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ELEVATIONAL GRADIENTS/LOCAL CHEMISTRY Volker A. Mohnen Professor, Department of Atmospheric Sciences State University of New York at Albany Albany, New York 12222 ABSTRACT Since some air pollutant concentrations are known to vary systematically with latitude, mountain sites were located at several latitudes, reflecting different transport distances and times from major air pollution source regions. MCCP's (Mountain Clouds Chemistry Project) specific locations are: Mt. Mitchell, NC; Whitetop Mt., VA; Shenandoah National Park, VA; Whiteface Mt., NY; Mt. Moosilauke, NY; and Howland Forest, ME. The sites were established during the late 1985-early 1986 time frame and all are currently functioning. Three of the sites are operating monitoring stations at several elevations. The Atmospheric Environment Service of Canada is collaborating with the United States EPA in this project by operating similar sites at Mt. Tremblant and Roundtop Mountain in Quebec Province. Experimental design at the EPA-funded Mountain Cloud Chemistry Project (MCCP) includes the selection of sites, establishment of protocols, coordination, data base oversight, and preparation of interpretive reports. The MCCP is designed to define the concentration of selected acidic compounds and associated oxidants in the gaseous and liquid phases for above-cloud base forests. To achieve the program objectives, the following measurements are made at one or all MCCP sites: (A) Gaseous chemical measurements: ozone-continuous (hourly average), sulfur dioxide- continuous (hourly average), oxides of nitrogen - continuous (hourly average), hydrogen peroxide (hourly average), ammonia/ammonium - filterpack (weekly), fine particulate sulfate aerosol - filterpack (weekly),sulfur dioxide - filterpack (weekly). (B) Aqueous chemical measurements: cloud water chemistry (hourly), cloud water hydrogen peroxide (hourly), rain chemistry from precipitating clouds (hourly), precipitation chemistry, NTN (weekly), throughfall/stemflow chemistry (weekly). (C) Physical measurements: cloud liquid water content, presence of cloud-continuous (hourly), precipitation amount (hourly). (D) Climatic measurements (forest stress and model evaluation parameters): air temperatures (hourly), wind speed and direction (hourly), relative humidity (hourly), insolation (hourly), barometric pressure (hourly). Measurement data collected at the MCCP sites are transferred to the MCCP Central Data Center. At the Data Center the data are loaded into a VAX/ORACLE database. These data are subsequently validated and certified in accordance with the procedures defined in the MCCP QA Plan. This database is exercised by a series of report generation software programs to yield the desired frequency distribution graphic reports and other statistical reports required by MCCP scientists or by the forest research community. 47
48 The following segments summarize the 1986-87 results. CLOUD EXPOSURE FREQUENCY Various techniques for monitoring the presence of clouds have been studied and tested during the 1986-87 field seasons and have been summarized in a MCCP interim technical report. For the 1988 field season, a uniform cloud detection technique has been implemented network- wide using an optical cloud detection instrument developed originally by the Netherlands Research Foundation. Table 1 summarizes cloud exposure statistics by site for the June-October 1987 period: Table 1. Cloud Exposure Statistics Average Cloud No. of Event Diurnal Cloud Frequency Cloud Duration Freauencv {%) _% time Events ~hrs) 1-6 AM ?-Noon 1-6 PM 7-Mdnt Whiteface 40 80 15 53 41 25 45 Whitelop 28 142 7 39 Mitchell 29 143 7 46 33 12 27 24 10 35 Moosilauke 21 52 13 26 21 15 23 Shenandoah 7 29 ~ 10 6 3 9
49 CLOUD CHEMISTRY EXPOSURE Table 2. Cloud Chemistry Exposure (Non-precipitating Clouds) for 1986 and 1987 All concentrations are in micromoles per liter. Std Dev Number of Site/Year Ion Max Mean Samules Hourlv l WHITEFACE 1986 H+ 1585 254 254 306 1987 H+ 1738 188 212 196 1 986 NH4+ 776 110 1 36 308 1987 NH4+ 493 123 108 198 1 986 S04- - 747 1 30 1 32 308 1 987 S04- - 940 1 20 1 29 1 98 1986 NO3- 804 82 95 308 1987 NO3- 547 95 97 198 SHENANDOAH 1986 H+ 589 237 157 25 1987 H+ 1202 212 210 57 1986 NH4+ 263 123 75 25 1 987 NH4+ 602 96 118 57 1986 S04-- 267 101 76 25 1987 S04-- 619 104 1 15 57 1986 NO3- 388 157 110 23 1987 NO3- 541 104 110 57 WHITETOP 1986 H+ 3020 274 395 153 1987 H+ 1413 308 240 153 1986 NH4+ 834 136 159 154 1987 NH4+ 1070 199 182 153 1986 S04-- 943 132 159 154 1987 S04-- 1070 199 156 152 1986 NO3- 1744 141 236 155 1987 NO3- 648 150 1 17 152 MT. MITCHELL 1 986 H+ 1 096 346 254 111 1987 H+ 724 374 140 38 1986 NH4+ 670 191 143 114 1987 NH4+ 430 154 89 38 1986 S04-- 670 191 143 1 14 1987 S04-- 435 198 83 38 1 986 NO3- 373 1 2 1 80 115 1987 NO3- 265 1 38 55 38 MOOSILAUKE 1987 H+ 1202 284 324 54 1 987 NH4+ 488 110 1 30 54 1987 S04-- 768 142 1 82 54 1 987 NO3- 548 114 1 23 54
50 OZONE Table 3. Frequency Distribution of Ozone (Summer) All values are given in parts per billion (ppb3. Standard Site Min Max Mean Deviation Whiteface Mtn 1986 15.3 98.5 44.1 13.S Whiteface Mtn 1987 6.0 105.0 46.0 14.3 Whitetop 1986 1.0 120.0 59.0 15.0 Mount Mitchell 1986 12.0 112.9 47.2 15.3 (Other sites had less extensive data records for 1986 and thus are omitted here.) HYDROGEN PEROXIDE HITEFACE MT. SUMMIT: 1985 AQUEOUS HYDROGEN PEROXIDE CONCENTRATIONS FROM 150 SAMPLE VALUES 100- 80- a: En lo 60- - 40- 20 o- 1_~ E// // ~/~ cr.,,,. vat ~///~ _ -T- 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-9090-100 >100 -ONCENTRATION ( PUB ) Fig. la. Frequency Distribution of Hydrogen Peroxide, 19XS
51 WHITETOP MT. SUMMIT: SUMMER 1 98 6 AQUEOUS HYDROGEN PEROXIDE CONCENTRATIONS 1 0 0- En A: En of .. 1 8 0- 6 0- 4O 2 0- O- it. ~,,,~ ~~ ~,,,~ V~ V'''~ V'''~ V'''' ~,,,~ V'''~ V'''~ I I I I I r I I ~ I 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-9090-100 >100 CONCENTHAT10~1 (PUB ) Fig. lb. Frequency Distribution of Hydrogen Peroxide, 19&6 A few cloud samples from all other sites have been shipped to the above sites for H2O2 analysis. Gas phase H202: because of the complexity in measuring the gas phase concentration of H2O2, only two sites are operating on a continuous basis. Table 4. Hydrogen Peroxide, 1986 No. of Standard Hourly Site Min Max Mean Deviation Samoles Whiteface Mtn 1986 BDL 2.82 0.72 0.55 424 Fall 1986 BDL 0.75 0.18 0.15 76 Whitetop 1986 BDL 2.60 0.80 0.50 183 Fall 1986 BDL 0.57 0.15 0.11 96
52 CLOUD LIQUID WATER CONTENT Cloud liquid water content (LWC) measurement is necessary to convert cloud water chemical concentrations into pollutant inputs. A filter sampler designed by TVA collects cloud water isokinetically on a filter which is weighed manually during cloud events to obtain hourly integrated LWC values. The frequency distribution of 1987 cloud LWC values was determined for all sites for both precipitating and non-precipitating cloud (see sample figure). Whiteface exhibits a broad, bimodal range (0.1-1.1 g/m3) compared with Mitchell and Moosilauke where most values are less than 0.5. Values measured at Shenandoah cover a broad range similar to that for Whiteface, but the sample size is significantly smaller. 30 20 PERCENT 1 5 FREQUENCY 1 0 5 - o MITCHELL 78 HOURLY SAMPI Fry . <.1 .1-.2.2-.3.3-.4.4-.5.5-.6.6-.7.7-.8.8-.9 .9- 1.0- >1.1 1.0 1.1 LIQUID WATER CONTENT (G/M3) Fig. 2. Frequency Distribution of Cloud Liquid Water Content For Non-Precipitating Cloud-1987 Field Season
53 DISCUSSION Concentrations of pollution related ions in cloud water (non-precipitating clouds) are significantly higher than in rain, typically by a factor of 5 to 10. There is a considerable interannual and intersite variation in cloud water ion concentration, reflecting the different weather patterns and meteorological conditions that give rise to clouds at the MCCP sites. In order to define "typical cloud exposure, we need to monitor clouds for several years as was originally envisioned and as provided for in the project plan. The mean hydrogen ion concentration of cloud water samples (non-precipitating clouds) are in the range of 174 to 374 micromoles per liter for all MCCP sites. Cloud exposure frequency over the first two field seasons ranged from about 10°h to 45% for the MCCP sites (see fig. 3~. There is no clear pattern evident of cloud frequency as a function of latitude or elevation. Current studies are focusing on the monitoring of cloud exposure at several mountain sub-sites to determine relationships with elevation, slope and aspect. CLOUD FREQUENCY AT MCCP SITES 1986 VS 1 987 WHITEFACE WHITETOP SITE MITCHELL MOOSllAUKE SHENANDOAH 25 1 40 38 35 46 1 1 0 10 20 PERCENT FREQUENCY Fig. 3. Cloud Frequency at MCCP sites 1986 vs 1987 1986 . 1987 30 40 50 The observed differences in liquid water content values among sites emphasizes the need to distinguish between cloud types (e.g., cap cloud vs. regional cloud) and cloud formation mechanisms.
54 Sulfur dioxide concentrations (not shown here) are very low at all sites except Shenandoah. At Whiteface Mountain for example, the mean SO2 concentration in 1987 was 1.3 ppb and 65 percent of the time the values were below the detection level of the instrument, i.e., 0.5 ppb. Ozone, hydrogen peroxide and sulfur dioxide show almost no diurnal variation in concentration at high elevation sites. Hydrogen peroxide is a typical summertime problem, being very low in spring and fall and non-detectable in winter (gas and aqueous phase). Total deposition fluxes computed for each 1986 Whitetop intensive study were converted to a monthly basis (i.e., kg ha ~i mom) for ease of comparison between studies of different lengths. Figure 4a illustrates the computed precipitation (wet) deposition fluxes. The length of a horizontal bar represents the computational uncertainty for each combination of chemical species and season. Dry deposition flux estimates for Whitetop are illustrated in Figure 4b. "Total SO 4" deposition is the sum of particle sulfate deposition and the sulfate equivalent of the deposited SO2 gas. Figure 4c illustrates cloud deposition flux estimates for Whitetop. Extrapolations to entire seasons yield the seasonal deposition totals listed in Table 5. In this extrapolation exercise, the seasons are defined as: spring=April-June, summer=June-September, fall=October-December. Caution must be taken in extrapolating these values further to annual values. 01 HzO2 so2- tIO3 C1- ~a NH; C:a 2. Mg2. 1 0 1 l loll 1 1 I.i _ . _ to Fig. 4a. Plots of Whitetop Mountain Wet Deposition Flux (kg hat m°l) Estimates during the Spring, Summer, and Fall intensive Field
55 001 03 R202 SO2 sop Total sop Total _ 01 1 1~1. 1.!.~.1~111 i ! ~ ~ , - j ~ . 1 lute | T ~1 1 1 ~!1 ~ T4.- '_ ; 10 _ _ _ . ., . 111- __., L ,: ~ AWL 1 1 ~! --1 ~ 1 l ~ Fig. 4b. Plots of Whitetop Mountain Dry Deposition Flux (kg hat m°l) Estimates During the Spring, Summer, and Fall Intensive Field Studies of 1989 0. 1 - HzO2 so2- ~O3 Cl- Na- NH; ~- c4~2. mg2. T ..... . 10 . ....... . . . _ _ _ _ 11 .l_.~. j ~ 1111~1 I, ~ 1 Fig. 4c. Plots of Whitetop Mountain Cloud Deposition Flux (kg hat M°1) Estimates During the Spring, Summer, and Fall Intensive Field Studies of 1986.
56 Table 5. 1986 Estimates Seasonal Deposition Budgets for Whitetop Mountain Total Chemical Deposition (kg ham) A113 Species Suring Summer Fall Seasons O3 34- 41 22- 34 <2oa <95 H2O2 - 9-18 3-6 _ so2 2-3 2-5 - _ so2 56- 123b 86- 167b 64- 131 b 206-421 b Total So2 - 59-128 (SO2 + so2 ~) Total NO3 tHNO3 + NO3) 88-174 - 59-128 38-78 38-79 106-229 C 1 - 1.5-3.2C 1.3-2.3C 1.6-3.1 C 4.4-8.6C Na+ 1 .2_2.5C 0.7_ 1.2C 0.6- 1.1 C 2.5-4.8C Nib+ 7-168 7-158 8-156 22-468 K+ 0.6-1.0C 0.4_0.6C 0.4_0 8C 1.4-2.4C ca2+ 0.6- 1.0C 1.4-2.0C 1.2_2.4C 3.2_5.4C Mg2+ 0.2_0.3C 0.3_0.6C 0.1 _0.3C 0.6- 1.2C a. Upper limit based on fall average Os concentrations and decline in daylight hours. b. Neglects the dry deposition of So2 ~ which was probably <1% of the total deposition. c. Neglects dry deposition which was computed to be typically <1% of the total deposition. d. Neglects the dry deposition of N~+and NH3.
