation are sulfate and nitrate (e.g. NAPAP, 1991). It is therefore important to understand the relationship between the aerosol, gaseous and aqueous phases of sulfur and nitrogen species in clouds. Sulfur is present in the atmosphere as gaseous sulfur dioxide and in the aerosol phase as sulfate. Cloudwater sulfate is the result of two processes; scavenging of aerosol sulfate, and absorption of SO followed by aqueous-phase oxidation. Aqueous SO may be oxidized by ozone, hydrogen peroxide, or trace metal catalysis, depending on the pH and the concentrations of oxidants in the cloudwater. H O is the principle oxidant at the low pH's ((4.5) typical of cloudwater in the northeastern United States (Seinfeld, 1986).
* Corresponding author. Fax: #1-518-473-2895. E-mail address: [email protected] (L. Husain)
The chemistry of nitrogen oxides in the atmosphere is more complex, and not yet fully understood. These species are important for a number of reasons. While SO emissions in both Europe and North America have been decreasing signi"cantly in recent years, emissions of nitrogen oxides have not, due to the increasing number of motor vehicles and a lack of regulatory pressure on industry (Lammel and Cape, 1996). The relative contribution of nitrogen species to acid precipitation is therefore increasing. Nitrogen oxides are important for a number of other reasons, as well. The photolysis of nitrous acid (HONO) produces the hydroxyl radical (OH), which is important to many processes in tropospheric chemistry. Nitrogen oxides are also factors in global climate forcing in both gas and aerosol phases (Lee et al., 1997). Several di!erent species of oxidized nitrogen may be present in the gas phase, e.g. NO ("NO#NO ), V HONO, HNO , N O . The solubility of these substan ces in water varies greatly, as does their stability in the atmosphere. N O , for example, is in equilibrium with NO and NO . Since NO is rapidly photolyzed during daylight hours, signi"cant concentrations of N O only
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 4 7 3 - 2
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occur at night. Aerosols may contain nitrate and nitrite, which are scavenged into cloud droplets. The major gas-phase contributor to cloudwater nitrate is HNO , which is rapidly and quantitatively absorbed due to its high solubility. N O reacts rapidly with water to form nitrate, but is usually present in much lower concentrations than nitric acid. NO also reacts with water to produce nitrate and nitrite according to the reaction 2NO (g)#H O(l)"NO\#NO\#2H>,
(1)
possibly followed by aqueous-phase oxidation of nitrite to nitrate. The importance of this reaction to acid precipitation has been a subject of debate. Initially it was thought to make an important contribution. Lee and Schwartz (1981a,b) made a theoretical and experimental analysis of this reaction and concluded that the reaction of NO with cloudwater should obey a second-order dependence on NO (g). This second-order dependence and the low Henry's law constant for NO indicated that the reaction should be too slow at ambient concentrations to provide a signi"cant source of cloudwater nitrate. More recent research has indicated that this may not be the case. Mertes and Wahner (1995) and Ponche et al. (1993) have measured the uptake of NO and found the apparent Henry's law constant to be much higher than that recommended by Lee and Schwartz. Bambaur et al. (1994) have studied the reaction of NO with water droplets in a laboratory cloud chamber and found a "rst-order dependence on the gas phase NO concen tration. They also found evidence of enhanced NO uptake and oxidation to nitrate when gaseous nitric acid is also present, as is likely in the atmosphere. The presence of oxidizing agents, such as H O and trace metals, may also enhance this process in cloud droplets. Other researchers have also found a "rst-order dependence on NO (Kle!man et al., 1998). In addition, extrapolation from laboratory measurements to "eld conditions is often not accurate. It is therefore desirable to obtain a direct measurement of nitrate production in cloudwater. There are several di$culties inherent in making in situ measurements of atmospheric reactions. The atmosphere is not a closed system, but rather is undergoing continual mixing and movement. There also may be multiple reaction paths creating the same product. Following the course of a reaction in the atmosphere is complicated by these factors. In order to determine whether a reaction is occurring in cloud droplets, it is necessary to partition the total concentration of the oxidized species in cloudwater into fractions derived from the aerosol and gas phases. Various methods for accomplishing this have been suggested. NH> has been used as an indicator of nucleation scavenging in order to estimate the nonaerosol fraction of both sulfate and nitrate in fog water (Liu et al., 1993; Minami and Ishikaza, 1996).
The use of NH> concentrations to determine nonaerosol sulfate and nitrate is limited by the possibility of artifacts in its collection and measurement. In addition NH> is not a conservative tracer; it reacts with many species present in the atmosphere. The technique developed in this laboratory (Husain, 1989) uses instead a conservative tracer such as selenium for the determination of aerosol and nonaerosol fractions. The technique has been successfully applied to determine the amount of SO oxidation in gas phase (Husain and Dutkiewicz, 1992) and cloud droplets in over 40 cloud systems at Whiteface Mountain, NY (Dutkiewicz et al., 1995; Burkhard et al., 1995; Husain et al., 1991). Selenium was chosen as the tracer for the studies presented here for several reasons. After emission it undergoes rapid gas-to-particle conversion ('90% within 10 km), so that at remote sites it is almost entirely partitioned in the aerosol phase (Kitto, 1987). The concentrations at Whiteface Mountain are usually high enough so that it can be measured with good precision, and there are no local sources. These conditions may not apply to other sites. The tracer technique may also be used with other trace metals, taking into account their chemical and physical properties. Arsenic and antimony have also been demonstrated as suitable tracers at WFM (Burkhard et al., 1995); other elements may prove useful at di!erent sites. The tracer technique can also be applied to nitrogen chemistry (or to any cloudwater species which is derived from both aerosol and gas phases). The concentration of cloudwater nitrate, [NO ] , can be expressed as [NO ] "a/L[NO ] #[NO ] #[NO ] , (2) where [NO ] is the concentration of aerosol nitrate in the ambient air, [NO ] is the aqueous concentration of nitrate in cloudwater due to contributions from gasphase HNO and N O , and [NO ] is the nitrate concentration due to oxidation of NO ; a is the scaveng ing coe$cient for aerosol NO , and L is the cloud liquid water content. The sole source of selenium in cloudwater is from the scavenging of aerosol selenium, which is present from the combustion of fossil fuels. It can thus be represented as [Se] "b/L[Se] , (3) where b is the scavenging coe$cient for selenium. Combining Eqs. (2) and (3) yields [NO ] #[NO ] "([NO /Se] !(a/b)[NO /Se] )[Se] . (4) At sites where the precloud NO and HNO concenV trations are suitable, [NO ] can be calculated from Eq. (4), if the aerosol and cloudwater NO /Se ratios, and the gas-phase HNO concentration are known. Since NO V oxidization occurs relatively rapidly in the gas phase, this
C.D. Judd, L. Husain / Atmospheric Environment 34 (2000) 2333}2341
would be at sites near NO sources. Nitrite has been V observed in both fog-and cloudwater (Cape et al., 1992; Fuzzi et al., 1992; Lammel and Metzig, 1998). The determination of [NO ] is limited by the precision of the [NO Se] measurement. This ratio can be determined with a precision of 7%. Therefore, for a S/N ratio of 2, [NO ] would have to be '14% of the total cloud water nitrate to be detectable. This is a minimum estimate, as the actual detectable level would depend on the relative magnitudes and precisions of the [NO ] and [NO /Se] terms. The contribution from [NO ] would probably have to be &20% of the total cloudwater nitrate to be detectable. The conditions where this might occur would only exist at highly polluted urban sites, where NO concentrations of tens of ppbs can occur. At more remote sites the amount of cloudwater nitrate which results from the absorption of nitric acid is usually much larger than that which could arise from other gasphase sources, or nitrite oxidation. Under these conditions [NO ] can be neglected and Eq. (4) reduces to [NO ] "([NO /Se] !(a/b)[NO /Se] )[Se] . (5) Here [NO ] is the aqueous nitrate concentration, which originated from gaseous nitric acid. For highly soluble species such as nitric acid, the partial pressure in the pre-cloud air can be obtained from, P "LRT[NO ] . (6) &,- This allows the calculation of the pre-cloud nitric acid partial pressure from aerosol and cloudwater data alone P "LRT([NO /Se] !(a/b)[NO /Se] )[Se] . &,- (7) The scavenging e$ciencies of aerosol particles should be largely dependent on their size and water solubilities. We have shown previously that the scavenging e$ciencies of SO and Se are similar enough to set the (a/b) ratio to unity (Husain et al., 1991). Since continental nitrate and sulfate size distributions are similar and they are both very soluble, it is reasonable to use (a/b) equal to unity for nitrate chemistry also. An advantage to this technique is that the aerosol samples may be collected from the interstitial air at the same site as the cloudwater sampling, or at a nearby site below the cloud. Although there are several direct methods for measuring gaseous nitric acid, none of them may be applicable during particular cloud study. Nitric acid is highly soluble and is rapidly adsorbed by cloud droplets; numerical simulation indicates that this occurs for both in-cloud and below-cloud air (Levine and Schwartz, 1982). In order to measure gas phase nitric acid concentrations directly during cloud events, it is necessary to "nd a suitable second site, which is su$ciently removed from the cloud, yet is close enough to ensure
2335
that the same air mass is being sampled. This may not be possible for all cloud chemistry studies. Even at locations with multiple sampling sites, there may be occasions when they are not measuring the same air mass due to meteorological conditions. This is the case in over 30% of the cloud events that we have sampled at WFM. Gas-phase concentrations measured at the cloud sampling site before arrival of the cloud are not necessarily representative of the cloud air parcel. Cloud events often last many hours and may have rapidly changing pollutant concentrations. Since the tracer technique allows the determination of the pre-cloud gas-phase HNO concen tration from measurements at a single site during a cloud event, it can extend the possibilities for studies of nitrogen chemistry in clouds. At Whiteface Mountain, HNO is usually the domi nant source of cloudwater nitrate. Nitric acid concentrations at WFM have been previously measured by Kelly (1989). He reported a summer average of 0.87 ppbv with values ranging up to &3 ppbv. The sampling period in that study was 24 h, so it is likely that higher concentrations occurred for shorter time periods. There is little data on other oxides of nitrogen at WFM. The Atmospheric Sciences Research Center at the State University of Albany has recently begun monitoring NO at WFM using a chemi-luminescence detector; concentrations are generally around 0.1 ppbv with occasional values of 0.3}0.4 ppbv (U. Roychowdhury, personal communication, 1997). Using values of 0.4;10\ for L (typical of WFM), 0.4 ppbv NO and 153C, and assuming that 10% of the NO reacts with water and that all the nitrite thus produced is oxidized, the calculated maximum value of [NO ] is &4 lM. The actual contribution is probably smaller due to the kinetics of the reaction and the low e!ective Henry's law constant of HONO at the pHs typical of Whiteface Mountain. The cloudwater NO concentrations measured are usually '100 lM. For most cloud events at Whiteface Mountain the contribution from in situ oxidation is negligible, and Eq. (7) can be used to calculate the pre-cloud nitric acid concentration. In this paper we have used the tracer technique in this manner to calculate the HNO concentrations at WFM for three cloud events occurring in the summers of 1997 and 1995. Under certain meteorological conditions, NO V may be transported rapidly enough from a source to WFM to allow measurement of any aqueous oxidation which is occurring. At more polluted sites nearer to NO V sources, the conditions necessary for determining the contribution from aqueous-phase oxidation might occur more often.
2. Experimental Whiteface Mountain is a high-elevation rural site located in the Adirondack Mountains of New York (443
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23 26 N, 733 5 34 W 1483 amsl.). The summit is above treeline and is exposed to air#ow from all directions; it is frequently immersed in clouds during the summer. The nearest urban centers are Montreal, QueH bec (130 km to the north), Albany, NY (180 km to the south), and Syracuse, NY (220 km to the southwest). Although the site is quite removed from major sources of pollutants, it is often in the path of polluted air masses originating in the Ohio Valley, and less frequently from Sudbury, Ontario. The air quality therefore varies from pristine to quite polluted. Sampling procedures and analytical methodology have been described previously (Husain et al., 1990,1991). Cloudwater samples were collected at the summit (1.5 km) of Whiteface Mountain (Summit) using a passive collector (Mohnen and Kadlecek, 1989). Cloudwater samples were collected continuously every 15 min at
Summit, but the sampling was discontinued while rain was falling. A few cloudwater samples were collected for longer periods (30 min}1 h) due to low collection rates. Cloud liquid water content was monitored using a forward light scattering instrument (Gerber PVM-100). Aerosols were collected on Whatman 41 "lters using high-volume samplers at both Summit (in-cloud air) and a second research station (Lodge) located at 0.6 km (precloud air). The aerosol sampling period was 2 h. At the Lodge site two samplers were operated in parallel, with their start times staggered by 1 h in order to provide an e!ective time resolution of 1 h. One of the Lodge samplers was synchronized with the Summit sampler to the extent that it was possible. A portion of each Whatman "lter was analyzed for SO\, NO\, NO\, and Cl\ by ion chromatography and for trace elements by neutron activation analysis (Dutkiewicz et al., 1987). Cloudwater
Table 1 Measured nitrate and selenium concentrations at Whiteface Mountain, NY Aerosol Sampling interval
2 Aug 1997 8:00}9:00 9:00}10:00 10:00}11:00 11:00}12:00 12:00}13:00 18}19 July 1995 23:00}0:00 0:00}1:00 1:00}2:00 2:00}3:00 3:00}4:00 4:00}5:00 5:00}6:00 6:00}7:00 7:00}8:00 8:00}9:00 9:00}10:00 10:00}11:00 23 July 1995 6:00}7:00 7:00}8:00 8:00}9:00 9:00}10:00 10:00}11:00 11:00}12:00 12:00}13:00 13:00}14:00 14:00}15:00 15:00}16:00 16:00}17:00
NO $S (nmol/m)
Cloudwater Se$S (pmol/m)
NO /Se$s
NO $s (lmol/l)
Se$s (nmol/l)
NO /Se$s
2.6$3.0 2.6$3.0 3.2$2.8 2.6$2.7 1.0$2.8
28.5$2.0 28.5$2.0 26.7$1.9 24.0$1.7 23.6$1.7
91$105 91$105 120$105 110$113 44$119
316$19 365$22 340$20 370$22 381$23
76.3$4.6 46.2$2.8 48.1$2.9 74.6$4.5 95.9$5.8
4142$353 7900$674 7069$601 4960$422 3973$338
2.7$0.5 (1.8 (1.8 2.6$0.5 2.4$0.5 2.1$0.5 2.0$0.5 (1.8 2.4$0.5 2.6$0.5 2.9$0.6 2.9$0.6
2.0$0.1 2.0$0.1 2.8$0.2 3.9$0.3 3.8$0.3 4.1$0.3 2.9$0.2 2.7$0.2 2.0$0.1 2.7$0.2 2.4$0.2 1.8$0.1
1350$259 (900 (643 667$138 632$141 512$128 690$179 (669 1200$257 963$198 1208$270 1667$346
48$2.9 26$1.6 12$0.7 14$0.8 20$1.2 19$1.2 10$0.6 6.0$0.4 6.8$0.4 9.4$0.6 22$1.3 24$1.5
6.7$0.4 4.5$0.3 2.8$0.3 3.1$0.2 5.1$0.3 6.0$0.4 4.4$0.3 3.2$0.2 3.3$0.2 3.5$0.2 4.5$0.3 5.6$0.3
7164$609 5778$524 4286$523 4516$389 3922$330 3167$291 2273$206 1875$171 2061$174 2686$230 4889$436 4286$353
2.3$0.5 (1.8 (1.8 (1.8 8.5$0.8 (1.8 4.1$0.6 3.8$0.6 2.3$0.5 1.9$0.5 (1.8
27.4$1.9 28.0$2.0 34.4$2.4 31.6$2.2 30.2$2.1 25.3$1.8 30.2$2.1 26.5$1.9 22.2$1.6 19.7$1.4 21.0$1.5
84$19 (64 (52 (57 281$33 (71 136$22 143$25 104$24 96$26 (86
525$32 669$40 452$27 351$21 434$26 411$25 413$25 226$14 229$14 87$5 181$11
159$9.5 262$15.7 94.8$5.7 60.8$3.6 77.8$4.7 68$4.1 71$4.3 48.8$2.9 54.1$3.2 33.2$2.0 52.5$3.2
3302$280 2553$216 4738$405 57733$487 5578$475 6044$513 5817$496 4631$390 4233$357 2620$222 3448$295
C.D. Judd, L. Husain / Atmospheric Environment 34 (2000) 2333}2341
samples were analyzed for ions in the same manner and for trace elements using hydride-generator atomic absorption spectroscopy. During some of the cloud events in 1997 a "lterpack system was also used to measure the gas-phase nitric acid concentration at the Lodge site. There were four "lters in series; a PTFE "lter to collect particulates, two nylon "lters to collect HNO , and a cel lulose "lter impregnated with KOH/glycerin to collect SO . The sampling period for the "lterpack system was 2 h. In previous studies at this site we have used several indicators to determine whether the two sites are sampling the same air mass. We have determined that ozone, total sulfur concentrations ("aerosol SO #SO at Lodge and aerosol SO #SO #cloudwater SO at Summit), and aerosol SO /Se and SO /As ratios are useful criteria for determining whether or not the two sites are coupled (Husain et al., 2000). During the 1997 "eld campaign our group measured SO at both sites using Thermo Environmental (TECO) model 43S pulsed #uorescence analyzers (detection limit 0.06 ppbv, precision $1%). Hourly ozone data was supplied by the New York State Department of Environmental Conservation (Delaware, 1998).
3. Results and discussion We report here the results of applying the tracer technique to three cloud events at WFM. The pre-cloud HNO partial pressure was determined for these events using Eq. (7). In addition to the cloudwater and aerosol measurements made during an event on 2 Aug 1997, the ambient gas-phase nitric acid was also measured directly at the Lodge site using nylon "lters. There were no nitric acid measurements made at WFM in the summer of 1995, but extensive cloudwater and aerosol measurements were made during the month of July. We have applied the tracer technique to calculate the ambient pre-cloud nitric acid concentrations for several clouds observed during this period, and report here the results of two events. Table 1 shows the measured aerosol and cloudwater nitrate and selenium concentrations for the three cloud events. The cloudwater values shown are averages of the samples collected during each aerosol sampling period The aerosol values shown are from the samples collected at the Lodge site. These samples were used because the aerosol loading at this site is higher, providing a lower experimental uncertainty, and the use of two samplers with overlapping sampling periods allows an e!ective time resolution of 1 h, versus 2 h at the Summit site. The nitrate concentration was below the detection limit for several of the samples. In these cases the detection limit was used to calculate the NO /Se ratio. The use of this ratio then provides an upper limit to [NO ] , in which
2337
Table 2 Calculated Values Sampling interval (NO ) $s (NO ) $s (l mol/l) (lmole/l) 2 Aug 1997 8:00}9:00 9:00}10:00 10:00}11:00 11:00}12:00 12:00}13:00 18}19 July 1995 23:00}0:00 0:00}1:00 1:00}2:00 2:00}3:00 3:00}4:00 4:00}5:00 5:00}6:00 6:00}7:00 7:00}8:00 8:00}9:00 9:00}10:00 10:00}11:00 23 July 1995 6:00}7:00 7:00}8:00 8:00}9:00 9:00}10:00 10:00}11:00 11:00}12:00 12:00}13:00 13:00}14:00 14:00}15:00 15:00}16:00 16:00}17:00
7.0$16.6 4.2$10.1 5.8$10.6 8.2$17.8 4.2$23.6
309$22.9 361$20.8 334$20.0 362$25.7 377$30.3
HNO (g)$s (ppb)
2.16$5.1 5.22$12.5 3.76$6.9 2.53$5.5 1.51$8.4
9.0$2.0 4.1 1.8 2.1$0.4 3.2$0.7 3.1$0.8 3.1$0.8 2.1 4.0$0.9 3.4$0.7 5.4$1.2 9.3$2.1
39.2$2.3 22.2 10.0 119.9$0.6 17.2$0.9 16.3$0.9 6.5$0.9 3.9 2.8$0.9 6.0$0.8 16.5$1.3 14.9$2.2
0.62$0.1 0.4 0.2 0.19$0.0 0.27$0.1 0.35$0.1 0.15$0.0 0.1 0.04$0.0 0.07$0.0 0.16$0.0 0.04$0.0
13.3$3.2 17.0 4.9 3.5 21.9$3.0 4.8 9.6$1.8 7.0$1.3 5.6$1.3 3.2$0.9 4.5
511$13.5 651.9 447.0 347.5 412$11.2 406 403$10.5 219$5.8 224$5.9 84$2.4 177
4.95$1.2 8.00 7.39 4.02 5.15$0.7 4.80 6.20$1.2 2.69$0.05 3.32$0.8 1.15$0.3 3.68
Values with no uncentainties shown were calculated from the detection limit as described in the text.
case Eq. (7) yields a minimum value for P . Table 2 &,- shows the calculated values for [NO ] and [NO ] based on the aerosol and cloudwater nitrate and selenium concentrations for the three clouds. 3.1. 2 August 1997 In the morning, a cold front was located in southeastern Canada, stretching from the border of Maine to northern Minnesota. The front advanced slowly during the day, reaching WFM at about 24:00. A cloud formed at the summit in advance of the front at 5:00 Cloudwater samples were collected from 8:00 to 14:00. A hindcast trajectory generated using the HYSPLIT 3 model (Draxler, 1992) shows that the air mass sampled during this period arrived from due west, passing
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Fig. 1. Whiteface Mountain cloudwater nitrate on 2 August 1997. (a) Total cloudwater nitrate, (NO ) , and calculated aerosol fraction (NO ) . The aerosol fraction has been scaled by a factor of 10. The di!erence between the two values corresponds to the gas phase contribution, (NO ) . (b) Nitric acid partial pressure, P , measured at the Lodge site, and cal&,- culated using Eq. (7).
Fig. 2. Whiteface Mountain cloudwater nitrate on 18}19 July, 1995. (a) Total cloudwater nitrate, (NO ) , and calculated aerosol fraction (NO ) . (b) Calculated nitric acid partial pres sure, P . &,-
3.2. 18}19 July 1995 through industrialized regions in Michigan and southern Ontario. The cloudwater nitrate and sulfate concentrations were high, indicating a polluted air mass. Aerosol SO /Se, NO /Se, and Cl/Se ratios, gas phase SO and O measurements, and total sulfur concentrations all indicate that the Lodge and Summit sites were sampling the same air mass. Fig. 1a shows the total cloudwater nitrate and the fraction derived from aerosol nitrate. The aerosol fraction has been scaled by a factor of 10 to show more detail. In this event the aerosol fraction was (5% of the total cloudwater nitrate. Fig. 1b shows the measured gaseous HNO concentration at the Lodge site and the concentration calculated from Eq. (7), averaged over the "lterpack sampling times. There is good agreement between the two techniques. Due to unusually dry weather and instrument problems, all the data necessary to compare the techniques were collected only for this one event in the 1997 "eld campaign. The agreement must therefore be regarded as preliminary at this time. We intend to make further comparisons in future "eld campaigns in order to further characterize and develop the technique.
A cold front passed through upstate New York in the afternoon of 18 July 1995. A cloud formed that night at about 23:00, and cloudwater samples were collected from 23:15 until 10:45 on 19 July 1995. The winds were WNW during this period at 10}15 m/s from 23:00 until 7:00, and then decreased to 5 m/s. The temperature was 9}113C at the summit. The HYSPLIT back trajectory indicates that the air mass arriving at WFM at the beginning of the event had passed over Lake Huron, and for the remainder of the event it had originated in central Ontario. The pollutant levels were low during this event, consistent with the trajectory, which originates in Canada and does not pass through any major industrialized areas. The cloudwater nitrate concentration started out at 60 lmol/l, and decreased to 6 lmol/l at about 6:00. It remained at this level with some variation until 7:30 and then increased again to a "nal value of 35 lmol/l at the end of the event. Fig. 2a shows the total cloudwater nitrate and the aerosol component. In this event the aerosol nitrate fraction ranged from 15 to 60% of the total, with a mean of 27%. Fig. 2b shows the calculated pre-cloud nitric acid partial pressure during this cloud.
C.D. Judd, L. Husain / Atmospheric Environment 34 (2000) 2333}2341
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Fig. 3. Trajectories for air parcels arriving at Whiteface Mountain summit on 23 July, 1995. The trajectories were calculated at the 850 millibar level using the HYSPLIT 3 program.
The initial nitric acid concentration was 0.9 ppbv, followed by a relatively steady decrease throughout the period to near zero at the end of the cloud. The time resolution in the "gure is to some degree arti"cial. Each point represents a calculation using a single-cloudwater sample. Since the aerosol sampling period was longer, there are four points calculated for each aerosol value. An aerosol sampling method with better time resolution, such as the one described by Buhr et al. (1995), would improve the actual time resolution. Comparison of Fig. 2a and b shows that the initial decrease in the cloudwater nitrate is due to a decrease in the gas-phase nitric acid concentration, while the "nal increase is mostly due to the changing liquid water content of the cloud. 3.3. 23 July 1995 The meteorological conditions on 23 July 1995 were quite complex. In the morning a weak cold front reached from an area of low pressure in southern Ontario to Missouri. A second cold front was located further north in Ontario and QueH bec, and moved south rapidly during the day, passing through New York State. A cloud formed at &6:00 ahead of the front, and cloudwater samples were collected at 15 min intervals until 17:45. The winds were from WSW during the cloud, shifting to the northwest at the end of the event. The HYSPLIT back trajectories are unusual. During the earlier part of the event the air mass originated in the Ohio valley, traveled eastward to the Philadelphia area, then turned sharply northward, passing near Binghamton, NY and proceeding northward to WFM. The trajectories for later in the event are similar in shape, but shifted progressively further eastward (Fig. 3). This was one of the most polluted air masses sampled at WFM; both sulfate and nitrate concentrations were very high. Fig. 4a shows the
Fig. 4. Whiteface Mountain cloudwater nitrate on 23 July, 1995. As in Fig. 2. The aerosol fraction has been scaled by a factor of 10.
total cloudwater nitrate concentrations and the aerosol fraction, which is scaled by a factor of 10 to show more detail. As in the event of 2 August 1997, the contribution from the aerosol component was negligible. Fig. 4b shows the calculated nitric acid concentration. At the beginning of the event the concentration was &5 ppbv. It increased rapidly to 10 ppbv, and then decreased during the rest of the cloud to a "nal value of 1 ppbv. In the above events the aerosol component of cloudwater varied from (5 to 60%. The principal gas-phase contribution was from HNO . Whiteface Mountain is far removed from any NO sources, so that the majority V of nitrogen oxides in the air reaching the site have been oxidized. In addition, the aerosol nitrate component is typically quite small. The amount of ammonia or other neutralizing species present is usually insu$cient to react with all the sulfate and nitrate, so the majority of the nitrate remains in the gas-phase. Under these conditions the application of the technique to determine the precloud nitric acid concentration is straightforward. At sites closer to emission sources the distribution of nitrogen oxides will be di!erent, and the tracer technique may be useful in determining the extent of nitrogen processing in cloud or fog droplets.
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4. Conclusions We have extended the tracer technique developed in this laboratory (Husain, 1989) to nitrogen chemistry in clouds. When gas-phase nitric acid is the dominant source of cloudwater nitrate, the technique can be used to calculate nitric acid mixing ratio in the precloud air. We have applied the technique to previously collected samples of aerosols and cloudwater from Whiteface Mountain, NY, in order to calculate the ambient nitric acid concentrations in the pre-cloud air for three cloud events. The calculated pre-cloud nitric acid partial pressure agrees with the measured value for a cloud occurring on 2 August 1997. We will be making more measurements to further test and develop the technique in future campaigns. Pre-cloud nitric acid concentrations were calculated for two clouds which occurred during July, 1995. The calculated HNO partial pressures had a range of 0}10 ppbv. Gas-phase HNO was the dominant source of cloudwater nitrate for all of these events. At other sites di!erent forms of nitrate may make a larger contribution. It would be desirable to test the technique at sites where the aerosol contribution to cloudwater nitrate is larger than at WFM. It would also be interesting to apply the technique to urban fog samples, where the NO concenV trations are much higher, and there may be measurable processing of NO species in fog droplets. V Acknowledgements The authors are grateful to the Atmospheric Sciences Research Center, State University of New York, for the use of their facilities for sample collection, and to Douglas Wolfe and Paul Casson for assistance in the operation of the sites. We also thank Adil Khan and Martin Buchter for assistance in sample collection, analytical measurements and data reduction. This research was funded by USEPA through grant R82342201.
References Bambauer, A., Brantner, B., Paige, M., Novakov, T., 1994. Laboratory study of NO reaction with dispersed and bulk liquid water. Atmospheric Environment 28, 3225}3232. Buhr, S.M., Buhr, M.P., Fehsenfeld, F.C., Holloway, J.S., Karst, M., Uwe, N., Richard, B., Parrish, D.D., Sievers, R.E., 1995. Development of a semi-continuous method for the measurement of nitric acid vapor and particulate nitrate and sulfate. Atmospheric Environment 29 (19), 2609}2624. Burkhard, E.G., Ghauri, B.M., Dutkiewicz, V.A., Husain, L., 1995. A multielement tracer technique for the determination of SO oxidation in clouds. Journal of Geophysical Research 100 (D12), 26051}26059.
Cape, J.N., Hargreaves, K.J., Stoveton-West, R., Fowler, D., Colvite, R.N., Choularton, T.W., Gallagher, M.W., 1992. Nitrite in orographic cloud as an indicator of nitrous acid in air. Atmospheric Environment 26A, 2301}2307. Draxler, R., 1992. Hybrid single particle lagrangian integrated trajectories (HYSPLIT): version NOAA Technical Memo ERLARL-195. Air Resources Laboratory, Silver Springs, MD. Delaware, W., 1998. Personal communication. Dutkiewicz, V.A., Burkhard, E.G., Husain, L., 1995. Availability of H O for oxidation of SO in clouds in the northeastern United States. Atmospheric Environment 29, 3281}3292. Dutkiewicz, V.A., Parekh, P.P., Husain, L., 1987. An evaluation of regional elemental signatures in the northeastern United States. Atmospheric Environment 21, 1033}1044. Fuzzi, S., Facchini, M.C., Orsi, G., et al., 1992. The Po Valley fog experiment 1989: an overview. Tellus 44B, 448}468. Husain, L., 1989. A tracer technique for determining in-cloud formation of SO\. Geophysical Research Letters 16, 57}60. Husain, L., Dutkiewicz, V.A., 1990. A long-term (1975}1988) study of atmospheric SO\ regional contributions and concentration trends. Atmospheric Environment 24A, 1175}1187. Husain, L., Dutkiewicz, V.A., 1992. Elemental Tracers for the Study of Homogeneous Gas Phase Oxidation of SO in the Atmosphere. Journal of Geophysical Research 97 (D13), 14635}14643. Husain, L., Dutkiewicz, V.A., Hussain, M.M., Khwaja, H.A., Burkhard, E.G., Mehmood, G., Parekh, P.P., Canelli, E., 1991. A study of heterogeneous oxidation in summer clouds. Journal of Geophysical Research 96 (D10), 18789}18805. Husain, L., Rattigan, O.V., Dutkiewicz, V.A., Das, Mita, Judd, C.D., Khan, A.R., Richter, R., Bala, R., Walcek, C.J., 2000. Case studies of the H O #SO reaction in clouds. Journal of Geophysical Research, in press. Kelly, T.J., McLaren, S.E., Kadlecek, J.A., 1989. Seasonal variations in atmospheric SO and NO in the Adirondacks. V W Atmospheric Environment 23, 1315}1332. Kitto, M.E., 1987. Receptor modeling of atmospheric particles and acidic gases. Ph.D. Thesis, Univ of MD, College Park, MD. Kle!man, J., Becker, H., Wiesen, P., 1998. Heterogeneous NO conversion processes on acid surfaces: possible atmo spheric implications. Atmospheric Environment 32 (16), 2721}2729. Lammel, G., Cape, J.N., 1996. Nitrous acid and nitrite in the atmosphere. Chemical Society Reviews, 361}369. Lammel, G., Metzig, G., 1998. On the occurrence of nitrite in urban fogwater. Chemosphere 37 (8), 1603}1614. Lee, D.S., Kohler, I., Grobler, E., Rohrer, F., Sausen, R., Gallardo-Klenner, L., Olivier, J.G.J., Dentener, F.J., Bouwman, A.F., 1997. Estimations of global NO emissions and their V uncertainties. Atmospheric Environment 31, 1735}1749. Lee, Y.-N., Schwartz, S.E., 1981a. Reaction kinetics of nitrogen dioxide at low partial pressure. Journal of Physical Chemistry 85, 840}848. Lee, Y.-N., Schwartz, S.E., 1981b. Evaluation of the rate of uptake of nitrogen dioxide by atmospheric and surface liquid water. Journal of Geophysical Research 86, 11971}11982.
C.D. Judd, L. Husain / Atmospheric Environment 34 (2000) 2333}2341 Levine, S.Z., Schwartz, S.E., 1982. In-cloud and below-cloud scavenging of nitric acid vapor. Atmospheric Environment 16, 1725}1734. Liu, P.S.K., Leaitch, W.R., MacDonald, A.M., Isaac, G.A., Strapp, J.W., Wiebe, H.A., 1 Sulfate production in summer clouds over Ontario, Canada. Tellus 45B, 368}389. Mertes, S., Wahner, A., 1995. Uptake of nitrogen dioxide and nitrous acid on aqueous surfaces. Journal of Physical Chemistry 99, 14000}14006. Minami, Y., Ishizaka, Y., 1996. Evaluation of chemical composition in fog water near the summit of a high mountain in Japan. Atmospheric Environment 30 (19), 3363}3376.
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Mohnen, V.A., Kadlacek, J.A., 1989. Cloud chemistry research at Whiteface Mountain. Tellus 41b, 79}91. NAPAP, 1991. National Acid Precipitation Assessment Program - State of Science and State of Technology Irving, P.M. (Ed)., National Acid Precipitation Assessment Program, Washington, DC 20503. Ponche, J.L., George, Ch., Mirabel, Ph., 1993. Mass transfer at the air/water interface: mass accommodation coe$cients of SO , HNO , NO , and NH . Journal of Atmospheric Chemistry 16, 1}21. Seinfeld, J.H., 1986. Atmospheric Chemistry and Physics of Air Pollution. Wiley, New York.