Joseph E. Sickles

The nitric acid shootout: field comparison of measurement methods

Eighteen instruments for measuring atmospheric concentrations of nitric acid were compared in an eight day field study at Pomona College, situated in the eastern portion of the Los Angeles Basin, in September 1985. The study design included collocated and separated duplicate samplers, and the analysis by each laboratory of a set of quality assurance filters, so that the experimental variability could be distinguished from differences due to measurement methods. For all sampling periods, the values for nitric acid concentrations reported by the different instruments vary by as much as a factor of four. The differences among measurement techniques increase with nitric acid loading, corresponding to a coefficient of variation of 40%. In contrast, samplers of the same design operated by the same group show variability of 11–27 %. Overall, the highest reported concentrations are observed with the filter packs and lower concentrations are observed by the annular denuders and tunable diode laser absorption spectrometers. When the nitric acid concentrations are high enough to be detected by the FTIR, the FTIR values are close to those obtained by the denuder difference method and to the mean value from the other sampler groups. In the absence of a reference standard for the entire study, measurement methods are compared to the average of four denuder difference method samplers (DDM). Filter pack samplers are higher than the DDM for both daytime and night-time sampling. Two different filter packs using Teflon® prefilters are higher than the DDM by factors of 1.25 and 1.4. The results from the three annular denuders do not agree; the ratios of means to the DDM value are 1.0,0.8 and 0.6. For the transition flow reactor method and for two dichotomous samplers operated as denuder difference samplers, the ratio of means to the DDM are 1.09 and 0.93, respectively. The tunable diode laser absorption spectrometers gave lower daytime and higher night-time readings compared to the DDM, especially during the last three days of the study. Averaged over the entire measurement period, the daytime ratio of TDLAS to DDM is 0.8 and the night-time ratio is 1.7.

Field comparison of methods for the measurement of gaseous and particulate contributors to acidic dry deposition.

Abstract A field study was conducted to compare methods for sampling and analysis of atmospheric constituents that are important contributors to acidic dry deposition. Comparisons are made of different measurements of ambient concentrations of gaseous HNO 3 , NO 2 , NH 3 and SO 2 and particulate NO 3 − , NH 4 + and SO 4 2− . Three multicomponent samplers were used: the Canadian filter pack (FP), the annular denuder system (ADS), and the transition flow reactor (TFR). A tunable diode laser absorption spectrometer (TDLAS) provided continuous reference measurements of NO 2 and HNO 3 . Nitrogen dioxide was also monitored with continuous luminol-based chemiluminescence monitors and with passive sampling devices (PSDs). The study was designed to provide a database for statistical comparison of the various methods with emphasis on the multicomponent samplers under consideration for use in a national dry deposition trends monitoring network. The study was conducted at the EPA dry deposition station in Research Triangle Park, NC between 29 September and 12 October 1986. Daily averaging and/or sampling times were employed for the 13-day study; weekly samples were also collected, but results from these samples are not compared in this paper. ADS, TFR and FP results are in good agreement for measurements of the sum of particulate and gaseous NO 3 − concentrations and of total particulate SO 4 2− . ADS, FP, and TDLAS measurements of HNO 3 are in good agreement, but TDLAS results differ with and are substantially less than those from the TFR. TDLAS measurements of NO 2 are highly correlated with those of the TFR, but show mixed comparisons with results from the two luminol-based monitors and with results from the two sets of PSDs. TFR and FP measurements of particulate NH 4 + are in good agreement, but FP results exceed those of the ADS when volatilization losses of NH 4 + in the ADS are ignored. At the low ambient NH 3 concentrations, results of ADS, TFR, and FP show considerable variability but no statistically discernable differences. ADS results for measurements of SO 2 exceed those from both the TFR and FP. Non-quantitative recovery of SO 2 from various elements in the TFR and FP may account for this discrepancy.

Precision of atmospheric dry deposition data from the Clean Air Status and Trends Network

Abstract A collocated, dry deposition sampling program was begun in January 1987 by the US Environmental Protection Agency to provide ongoing estimates of the overall precision of dry deposition and supporting data entering the Clean Air Status and Trends Network (CASTNet) archive. Duplicate sets of dry deposition sampling instruments were installed adjacent to existing instruments and have been operated for various periods at 11 collocated field sites. All sampling and operations were performed using standard CASTNet procedures. The current study documents the bias-corrected precision of CASTNet data based on collocated measurements made at paired sampling sites representative of sites across the network. These precision estimates include the variability for all operations from sampling to data storage in the archive. Precision estimates are provided for hourly, instrumental ozone (O 3 ) concentration and meteorological measurements, hourly model estimates of deposition velocity ( V d ) from collocated measurements of model inputs, hourly O 3 deposition estimates, weekly filter pack determinations of selected atmospheric chemical species, and weekly estimates of V d and deposition for each monitored filter pack chemical species and O 3 . Estimates of variability of weekly pollutant concentrations, expressed as coefficients of variation, depend on chemical species: NO 3 − ∼8.1%; HNO 3 ∼6.4%; SO 2 ∼4.3%; NH 4 + ∼3.7%; SO 4 2− ∼2.3%; and O 3 ∼1.3%. Precision of estimates of weekly V d from collocated measurements of model inputs also depends on the chemical species: aerosols ∼2.8%; HNO 3 ∼2.6%; SO 2 ∼3.0%; and O 3 ∼2.0%. Corresponding precision of weekly deposition estimates are: NO 3 − ∼8.6%; HNO 3 − ∼5.2%; SO 2 ∼5.6%; NH 4 + ∼3.9%; SO 4 2− ∼3.5%; and O 3 ∼3.3%. Precision of weekly concentration, V d estimates, and deposition estimates are comparable in magnitude and slightly smaller than the corresponding hourly values. Annual precision estimates, although uncertain due to their small sample size in the current study, are consistent with the corresponding weekly values.

Wet deposition from clouds and precipitation in three high-elevation regions of the Eastern United States

Abstract Three regions are identified in the Eastern United States (US) that contain substantial land area at high elevations: the mid-Appalachian region (MAR), Eastern New York state (ENY), and the New England region (NER). Approximately 75% of the land cover in these areas is forested, with 5.6–29% of the total acreage above 600 m and subject to cloud deposition. Measurements of cloud deposition are scarce. A 6-year data record of measurements at two high-elevation locations is considered, and scaling factors are developed to enable the rough estimation of area-wide cloud deposition at various elevations in each region. Estimates of precipitation and associated ion deposition are made at 12 arc-second resolution for the Eastern US and are used to obtain elevation-resolved precipitation-mediated deposition for the three study regions. At high elevations, clouds account for a substantial proportion of wet deposition (i.e., the sum of that from clouds and precipitation). For the total land area above 600 m , clouds may account for 20–60% of the total wet ion deposition, with the exact proportion depending on both location and ion species. At elevations above 600 m , but below the climatic tree line, the ratio of cloud-to-precipitation-mediated deposition is higher in NER and ENY than in MAR. At the highest elevations of each study region, clouds may account for over 80% of the wet ion deposition. Although the wet deposition of ammonium (NH4+), sulfate (SO42−), nitrate (NO3−), and hydrogen (H+) ions is enhanced at higher elevations by clouds over precipitation, this enhancement is the largest for ammonium. This study illustrates the major and perhaps dominant role that clouds may play by delivering considerable ion loads to montane ecosystems in selected elevation ranges where these ecosystems may be especially vulnerable.

A Summary of Airborne Concentrations of Sulfur- and Nitrogen-Containing Pollutants in the Northeastern United States

Airborne concentrations of SO2, SO42-, HNO3, NO3-, NH4+, and O3 were monitored over the six-year period from September 1, 1989, through August 31, 1995, at 10 largely rural Clean Air Status and Trends Network (CASTNet) sites in the northeastern United States. Each of the sulfur- and nitrogen-containing air pollutants monitored by CASTNet displays regular, seasonal cyclical behavior and also exhibits a relatively strong high-to-low spatial concentration gradient from southwest to northeast. On average, more than 70% of the measured airborne sulfur is present as SO2, except during the summer, when the figure drops to about 50%. During the summer, the SO2 concentration is the lowest, SO42- is the highest, and the fraction of airborne sulfur present as SO42- varies considerably with location, ranging from an average of 42% at five sites in Pennsylvania to 70% at two sites in New England. Studywide, more than 70% of the measured, oxidized, airborne nitrogen (N) is present as HNO3, except during the winter, when the figure drops to about 60%. The concentrations of gaseous SO2 and HNO3 are usually comparable but not always larger than the corresponding concentrations of measured sulfur and nitrogen aerosols. Nevertheless, the relatively faster deposition velocities for gases are sufficient to ensure that SO2 and HNO3 are usually the dominant contributors to dry sulfur and nitrogen deposition. Observed changes of 1990-1995 annual average airborne sulfur and N concentrations at 10 CASTNet sites in the Northeast are generally consistent with changes in emissions estimated to have occurred in the Northeast over the same period.

On the feasibility of using the tropospheric ozone residual for nonclimatological studies on a quasi-global scale

The tropospheric ozone residual (TOR) was estimated using total ozone mapping spectrometer (TOMS), Stratospheric Aerosol and Gas Experiment (SAGE), and solar backscatter ultraviolet (SBUV) data for nonclimatological periods (i.e., daily values, weekly averages, and monthly averages). The TOR is the difference between the total ozone from TOMS and the stratospheric ozone estimated using either SAGE or SBUV ozone data and the National Meteorological Center tropopause pressure data. Comparison of the TOMS/SAGE TOR with ozonesonde data and with area-averaged surface ozone data for nonclimatological periods was very poor. The month-to-month variations of the TOMS/SBUV TOR, on the other hand, compared very well with the month-to-month variations of the area-averaged surface ozone concentrations, and the day-to-day variations of the tropospheric ozone using ozonesonde data from five stations compared reasonably well with the day-to-day variations of the TOR. However, the RMS difference between the TOMS/SBUV TOR and the tropospheric ozone from ozonesonde data was about 14.4 Dobson units (DU) and the mean difference about 8 DU, though the mean difference was about 1.5 DU at Boulder, Colorado.

Fate of nitrous acid on selected collection surfaces

Glass fiber filters were spiked with a solution containing nitrite (NO2−) and subsequently exposed to various laboratory-prepared atmospheres. Substantial oxidation of NO2− to nitrate (NO3−) was observed in the presence of ozone. Na2CO3-coated glass fiber filters and nylon filters were used to sample ambient air and air from a nitrous acid (HNO2) generator. Substantial amounts of HNO2 collected by Na2CO3-coated filters were oxidized to NO3− upon exposure to ambient air, HNO2 was reversibly and nonquantitatively collected by nylon filters, and appreciable amounts of HNO2 collected by nylon filters were oxidized to NO3− upon exposure to ambient air. Ozone may have been the responsible oxidant. These findings may have important implications on the interpretation of results of integrative ambient air samplers for HNO2 and nitric acid (HNO3) and on the design of these samplers.

Sampling and analysis of ambient air near Los Angeles using an annular denuder system

Abstract The annular denuder system (ADS) was used to measure HNO3, HNO2, NH3, SO2, and fine particle NO3−, SO42− and NH4+ during the 1985 Nitrogen Species Methods Comparison Study. This study was conducted from 11 to 19 September 1985 on the Pomona College Campus at Claremont, California near Los Angeles. This paper describes the performance of the ADS and presents selected results obtained using the system during the study. Most of the variability of the ADS results is associated with sampling rather than analytical operations. Based on collocated operation, the precision depends on the species sampled. Precision estimates (median CVs) for SO2 HNO3 and SO42−, as determined from 4- and 6-h collocated sampling are 8.0,14.3 and 4.1 %. Weighted averages of 4- and 6-h concentrations agree well with the corresponding 10- and 12-h averages for most species. An apparent low bias in the HNO3 results may be associated with interactions between the gas sample and the large Teflon® surface area upstream of the annular denuder.

A climatology of total ozone mapping spectrometer data using rotated principal component analysis

The spatial and temporal variability of total column ozone (Ω) obtained from the total ozone mapping spectrometer (TOMS version 7.0) during the period 1980–1992 was examined through the use of a multivariate statistical technique called rotated principal component analysis. Utilization of Kaisers varimax orthogonal rotation led to the identification of 14, mostly contiguous subregions that together accounted for more than 70% of the total Ω variance. Each subregion displayed statistically unique Ω characteristics that were further examined through time series and spectral density analyses, revealing significant periodicities on semiannual, annual, quasi-biennial, and longer term time frames. This analysis facilitated identification of the probable mechanisms responsible for the variability of Ω within the 14 homogeneous subregions. The mechanisms were either dynamical in nature (i.e., advection associated with baroclinic waves, the quasi-biennial oscillation, or El Nino-Southern Oscillation) or photochemical in nature (i.e., production of odd oxygen (O or O3) associated with the annual progression of the Sun). The analysis has also revealed that the influence of a data retrieval artifact, found in equatorial latitudes of version 6.0 of the TOMS data, has been reduced in version 7.0.

A 5-year evaluation of the representativeness of the tropospheric ozone residual at nonclimatological periods

Daily values, monthly averages, and seasonal averages of the tropospheric ozone residual (TOR) were estimated using the Total Ozone Mapping Spectrometer (TOMS) and the Solar Backscattered Ultraviolet (SBUV) data for the five-year period 1985–1989. Comparisons were made at these various timescales between TOR and tropospheric ozone using data from eight ozonesonde stations within the region 50°N to 50°S and between normalized departures of the TOR and surface ozone data averaged over four 5° latitude by 5° longitude squares within the eastern United States. These comparisons were accomplished to determine the usefulness of the TOR to represent tropospheric ozone at nonclimatological periods. The results indicated that the annual cycle of the TOR determined using monthly and seasonally averaged values provides a realistic depiction of the annual cycle of tropospheric ozone in the northern hemisphere; that is, approximately a 2% mean error and an 81% correlation. However, in a limited number of comparisons (Hilo and Natal), the annual cycle of the TOR represented the annual cycle of tropospheric ozone at tropical latitudes poorly; that is, approximately a 38% mean error and a 59% correlation.