Environ. Sci. Technol. 2002, 36, 4981-4989
Aircraft Measurements of Nitrogen and Phosphorus in and around the Lake Tahoe Basin: Implications for Possible Sources of Atmospheric Pollutants to Lake Tahoe QI ZHANG,† JOHN J. CARROLL,* ALAN J. DIXON, AND CORT ANASTASIO* Atmospheric Science Program, Department of Land, Air and Water Resources, University of California, One Shields Avenue, Davis, California 95616-8627
Atmospheric deposition of nitrogen (N) and phosphorus (P) into Lake Tahoe appears to have been a major factor responsible for the shifting of the lake’s nutrient response from N-limited to P-limited. To characterize atmospheric N and P in and around the Lake Tahoe Basin during summer, samples were collected using an instrumented aircraft flown over three locations: the Sierra Nevada foothills east of Sacramento (“low-Sierra”), further east and higher in the Sierra (“mid-Sierra”), and in the Tahoe Basin. Measurements were also made within the smoke plume downwind of an intense forest fire just outside the Tahoe Basin. Samples were collected using a denuder-filter pack sampling system (DFP) and analyzed for gaseous and water-soluble particle components including HNO3/ NO3-, NH3 /NH4+, organic N (ON), total N, SRP (soluble reactive phosphate) and total P. The average total gaseous and particulate N concentrations (( 1σ) measured over the low- and mid-Sierra were 660 (( 270) and 630 (( 350) nmol N/m3-air, respectively. Total airborne N concentrations in the Tahoe samples were one-half to one-fifth of these values. The forest fire plume had the highest concentration of atmospheric N (860 nmol N/m3-air) and a greater contribution of organic N (ON) to the total N compared to nonsmoky conditions. Airborne P was rarely observed over the low- and mid-Sierra but was present at low concentrations over Lake Tahoe, with average (( 1σ) concentrations of 2.3 ( 2.9 and 2.8 ( 0.8 nmol P/m3-air under typical clear air and slightly smoky air conditions, respectively. Phosphorus in the forest fire plume was present at concentrations ∼10 times greater than over the Tahoe Basin. P in these samples included both fine and coarse particulate phosphate as well as unidentified, possibly organic, gaseous P species. Overall, our results suggest that out-of-basin emissions could be significant sources of nitrogen to Lake Tahoe during the summer and that forest fires could be important sources of both N and P.
* Corresponding author phone: (530)754-6095; fax: (530)752-1552; e-mail:
[email protected] (J.J.C.) and
[email protected] (C.A.). † Present address: Cooperative Institute for Research in Environmental Science (CIRES), 216 UCB, University of Colorado, Boulder, CO 80309-0216. 10.1021/es025658m CCC: $22.00 Published on Web 10/26/2002
2002 American Chemical Society
1. Introduction Lake Tahoe, an ultraoligotrophic lake with exceptional transparency, is located in a relatively small bowl-shaped basin near the crest of the Sierra Nevada (39°N, 120°W, elevation 1898 m; Figure 1) (1). The lake is broad and deep (surface area ) 512 km2, mean depth ) 313 m) and has a relatively small watershed (area ) 812 km2). The surrounding N-limited forested watershed (2) and the granitic geology of the basin yield relatively small amounts of nutrients in runoff into the lake (3). These characteristics, in combination with the low ratio of watershed area to lake surface area (1.6), give Lake Tahoe extremely low natural productivity and very high clarity. These characteristics also make the lake very sensitive to direct atmospheric deposition. Recent estimates conclude that over half of the annual loading of nitrogen (N) and 25% of the phosphorus (P) (4-6) come from direct atmospheric deposition. Delivery of N from atmospheric deposition and other sources has increased the lake’s level of fixed nitrogen and shifted its nutrient response from N-limited to predominantly P-limited (3, 6, 7). Airborne N and P compounds deposited to Lake Tahoe could originate either within the basin or be transported from sources located outside of the basin. During the afternoon and evening hours of the warm season (approximately May-September), low altitude winds in Central California tend to be westerly, flowing from the coast through various gaps in the western coastal mountains, then into the Central Valley and continuing east up the slopes of the Sierra Nevada (8, 9). This pattern occurs on 72% of the warm season days (10). At night, the flow in the mountains is reversed, and the predominant flows are downslope on the western side of the Sierra Nevada. The upslope, daytime flow of air is impacted by various agricultural, urban and transportation sources of air pollutants, resulting in significant concentrations of primary and secondary air pollutants being transported to the Sierra Nevada (11, 12). Although the more polluted Central Valley air should be diluted during transport up the mountain slopes, it is likely that significant levels of pollutants from these out-of-basin sources reach the Lake Tahoe Basin and contribute to pollutant deposition to the lake. To examine this issue, we have characterized the concentrations of airborne nitrogen and phosphorus compounds upwind of, and over, the lake. To minimize effects of near-ground local sources and to obtain regionally representative samples, measurements and sampling were performed using an instrumented light aircraft.
2. Experimental Methods 2.1. Sample Collection. 2.1.1. Sampling Equipment. Samples were collected using a Cessna 182 aircraft which continuously recorded position, air speed, altitude, temperature, relative humidity, approximate wind speed and direction, concentrations of ozone, NO and NOy, and number concentrations of particles (13). NOy was measured by a Thermo Environmental Instruments 42C analyzer and is defined operationally as the amount of NO resulting from passing the ambient sample through a molybdenum catalyst at 325 °C. Gaseous and particulate N & P species were collected using a URG denuder-filter pack system (hereafter DFP). The first portion of this system consisted of a fabricated isokinetic Teflon nozzle inlet followed by a cyclone separator to remove coarse particles. The average DFP flow rate in this study was 21 slpm (standard liter per minute), corresponding to a cyclone cut point of ∼4.5 µm; average flow rates for individual samples ranged from 18 to 33 slpm, corresponding to cut points of 3.4-5.0 µm. Thus particles collected in the cyclone will be VOL. 36, NO. 23, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Map of the study area. Dark solid lines represent flight paths for each sampling location. The approximate location and extent of the forest fire plume during sampling are also shown. considered “coarse”, while those collected on the Teflon filter will be referred to as fine particles. Air was drawn through the DFP using an engine-driven vacuum pump, and the flow rate was measured between the end of the filter pack and the vacuum pump. Downstream of the cyclone were two duplicate sets of denuder/filter packs (described below) arranged in parallel. Each set was activated separately, allowing two independent samples to be obtained per flight. The sampler intake was located outside the cabin well clear of the fuselage surface and engine exhaust. Each DFP contained three denuders, coated to collect (in order) gaseous HNO3, HNO2, and NH3. Prior to use, denuders were coated either with 0.1% (w/v) NaCl in methanol (for HNO3 collection), 1% (w/v) Na2CO3 in 50:50 (v:v) methanolH2O (for HNO2), or 1% (w/v) citric acid in methanol (for NH3). Denuders were prepared by adding 10 mL of the coating solution, shaking gently, pouring out the excess solution, and drying with purified air. To minimize contamination, denuders were prepared within 36 h of sampling. The filter pack of each DFP contained a Teflon filter (Zefluor, 2 µm pore size) to collect fine particles, followed by a Nylon filter (Nylasorb, 1 µm) and a citric acid impregnated Whatman filter to collect any HNO3 or NH3, respectively, that volatilized from the upstream Teflon filter. All filters were 47 mm diameter and were precleaned by repeatedly sonicating and shaking in Milli-Q water followed by rinsing with copious Milli-Q. Whatman filters were prepared by soaking precleaned filters in 50 mL of methanol solution containing 1.5% (w/v) citric acid and 1.5% (w/v) glycerin, decanting the solution, and drying in an ammonia-free vacuum desiccator. Coated Whatman filters were kept individually in clean Petri dishes, sealed in clean plastic bags containing citric acid-coated Kimwipes, and stored in the dark at ∼4 °C for up to 3 weeks. After preparation, denuders and filter packs were capped and kept in sealed plastic bags until immediately prior to deployment on the aircraft. 2.1.2. Sampling Times and Locations. Aircraft flight tracks during DFP sample collection are shown in Figure 1 and described in Table 1. Low- and mid-Sierra flights were conducted on afternoons when the wind flow was predominantly upslope. Lake Tahoe samples were collected twice 4982
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per day (one morning and one afternoon) for two consecutive days during each sampling trip. 2.2. Sample Processing. As soon as possible after each Sierra flight, the intact DFP unit was brought to the laboratory and the components were extracted immediately. For the Tahoe Basin flights, samples from the first day were capped, sealed in plastic bags and stored in dry ice until they could be processed upon return to Davis the next evening. Denuders were extracted with 6.0 mL of Milli-Q water (g18.2 MΩ-cm), while filters were extracted by shaking (3 h at ∼4 °C) in high-density polyethylene bottles containing Milli-Q. For the Teflon filters, one-half was wetted with 100 µL of ethanol and extracted with 4.0 mL of Milli-Q water (for inorganic N and P analyses), while the other half (for organic N and P analyses) used the same procedure without ethanol (which interfered with the organic N analyses). Each filter half was extracted twice consecutively (see Section 2.4.). Nylon and Whatman filters were extracted with 10.0 mL of Milli-Q. Filter extracts were not filtered since they contained no discernible particles. Significant amounts of coarse particles were observed in the cyclone only for the forest fire plume samples. These particles were rinsed from the cyclone using 2.0 mL of Milli-Q, and this solution was sonicated for 60 min and filtered (0.22 µm Teflon) to remove insoluble particles. 2.3. Sample Analysis. Concentrations of NH4+, NO3-, NO2-, SO42- and PO43- were analyzed using a Dionex DX-120 Ion Chromatograph with conductivity detection. Organic nitrogen (ON) was determined as the difference in inorganic N concentrations in a given sample before and after adjustment to pH ≈ 3 and illumination with 254 nm light (to convert ON to inorganic forms) (14, 15). Since Teflon filter extracts were not filtered, reported concentrations of particulate ON include both water-soluble and some portion of the less soluble species. The same UV-photooxidation method was used to measure total phosphorus (TP). However, since the photooxidation technique might liberate inorganic particulate P as well as mineralize organic P, we refer to the difference between TP and SRP values as “other phosphorus” (OP) rather than as organic P. Additional details of the analytical procedures are given by Zhang and Anastasio (14).
TABLE 1. Sampling Flight Informationa averages over sampling period date
location
8-31-98 5-24-01 5-24-01 6-15-01 6-15-01 7-05-01 7-17-01 7-17-01 7-18-01 7-18-01 8-02-01 8-02-01 8-03-01 8-03-01 8-15-01 8-15-01 8-16-01 8-16-01 9-05-01 9-05-01
mid-Sierra low-Sierra mid-Sierra low-Sierra mid-Sierra low-Sierra Tahoe Tahoe Tahoe Tahoe Tahoe Tahoe Tahoe Tahoe Tahoe Tahoe Tahoe Tahoe forest fire forest fire
time (PST) start end 14:00 13:12 14:39 12:40 14:06 12:05 9:13 13:18 8:49 12:54 8:45 13:09 8:31 13:06 8:48 13:16 8:26 12:48 11:10 13:22
15:47 14:13 15:46 13:43 15:12 14.09 11:09 15:16 10:48 14:55 10:45 15:09 10:31 15:05 10:45 15:16 10:26 14:48 12:16 14:24
sample vol. (L)
air T (°C)
R.H. (%)
press. (mb)
NOy nmol-N/m3
O3 (ppbv)
N(>0.3)b x106/m3
N(>3.0)c x104/m3
2263 973 853 1322 1169 2880 2200 2424 2657 2292 2552 2687 2472 2487 2473 2312 1931 3960 1871 1447
18.2 26.8 19.3 26.5 19.8 29.0 10.8 12.8 11.9 14.9 17.4 19.9 16.0 17.2 18.9 22.2 19.8 23.2 11.0 11.5
21.0 21.2 29.2 18.5 24.8 24.4 42.3 43.4 41.2 39.2 24.8 23.7 34.0 33.2 13.6 15.4 16.8 13.7 24.9 22.3
785.9 922.3 824.0 920.9 822.4 917.8 768.0 769.2 771.0 770.3 774.8 773.6 771.6 770.3 772.6 771.9 775.5 772.5 724.5 724.7
205 37 17 152 68 263 26 55 16 48 48 51 71 73 70 79 86 85 298 270
78 75 70 92 87 87 56 60 47 55 78 71 67 71 88 80 85 84 69 71
14 18 14 9 8 19 8 11 6 6 9 9 10 10 9 9 11 14 55 57
42.4 6.3 5.3 3.9 3.1 35.0 3.2 3.1 2.5 3.3 2.7 2.8 3.2 3.4 4.7 3.6 3.2 7.6 32.1 28.9
a Data listed here were collected real-time (0.5 Hz) and were averaged over the period of each DFP collection period. The average mean elevations (above sea level) for sampling sites were: low-Sierra ∼ 400 m; mid-Sierra ∼ 770 m; Tahoe ∼ 1900 m; forest fire: ∼ 1900 m. The aircraft was flown 300-350 m above ground level except in the forest fire plume (∼1000 m above the ground level). b Number of particles with diameter >0.3 µm as measured by an optical particle counter (OPC). c Number of particles with diameter >3.0 µm as measured by an OPC.
TABLE 2. Average Concentrations of Nitrogen and Phosphorus Species (( 1σ) on Field Blanks and the Average Sample/Blank Ratios field blank concentrationa
denuder filtersd
average sample/blank ratiob
NO2-
NO3-
NH4+
ONc
SRP
TP
NO2-
NO3-
NH4+
ON
1.8 ( 1.1
