Candis S. Claiborn

Ozone climatology at high elevations in the southern Appalachians

Ozone measurements are reported for two high-elevation sites located in the Mt. Mitchell State Park in North Carolina (site 1 on Mt. Gibbs, ---2006 m, and site 2 on Commissary Ridge, ---1760 m). These measurements are also compared to those from a nearby, low-elevation site (Fairview, ---850 m). The measurements were made from May through September during the years 1986 and 1988 and from May through October during 1987, at sites 1 and 2. Measurements were also made from May through September 1989 at site 1 only. During the monitoring season at site 1 the mean ozone concentrations were 50, 51, 66, and 52 ppbv for 1986, 1987, 1988, and 1989 field seasons, respectively, while at site 2 the mean ozone concentrations were 49, 49 and 52 ppbv for 1986, 1987, and 1988. (It has been shown that exposure to ozone concentrations of ->50 ppbv is sufficient to cause damage to certain species of vegetation.) The daily maximum, 1-hour average, and 24-hour average concentrations were found to be greatest during summer months (late May through early July), with lower concentrations during fall (August and September), suggesting a correlation with the seasonal photochemical cycle. It is suggested that excess hydrocarbons released during budbreak may contribute to the seasonal signal in the ozone data. During the 1988 monitoring season there were three periods of very high ozone levels (>80 ppbv) which lasted over 100 hours. During these long episodes there were 48 hours during which ozone concentrations exceeded the current National Ambient Air Quality Standard (NAAQS) of 0.12 ppmv. High hourly averaged SO2 (---25 ppbv) and NOx ( 11 ppbv) levels were also found during these episodes. Meteorological analyses show an association between periods of high ozone concentrations and synoptic-scale patterns. Such high gaseous pollutant concentrations were not observed during the previous two field seasons. Also, no exceedances of the NAAQS were observed during the field season of 1989. No discernible diurnal cycle in the ozone concentrations was observed at site 2; however, a reversed diurnal cycle (nighttime maximum) was evident at site 1. Also, average ozone concentrations increased from lower elevations to higher elevations. Evidence suggests that the relationship between the mountaintop and the height of the mixing layer, coupled with horizontal transport of ozone in the lower troposphere, may be important in explaining the nocturnal maximum at site 1 and the observed altitudinal gradient in ozone.

Measurements of atmospheric hydrogen peroxide in the gas phase and in cloud water at Mt. Mitchell, North Carolina

Measents of atmospheric hydrogen peroxide in the gas phawae made during four latemire observon periods at the Mt. Mitchell State Park, Noah Carolina, during the growing sea, on (May through September) of 1988. Cloud water hydrogen peroxide was measured during the entire field semon of 1988 and during the late rummet and fall of 1987 (Augtat and October). Cloud water concentrations were found to be similar to those reported from another high-elevation location in the southeastvaUnited Statee. Cloud wattamplee coilacted during these periods showed a wide range of levels (- 0 - 219/aM/L) and average values of 38/zM/L, and 44 /zM/L for the entire sampling seasons of 1988 and 1987. respectively. Significant seasonal variation was noted both in 1987 and 1988, with cloud water levels of hydrogen peroxide much higher in the summer than in the fall. Gas-phase hydrogen peroxide levels ranged from the detection limit (0.1 ppbv) to above 4 ppbv. Gas-phase hydrogen peroxide demonstrated a nighttime maximum in the summer but not in the fall. The measurements taken in the fall were significantly lower than those taken during the summer, possibly due at least in part to seasonal variation. Atmospheric hydrogen peroxide levels were found to be incrv, asing during stagnating high- pressure systems and were found to correspond to the back trajectory of the air mass with the highest conecatratiom corresponding to conLineatal air masses. The hydrogen peroxide concentration was also found to be affby radical formation from ozone and by lcpmcassee such as wet and dry deposition. LVIRODUON

Trends, seasonal variations, and analysis of high-elevation surface nitric acid, ozone, and hydrogen peroxide

Abstract Atmospheric photochemical oxidants nitric acid, ozone, and hydrogen peroxide were monitored in ambient air at Mt Mitchell State Park, North Carolina. Ozone measurements made from May to September during 1986–1990 are reported for two high-elevation sites (Site I on Mt Gibbs, approximately 2006 m; and Site 2 on Commissary Ridge, approximately 1760 m). These measurements are also compared to those from a nearby, low-elevation site (Fairview, approximately 830 m). Average ozone concentrations increased from lower to higher elevations. Meteorological analysis shows an association between periods of high ozone concentrations and synoptic-scale patterns. No discernible diurnal cycle in the ozone concentrations was observed at Site 2; however, a reversed diurnal cycle (nighttime maximum) was evident at Site 1. Gas-phase hydrogen peroxide and nitric acid concentration were measured at Site I during 1988 and 1989, and typically range from 0 to 4 ppbv, and 0–2 ppbv, respectively. Seasonal analysis shows that the ozone maximum occurs during spring coincident with the spring maximum at Whiteface Mountain, NY, Mauna Loa in Hawaii, and at Alpine stations in Europe, suggesting that ozone production is a hemispheric rather than local phenomenon and that the underlying phenomenon affects perhaps the entire Northern Hemisphere. The diurnal cycle of gaseous hydrogen peroxide was similar to the high-elevation ozone signal, while gaseous nitric acid concentration peaked during the day. This apparent discrepancy in the diurnal cycle between the three atmospheric photochemical oxidants at high elevation may be due to a difference in the behavior of the altitudinal gradients of those oxidants resulting from a combination of photochemistry, meteorology and dynamic processes.

Regional measurements and modeling of windblown agricultural dust: The Columbia Plateau PM10 Program

The Columbia Plateau PM10 Program [CP3] is a multi-investigator study of windblown dust in the Pacific Northwest with an emphasis upon the role of agricultural lands in regional dust storms. Ambient measurements of PM10 within the source areas of the central basin of Washington during several autumn dust periods show that typical background concentrations [nonwind-event periods] decrease from an average of 34 μg m−3 in early fall to 10 μg m−3 in late fall. During wind events, ambient concentrations at downwind urban receptors can exceed 500 μg m−3 on an hourly basis, with 24 hour averaged values as high as 300 μg m−3. Particle counts during wind events are enhanced by as much as a factor of 5 for particle sizes greater than 5 μm, and also for sizes between 1 and 5 μm compared to nonwindy periods. Analysis of the synoptic conditions which exist during these dust storms showed a common situation where a surface low is moving rapidly across British Columbia while a surface high is positioned in the Great Basin of Nevada. A regional windblown dust air quality model, developed for the CP3 study, predicts large dust plumes stretching across eastern Washington with maximum concentrations in the source regions exceeding 10,000 μg m−3. Total mass emissions during a storm are estimated to equal 100 Gg dy−1, which represents about 1% of recent estimates of the global annual dust emission rate. In the initial applications of the model, available PM10 observations are used to calibrate the dust emission algorithm. Changes in the dust constant for two modeled events are consistent with changes in soil cover and accumulated precipitation between an early fall event and a late fall event. The estimated fluxes are in a range similar to those in the literature but appear to be much less than estimated from global modeling of recently disturbed soils.

Measurements of the dry deposition of peroxides to a Canadian boreal forest

The dry deposition rates of hydrogen peroxide (H2O2) and total organic peroxides (ROOH) were measured above a coniferous forest in Saskatchewan, Canada. Deposition velocities νd were obtained from gradient measurements using the modified Bowen ratio method. A diurnal pattern was observed, with highest deposition velocities occurring during the day. Daytime deposition velocities were approximately 5 cm s−1 for H2O2 and 1.6 cm s−1 for ROOH. Nighttime deposition velocities were much smaller, approximately 1 and 0.5 cm s−1, respectively. A slight seasonal trend observed in νd can be attributed to meteorological rather than physiological factors. The seasonal variation of H2O2 and ROOH concentrations, however, effects a significant seasonal variation in flux. Highest concentrations and therefore highest fluxes were observed during midsummer. On a diurnal scale, maximum deposition velocities coincide with high concentrations only during midday. Thus the highest fluxes occur primarily from 1100 to 1500 hours. The transport of H2O2 appears to be similar to that of other soluble, reactive trace gases, such as HNO3 and NH3, and a small surface resistance is suspected.

April 1998 Asian dust event over the Columbia Plateau

Surface-based radiometers can be used to assess the atmospheric aerosol burden. During 1998, two multifilter rotating shadow-band radiometers (MFRSRs), operated by Washington State University (WSU) and by the USDA UV-B program, were used to collect data on the Columbia Plateau atmosphere. Analysis of these data by an automated Langley algorithm provided retrievals for total optical thickness, allowing for calculation of aerosol optical thickness (AOT) and the top-of-atmosphere (TOA) instrument signal. Statistical evaluation of the TOA signal permitted recalculation of optical thickness using the Bouguer-Lambert-Beer law and resulted in improved estimates of aerosol optical thickness (AOT). Results for AOT and the associated Angstrom parameters are presented here for an April 1998 dust event for two colocated Columbia Plateau sites. AOT at 500 nm went from background levels (seasonally dominated by regional windblown dust) of ∼0.2 to more than 0.4 during the event maximum on April 27, not returning to normal levels until April 30. Comparison of 500-nm AOT between the two MFRSR showed a root-mean-square (RMS) difference of 0.016. The Angstrom exponent α reached a minimum of ∼0.2, and the β coefficient reached a maximum of ∼0.35, both on April 27, coincident with the AOT maximum. Contemporaneous aerosol sampling in Spokane, Washington, provided (1) elemental data that strongly support our interpretation of this event as an influx of Asian dust without significant sulfur enrichment and (2) event maximum PM10 measurements ∼80 μg/m3 consistent with Pullman event maximum AOT results, assuming a 3–4 km thick dust layer.

Dynamic chemical characterization of montane clouds

Abstract Cloud water collections have been made on Mt. Mitchell using a nearly real-time cloud and rain acidity/conductivity (CRAC) analyzer. Results are reported for integrating times of approximately 5 min during several cloud events in the summer and fall of 1987. Both pH and ionic strength during cloud events were found to be much more variable than previously indicated by cloud collection. Maximum values of H + and SO 4 2− ion concentrations in 5-min samples were as much as 2.5 times greater than those measured in 1-h integrated collections. These results are not influenced by instrumental variability to any measurable extent. Results from repeated quality control samples were highly reproducible, and agreement between integrated collection data and the average values of 5-min sequential samples was also very good.

Chemical climatology of high elevation spruce-fir forests in the southern Appalachian mountains

The physical and chemical climatology of high elevation (> 1500 m) spruce-fir forests in the southern Appalachian mountains was studied by establishing a weather and atmospheric chemical observatory at Mt Mitchell State Park in North Carolina (35 degrees 44 05 N, 82 degrees 17 15W). Data collected during the summer and autumn (May-October) of 1986, 1987, and 1988 are reported. All measurements were made on or near a 16.5 m walk-up tower extending 10 m above the forest canopy on Mt Gibbes (2006 m msl), which is located approximately 2 km SW of Mt Mitchell. The tower was equipped with standard meteorological instrumentation, a passive cloud water collector, and gas pollutant sensors for O3, SO2, NOx. The tower and nearby forest canopy were immersed in clouds 25 to 40% of the time. Non-precipitating clouds were very acidic (pH 2.5-4.5). Precipitating clouds were less acidic (pH 3.5-5.5). The dominant wind directions were WNW and ESE. Clouds from the most common wind direction (WNW) were more acidic (mean pH 3.5) than those from the next most common wind direction (ESE, mean pH 5.5). Cloud water acidity was related to the concentration of SO4(2-), and NO3- ions. Mean concentration of H+, NH4+, SO4(2-), and NO3- ions in the cloud water varied from 330-340, 150-200, 190-200 and 120-140 micromol litre(-1) respectively. The average and range of O3 were 50 (25-100) ppbv (109) in 1986, 51 (26-102) ppbv in 1987, and 66 (30-140) during the 1988 field seasons, respectively. The daily maximum, 1-h average, and 24-h average concentrations were all greatest during June through mid-August, suggesting a correlation with the seasonal temperature and solar intensity. Throughfall collectors near the tower were used to obtain a useful estimate of deposition to the forest canopy. Between 50-60% of the total deposition of SO4(2-) was due to cloud impact.

Exceedances of the National Ambient Air Quality Standard for Ozone Occurring at a “Pristine” Area Site

The authors present ozone measurements for high elevation pristine area sites in the Mt. Mitchell, North Carolina State Park located in the southeastern United States. These data have been collected as part of an on-going project, one of the goals of which is to assess rural ozone exposure. They may also provide insight into those processes which affect rural ozone levels, and perhaps might guide policy makers in developing strategies for rural ozone control.

Dynamic atmospheric chamber systems: applications to trace gas emissions from soil and plant uptake

Atmospheric emissions, transport, transformation and deposition of trace gases may be simulated through chambers. The dynamic flow-through chamber system has been developed in response to a need to measure emissions of nitrogen, sulphur and carbon compounds for a variety of field applications. Oxides of nitrogen (NO, NO2, NOY) emissions have been measured from agricultural fertilised/unfertilised soils. Ammonia-nitrogen (NH3–N) and reduced organic sulphur compound emissions have been measured using this same technique across a gas-liquid and soil-atmosphere interface at swine waste treatment anaerobic storage lagoons and in agricultural fields. Similar chamber systems have also been deployed to measure the uptake of nitrogen, sulphur, ozone and hydrogen peroxide gases by crops and vegetation to examine atmospheric-biospheric interactions. Emission measurements compare well with a coupled gas-liquid transfer with chemical reaction model as well as a US Environmental Protection Agency (EPA) WATER9 model.