Douglas D. Davis

Pacific Exploratory Mission in the Tropical Pacific: PEM-Tropics B, March-April 1999

The Pacific Exploratory Mission - Tropics B (PEM-Tropics B) was conducted by the NASA Global Tropospheric Experiment (GTE) over the tropical Pacific Ocean in March-April 1999. It used the NASA DC-8 and P-3B aircraft equipped with extensive instrumentation for measuring numerous chemical compounds and gases. Its central objective was to improve knowledge of the factors controlling ozone, OH, aerosols, and related species over the tropical Pacific. Geographical coverage ranged from 38°N to 36°S and 148°W to 76°E. Major deployment sites included Hilo, Hawaii, Christmas Island, Tahiti, Fiji, and Easter Island. PEM-Tropics B was a sequel to PEM-Tropics A, which was conducted in September-October 1996 and encountered considerable biomass burning. PEM-Tropics B, conducted in the wet season of the southern tropics, observed an exceedingly clean atmosphere over the South Pacific but a variety of pollution influences over the tropical North Pacific. Photochemical ozone loss over both the North and the South Pacific exceeded local photochemical production by about a factor of 2, implying a major deficit in the tropospheric ozone budget. Dedicated flights investigated the sharp air mass transitions at the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). Extensive OH observations permitted the first large-scale comparisons with photochemical model predictions. High concentrations of oxygenated organics were observed ubiquitously in the tropical Pacific atmosphere and may have important implications for global HOx and NOx budgets. Extensive equatorial measurements of dimethyl sulfide and OH suggest that important aspects of marine sulfur chemistry are still poorly understood.

Springtime photochemistry at northern mid and high latitudes

which increases rapidly during spring. Unlike in other tropospheric experiments, observed H2O2 concentrations are a factor of 2–10 lower than those simulated by the model. The required scavenging timescale to reconcile the model overestimates shows a rapid seasonal decrease down to 0.5–1 day in May, which cannot be explained by known mechanisms. This loss of H2O2 implies a large loss of HOx resulting in decreases in O3 production (10–20%) and OH concentrations (20–30%). Photolysis of CH2O, either transported into the region or produced by unknown chemical pathways, appears to provide a significant HOx source at 6–8 km at high latitudes. The rapid increase of in situ O3 production in spring is fueled by concurrent increases of the primary HOx production and NO concentrations. Long-lived reactive nitrogen species continue to accumulate at mid and high latitudes in spring. There is a net loss of NOx to HNO3 and PAN throughout the spring, suggesting that these long-term NOx reservoirs do not provide a net source for NOx in the region. In situ O3 chemical loss is dominated by the reaction of O3 and HO2, and not that of O( 1 D) and H2O. At midlatitudes, there is net in situ chemical production of O3 from February to May. The lower free troposphere (1–4 km) is a region of significant net O3 production. The net production peaks in April coinciding with the observed peak of column O3 (0–8 km). The net in situ O3 production at midlatitudes can explain much of the observed column O3 increase, although it alone cannot explain the observed April maximum. In contrast, there is a net in situ O3 loss from February to April at high latitudes. Only in May is the in situ O3 production larger than loss. The observed continuous increase of column O3 at high latitudes throughout the spring is due to transport from other tropospheric regions or the stratosphere not in situ photochemistry. INDEX TERMS: 0317 Atmospheric Composition and Structure: Chemical kinetic and photochemical properties; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; KEYWORDS: springtime, ozone, HOx, oxidation, reactive nitrogen

Photofragmentation-laser induced fluorescence: a new method for detecting atmospheric trace gases

A new method for the in situ detection of nonfluorescing molecular species is proposed: photofragmentation-laser induced fluorescence (PF-LIF). In this approach, the species to be detected is first laser photolyzed at a wavelength lambda(1), producing one or more vibrationally excited photofragments. Before vibrational relaxation occurs, one of these photofragments is pumped into a bonding excited state by a second laser pulse centered at wavelength lambda (2). Fluorescence is sampled at a wavelength lambda (3), where lambda (3) < lambda(2) and lambda(1) This pumping configuration thus permits massive discrimination against Rayleigh and Raman scattering as well as white noise fluorescence from the laser wavelengths lambda(1) and lambda(2). The technique should be both highly sensitive and selective for numerous atmospheric trace gases. Specific sampling schemes for detecting NO(2), NO(3), and HNO(2) are proposed. Various noise sources and chemical interferences are discussed. Specific calculations that estimate the sensitivity of the PF-LIF system for detecting NO(2), NO(3), and HNO(2) are given.

Non‐sea salt sulfate and other aerosol constituents at the South Pole during ISCAT

Bulk aerosol samples collected at the South Pole were analyzed for Na, SO42−, Cl−, NO3−, methanesulfonate (MSA), 210Pb and 210Po. Sea salt concentrations were relatively high compared with previous reports, with Na, averaging 45 ng m−3 Chloride was depleted by ∼65% compared with its ratio to Na in seawater. Non-sea salt sulfate (NSS) averaged 212 ng m−3, and less than 5% of the sulfate was from sea salt. The mean concentration of MSA (an indicator of marine biogenic sulfur) was 12 ng m−3, and the MSA/NSS mass ratio (0.059) was similar to that in Antarctic ice. Although MSA and NSS were correlated, the data set is not sufficiently robust to draw a quantitative conclusion concerning the fraction of NSS that is biogenic. Data for 210Po and 210Pb showed no evidence for strong volcanic influences on sulfate, and NSS also was not correlated with NO3−, a possible tracer of continental emissions.

A study of DMS oxidation in the tropics : Comparison of Christmas Island field observations of DMS, SO2, and DMSO with model simulations

This study reports comparisonsbetween model simulations, based on current sulfurmechanisms, with the DMS, SO2 and DMSOobservational data reported by Bandy et al.(1996) in their 1994 Christmas Island field study. For both DMS and SO2, the model results werefound to be in excellent agreement with theobservations when the observations were filtered so asto establish a common meteorological environment. Thisfiltered DMS and SO2 data encompassedapproximately half of the total sampled days. Basedon these composite profiles, it was shown thatoxidation of DMS via OH was the dominant pathway withno more than 5 to 15% proceeding through Cl atoms andless than 3% through NO3. This analysis wasbased on an estimated DMS sea-to-air flux of 3.4 ×109 molecs cm-2 s-1. The dominant sourceof BL SO2 was oxidation of DMS, the overallconversion efficiency being evaluated at 0.65 ± 0.15. The major loss of SO2 was deposition to theoceans surface and scavenging by aerosol. Theresulting combined first order k value was estimated at 1.6 × 10-5 s-1. In contrast to the DMSand SO2 simulations, the model under-predictedthe observed DMSO levels by nearly a factor of 50. Although DMSO instrument measurement problems can notbe totally ruled out, the possibility of DMSO sourcesother than gas phase oxidation of DMS must beseriously considered and should be explored in futurestudies.

Operational Overview of the NASA GTE/CITE 3 Airborne Instrument Intercomparisons for Sulfur Dioxide, Hydrogen Sulfide, Carbonyl Sulfide, Dimethyl Sulfide, and Carbon Disulfide

This paper reports the overall experimental design and gives a brief overview of results from the third airborne Chemical Instrumentation Test and Evaluation (CITE 3) mission conducted as part of the National Aeronautics and Space Administrations Global Tropospheric Experiment. The primary objective of CITE 3 was to evaluate the capability of instrumentation for airborne measurements of ambient concentrations of SO2, H2S, CS2, dimethyl sulfide, and carbonyl sulfide. Ancillary measurements augmented the intercomparison data in order to address the secondary objective of CITE 3 which was to address specific issues related to the budget and photochemistry of tropospheric sulfur species. The CITE 3 mission was conducted on NASAs Wallops Flight Center Electra aircraft and included a ground-based intercomparison of sulfur standards and intercomparison/sulfur science flights conducted from the NASA Wallops Flight Facility, Wallops Island, Virginia, followed by flights from Natal, Brazil. Including the transit flights, CITE 3 included 16 flights encompassing approximately 96 flight hours.

Real-time visualization of scalably large collections of heterogeneous objects

This paper presents results for real-time visualization of out-of-core collections of 3D objects. This is a significant extension of previous methods and shows the generality of hierarchical paging procedures applied both to global terrain and any objects that reside on it. Applied to buildings, the procedure shows the effectiveness of using a screen-based paging and display criterion within a hierarchical framework. The results demonstrate that the method is scalable since it is able to handle multiple collections of buildings (e.g., cities) placed around the earth with full interactivity and without extensive memory load. Further the method shows efficient handling of culling and is applicable to larger, extended collections of buildings. Finally, the method shows that levels of detail can be incorporated to provide improved detail management.

Single photon laser-induced fluorescence detection of NO and SO 2 for atmospheric conditions of composition and pressure

Reported here are laboratory results from a laser-induced fluorescence (LIF) study of the molecules NO and SO(2) in which both the selectivity and sensitivity of the LIF method are examined. The laser excitation of these molecules occurred at 226 and 222 nm, respectively. The laser system employed consisted of a Nd: YAG-driven Quanta-Ray PDL dye laser, the fundamental of which was frequency doubled, and this output, in turn, was then frequency mixed with the Nd:YAG fundamental at 1064 nm. Two different dyes were required for generating the 226- and 222-nm wavelengths. To make these results as relevant as possible to the ultimate development of an atmospheric airborne field sampling system all experiments were carried out in atmospheric conditions of pressure and composition. In addition to the experimental data provided there has also been presented a theoretical assessment of the signal strength for both the NO and SO(2) LIF systems, and these results have been compared with the experimentally measured values. Current state-of-the-art technology would suggest that both NO and SO(2) can be measured by the LIF technique in atmospheric conditions at concentration levels of a few pptv.

A temperature dependence kinetics study of the reactions of Cl (2P3/2) with O3, CH4, and H2O2

The technique of flash photolysis–resonance fluorescence has been utilized to study the temperature dependences of two chlorine atom reactions of considerable fundamental importance to stratospheric chemistry. These reactions have been studied using a wide range of experimental conditions to ensure the absence of complicating secondary processes. The reactions of interest with their corresponding rate constants are expressed in units of cm3 molecule−1  s−1: Cl+O3→ClO O2 (k1), ΔU°298=−164 kJ mol−1, k1 = (3.08±0.30) ×10−11 exp[−(576±60/RT)], (220–350) K; Cl+CH4→CH3+HCl (k2), ΔU°298=−164 kJ mol−1, k2 = (7.44±0.75) ×10−12 exp[−(2437±110/RT)], (218–401) K. In addition, the followong reaction was studied at 300 K: Cl+H2O2→HCl+HO2 (K3), ΔU°298=−56.8 kJ mol−1, k3?5.8×10−13 (±factor 2.0), 300 K. A direct implication of the new rate data is the need to revise downward by a factor of 2.4 to 3 the magnitude of the ozone perturbation predicted by earlier model calculations due to the presence of ClOx species in the st...

A quantum yield determination of O/1D/ production from ozone via laser flash photolysis

The quantum yield for O(1D) production from ozone photolysis has been measured at 298 K from 293.0 to 316.5 nm. The O(1D) was monitored by its reaction with N2O to form excited NO2*. The photolysis source was a frequency doubled flashlamp pumped dye laser which provided tunable uv in the desired spectral region with 0.1 nm linewidth. The results show φ to be constant below 300 nm, taken to be unity, with a sharp decrease centered at 308 nm and a value of less than 0.1 above 313.5 nm.