Winston T. Luke

Thunderstorms: An Important Mechanism in the Transport of Air Pollutants

Acid deposition and photochemical smog are urban air pollution problems, and they remain localized as long as the sulfur, nitrogen, and hydrocarbon pollutants are confined to the lower troposphere (below about 1-kilometer altitude) where they are short-lived. If, however, the contaminants are rapidly transported to the upper troposphere, then their atmospheric residence times grow and their range of influence expands dramatically. Although this vertical transport ameliorates some of the effects of acid rain by diluting atmospheric acids, it exacerbates global tropospheric ozone production by redistributing the necessary nitrogen catalysts. Results of recent computer simulations suggest that thunderstorms are one means of rapid vertical transport. To test this hypothesis, several research aircraft near a midwestern thunderstrom measured carbon monoxide, hydrocarbons, ozone, and reactive nitrogen compounds. Their concentrations were much greater in the outflow region of the storm, up to 11 kilometers in altitude, than in surrounding air. Trace gas measurements can thus be used to track the motion of air in and around a cloud. Thunderstorms may transform local air pollution problems into regional or global atmospheric chemistry problems.

A tropical Atlantic paradox: Shipboard and satellite views of a tropospheric ozone maximum and wave-one in January-February 1999

During the Aerosols99 trans-Atlantic cruise from Norfolk, VA, to Cape Town, South Africa, daily ozonesondes were launched from the R/V Ronald H Brown between 17 January and 6 February 1999. A composite of tropospheric ozone profiles along the latitudinal transect shows 4 zones, nearly identical to the ozone distribution during a January-February 1993 trans-Atlantic cruise [Weller et al., 1996]. Sondes from the cruise and Ascension Island (8S, 14.5W), as well as the Earth-Probe (EP)/TOMS satellite instrument, show elevated tropospheric ozone (> 35 Dobson Units) throughout the south Atlantic in January 1999. Ozone layers associated with biomass burning north of the ITCZ (Intertropical Convergence Zone) are prominent at 0-5 km from 10-ON, but even higher ozone (100 ppbv, 5-15 km) occurred south of the ITCZ, where it was not burning - an ozone paradox that contributes to a wave-one zonal pattern in tropospheric ozone. Back trajectories, satellite observations and shipboard tracers suggest that the south Atlantic ozone results from a combination of interhemispheric transport, aged stratospheric-upper tropospheric air, and possibly from ozone supplied by lightning nitric oxide.

Evaluation of a commercial pulsed fluorescence detector for the measurement of low-level SO2 concentrations during the Gas-Phase Sulfur Intercomparison Experiment

A modified pulsed fluorescence (PF) detector (Thermo Environmental Instruments, Model 43s) was used to measure low levels of SO2 in a rigorous, blind intercomparison experiment (Gas-Phase Intercomparison Experiment (GASIE)). The PF detector was able to detect as little as 30 pptv SO2 in a 25-min sampling interval. The coefficients of variation for measurements of approximately 30, 60, 200, 330, and 600 pptv were approximately 40, 9, 6.5, 3, and 3%, respectively. Overall uncertainty of the measurements at 30 pptv approaches ±100%. As inferred from GASIE results, the response of the PF detector may be reduced (quenched) by approximately 7% and 15% at water vapor mixing ratios of 1 and 1.5 mole percent (relative humidities of 35–50% at 20–25°C and 1 atm), respectively. These results are uncertain, however, due to lack of extensive data. Post-GASIE tests point to moderate interferences from NO (rejection ratio of 35), CS2 (rejection ratio of 20), and a number of highly fluorescent aromatic hydrocarbons such as benzene, toluene, o-xylene, m-xylene,p-xylene, m-ethyltoluene, ethylbenzene, and 1,2,4-trimethylbenzene. Rejection ratios for these compounds increase from approximately 17–123 to circa 1200–3800 as the sample flow rate is decreased from 2000 to 300 standard cubic centimeters per minute (sccm), and the hydrocarbons are more efficiently removed by the instruments proprietary hydrocarbon “kicker” membrane. At a flow rate of 300 sccm and a pressure drop of 645 torr across the kicker, the interference from ppmv levels of many aromatic hydrocarbon was eliminated entirely. None of the tested interferants were removed by the carbonate-impregnated paper filter used to zero the instrument during GASIE; thus they induced no net response in the PF detector. These results illustrate the importance of using a selective zeroing method to scrub SO2 without removing potential interferants from the sample flow, thus preserving the overall composition of the sampling matrix.

Tropospheric Chemistry Over the Lower Great Plains of the United States 2. Trace Gas Profiles and Distributions

Convective clouds and thunderstorms redistribute air pollutants vertically, and by altering the chemistry and radiative balance of the upper troposphere, these local actions can have global consequences. To study these effects, measurements of trace gases ozone, O3, carbon monoxide, CO, and odd nitrogen were made aboard the NCAR Sabreliner on 18 flights over the southern Great Plains during June 1985. To demonstrate chemical changes induced by vertical motions in the atmosphere and to facilitate comparison with computer model calculations, these data were categorized according to synoptic flow patterns. Part 1 of this two-part paper details the alternating pulses of polar and maritime air masses that dominate the vertical mixing in this region. In this paper, trace gas measurements are presented as altitude profiles (0–12 km) with statistical distributions of mixing ratios for each species in each flow pattern. The polar flow regime is characterized by northwesterly winds, subsiding air, and convective stability. Concentrations of CO and total odd nitrogen (NOy) are relatively high in the shallow planetary boundary layer (PBL) but decrease rapidly with altitude. Ozone, on the other hand, is uniformly distributed, suggesting limited photochemical production; in fact, nitric oxide, NO, mixing ratios fell below 10 ppt (parts per 1012 by volume) in the midtroposphere. The maritime regime is characterized by southerly surface winds, convective instability, and a deep PBL; uniformly high concentrations of trace gases were found up to 4 km on one flight. Severe storms occur in maritime flow, especially when capped by a dry layer, and they transport large amounts of CO, O3, and NOy into the upper troposphere. Median NO levels at high altitude exceeded 300 ppt. Lightning produces spikes of NO (but not CO) with mixing ratios sometimes exceeding 1000 ppt. This flow pattern tends to leave the midtroposphere relatively clean with concentrations of trace gases similar to those observed in the polar category. During frontal passage both stratiform and convective clouds mix pollutants more uniformly into the middle and upper levels; high mixing ratios of CO are found at all altitudes, and O3 levels are highest of any category, implicating photochemical production. These results illustrate the importance of convection in tropospheric chemistry. Use of average trace gas profiles or eddy diffusion parameterized vertical mixing can lead to errors of 30 to 50% in O3 and CO concentrations and an order of magnitude for odd nitrogen.

Rate of NO2 photolysis from the surface to 7.6 km altitude in clear‐sky and clouds

The photolysis of NO2 by solar UV radiation produces ozone in the troposphere. Radiative transfer models predict an increase in actinic UV flux with altitude, but this has never been well documented experimentally. This paper presents direct airborne measurements of both upwelling and downwelling flux (2π sr each) from 0.2 to 7.6 km for clear-sky conditions and 58° solar zenith angle. The downwelling flux increases by 33% and upwelling flux increases by a factor of four going from the surface to 7.6 km. Excellent agreement was found with models using detailed solutions to the radiative transfer equation or the six-stream approximation, while a number of two-stream computer models agreed within 10 to 25%. Compared to clear-sky measurements, in-cloud enhancement of up to 58% was also observed.

Results of the Gas-Phase Sulfur Intercomparison Experiment (GASIE): Overview of experimental setup, results and general conclusions

Seven techniques for the field measurement of trace atmospheric SO2 were compared simultaneously over 1 month in 1994 using samples produced in situ by dynamic dilution. Samples included SO2 in dry air, in humid air, and in air with potentially interfering gases added. In addition, 2 days of comparison using diluted ambient air were conducted. Six of the seven techniques compared well, with good linear response and no serious interferences but with a range of calibration differences of about 50%.

Measurements of sulfur dioxide during GASIE with the mist chamber technique

This paper highlights the performance of the mist chamber/ion chromatography (MC/IC) technique for measuring atmospheric sulfur dioxide (SO2) during the Gas-Phase Sulfur Intercomparison Experiment (GASIE). The technique was found to be free of interference from CO, CO2, CH4, NOx, O3, CH3SCH3, and H2O vapor. Repeated measurements at various mixing ratios of SO2 indicated that the coefficient of variation in the MC/IC measurement is 3–5% at 300–500 parts per trillion by volume (pptv), 4–7% at 150 pptv, 10% at 50 pptv, and as great as 20% near 20 pptv. In ambient air diluted five- to tenfold with zero air, the MC/IC technique tracked the other methods over the range of 30–3500 pptv. This agreement reinforced the conclusions obtained during the other test phases of GASIE. The MC/IC method has desirable features such as simplicity, small size, and weight of required equipment, sample integration times of 10 min or less, low pptv detection capabilities, and relatively modest implementation costs. In addition, a suite of important soluble gases can be measured simultaneously including nitric, formic, and acetic acids.

A comparison of airborne and surface trace gas measurements during the Southern Oxidants Study (SOS)

The NOAA Twin Otter conducted more than a dozen overflights of ground-level air quality monitoring stations during the 1995 Southern Oxidants Study (SOS) Nashville/Middle Tennessee Ozone Project Field Intensive. Surface and aircraft observations of ozone and ozone precursors were examined to identify systematic sampling errors, and to assess the degree to which surface measurements may be considered representative of the larger planetary boundary layer (PBL). Overall agreement between surface and aircraft trace gas measurements was excellent in the well developed mixed layer, especially in rural-regional background air and under stagnant conditions, where surface concentrations change only slowly. On July 2, surface level measurements were representative of the larger mixed layer over distances as far as 70 km in background air, and 30 km in the weakly advected urban plume. Vertical variations in trace gas concentrations were often minimal in the well-mixed PBL, and measurements at the surface always agreed well with aircraft observations up to 460 m above ground level. Under conditions of rapidly varying surface concentrations (e.g., during episodes of power plant plume fumigation and early morning boundary layer development), agreement between surface and aloft is dependent upon the spatial (aircraft) and temporal (ground) averaging intervals used in the comparison. Under these conditions, surface sites may be representative of the PBL only to within a few kilometers. Under clear skies in the well-mixed PBL, regression of aircraft trace gas data collected within 5 km of the ground sites against 15–20 min average surface concentrations centered on the times of the overflights yielded the following relationships: [O3]aircraft = ([O3]surface × 0.9374) + 4.86 ppbv (r2 = 0.9642); [CO]aircraft = ([CO]surface × 0.8914) + 16.4 ppbv (r2=0.9673); [SO2]aircraft = ([SO2]surface × 0.9414) − 0.069 ppbv (r2 = 0.9945). Although O3, CO, and SO2 measurements at the surface and aloft generally agreed well, isolated examples of unexplained measurement discrepancies emerged, illustrating the need for side-by-side instrument comparisons. NOY measurements agreed poorly between surface and aircraft: [NOY]aircraft = ([NOY]surfacex × 0.9184) + 4.56 ppbv (r2 = 0.9188).

Intercomparison of measurements of sulfur dioxide in ambient air by carbonate‐impregnated filters and Teco pulsed‐fluorescence analyzers

In two previous University of Washington field programs, airborne measurements of SO2 using carbonate-impregnated filters and a Teco pulsed-fluorescence analyzer showed excellent agreement over a range of ambient concentrations from 2 to 127 ppbv. As part of the Gas-Phase Sulfur Intercomparison Experiment (GASIE), ambient air, diluted fivefold to tenfold with zero air, was sampled in the concentration range of 0.02 to 4 ppbv. With the bulk of the measurements in the range of 40 to 230 parts per trillion by volume (pptv), agreement between the two techniques was again very good (regression equation: Teco =1.07(filter)+4.5 pptv, r=0.93). Using careful precleaning, impregnation, storage, and handling techniques for the filter substrates, at sub-100 pptv concentrations, the filter method is capable of an accuracy of better than ±10% with a ±7 pptv uncertainty (due to blank variability) for 6 m3 samples. In addition, the Teco model 43S is capable of rather precise measurements of sub-100 pptv concentrations (approximately ±16 pptv) provided a suitable averaging time is employed (at least 10 min).

Modification of a commercial cavity ring-down spectroscopy NO2 detector for enhanced sensitivity

Nitrogen dioxide (NO(2)) plays a central role in atmospheric chemistry, air pollution, and biogeochemical cycles. Many analytical techniques have been developed to detect NO(2), but only chemiluminescence-based instruments are commonly, commercially available. There remains a need for a fast, light, and simple method to directly measure NO(2). In this work we describe the modification and characterization of a small, commercially available cavity ring-down spectroscopy (CRDS) NO(2) detector suitable for surface and aircraft monitoring. A metal oxide scrubber was added to remove NO(2), and provide a chemical zero, improving the detection limit (3sigma of the background noise) from several parts per billion by volume (ppbv) to 0.06 ppbv, integrated over 60 s. Known interferences by water and particles were removed using Nafion tubing and a 1 microm Teflon filter, respectively. A 95% response time of 18+/-1 s was observed for a step change in concentration. The CRDS detector was run in parallel to an ozone chemiluminescence device with photolytic conversion of NO(2) to NO. The two instruments measured ambient air in suburban Maryland. A least-squares fit to the comparison data resulted a slope of 0.960+/-0.002 and R of 0.995, showing agreement within experimental uncertainty.