Julia Lee-Taylor

Distributions of brominated organic compounds in the troposphere and lower stratosphere

A comprehensive suite of brominated organic compounds was measured from whole air samples collected during the 1996 NASA Stratospheric Tracers of Atmospheric Transport aircraft campaign and the 1996 NASA Global Tropospheric Experiment Pacific Exploratory Mission-Tropics aircraft campaign. Measurements of individual species and total organic bromine were utilized to describe latitudinal and vertical distributions in the troposphere and lower stratosphere, fractional contributions to total organic bromine by individual species, fractional dissociation of the long-lived species relative to CFC-11, and the Ozone Depletion Potential of the halons and CH3Br. Spatial differences in the various organic brominated compounds were related to their respective sources and chemical lifetimes. The difference between tropospheric mixing ratios in the Northern and Southern Hemispheres for halons was approximately equivalent to their annual tropospheric growth rates, while the interhemispheric ratio of CH3Br was 1.18. The shorter-lived brominated organic species showed larger tropospheric mixing ratios in the tropics relative to midlatitudes, which may reflect marine biogenic sources. Significant vertical gradients in the troposphere were observed for the short-lived species with upper troposphere values 40–70% of the lower troposphere values. Much smaller vertical gradients (3–14%) were observed for CH3Br, and no significant vertical gradients were observed for the halons. Above the tropopause, the decrease in organic bromine compounds was found to have some seasonal and latitudinal differences. The combined losses of the individual compounds resulted in a loss of total organic bromine between the tropopause and 20 km of 38–40% in the tropics and 75–85% in midlatitudes. The fractional dissociation of the halons and CH3Br relative to CFC-11 showed latitudinal differences, with larger values in the tropics.

Number of Nevi and Early-Life Ambient UV Exposure Are Associated with BRAF-Mutant Melanoma

Malignant melanomas often contain BRAF or NRAS mutations, but the relationship of these mutations to ambient UV exposure in combination with phenotypic characteristics is unknown. In a population-based case series from North Carolina, 214 first primary invasive melanoma patients in the year 2000 were interviewed regarding their risk factors. Ambient solar UV exposures were estimated using residential histories and a satellite-based model. Cases were grouped on the basis of BRAF and NRAS somatic mutations, determined using single-strand conformation polymorphism analysis and radiolabeled DNA sequencing, and the risk profiles of these groups were compared. Mutually exclusive BRAF-mutant and NRAS-mutant cases occurred at frequencies of 43.0% and 13.6% with mean ages at diagnosis of 47.3 and 62.1 years, respectively. Tumors from patients with >14 back nevi were more likely to harbor either a BRAF mutation [age-adjusted odds ratio (OR), 3.2; 95% confidence interval (95% CI), 1.4-7.0] or an NRAS mutation (age-adjusted OR, 1.7; 95% CI, 0.6-4.8) compared with patients with 0 to 4 back nevi. However, BRAF-mutant and NRAS-mutant tumors were distinctive in that BRAF-mutant tumors were characteristic of patients with high early-life ambient UV exposure (adjusted OR, 2.6; 95% CI, 1.2-5.3). When ambient UV irradiance was analyzed by decadal age, high exposure at ages 0 to 20 years was associated with BRAF-mutant cases, whereas high exposure at ages 50 and 60 years was characteristic of NRAS-mutant cases. Our results suggest that although nevus propensity is important for the occurrence of both BRAF and NRAS-mutant melanomas, ambient UV irradiance influences risk differently based on the age of exposure. The association of BRAF mutations with early-life UV exposure provides evidence in support of childhood sun protection for melanoma prevention. (Cancer Epidemiol Biomarkers Prev 2007;16(5):991–7)

Global methane emission estimates from ultraviolet irradiation of terrestrial plant foliage

SUMMARY *Several studies have reported in situ methane (CH(4)) emissions from vegetation foliage, but there remains considerable debate about its significance as a global source. Here, we report a study that evaluates the role of ultraviolet (UV) radiation-driven CH(4) emissions from foliar pectin as a global CH(4) source. *We combine a relationship for spectrally weighted CH(4) production from pectin with a global UV irradiation climatology model, satellite-derived leaf area index (LAI) and air temperature data to estimate the potential global CH(4) emissions from vegetation foliage. *Our results suggest that global foliar CH(4) emissions from UV-irradiated pectin could account for 0.2-1.0 Tg yr(-1), of which 60% is from tropical latitudes, corresponding to < 0.2% of total CH(4) sources. *Our estimate is one to two orders of magnitude lower than previous estimates of global foliar CH(4) emissions. Recent studies have reported that pectin is not the only molecular source of UV-driven CH(4) emissions and that other environmental stresses may also generate CH(4). Consequently, further evaluation of such mechanisms of CH(4) generation is needed to confirm the contribution of foliage to the global CH(4) budget.

A preliminary three-dimensional global model study of atmospheric methyl chloride distributions

A global three-dimensional atmospheric model of methyl chloride (CH3Cl) is presented. When incorporating known terrestrial and oceanic source terms, the tropospheric budget of CH3Cl is unbalanced. We show that a reduction in the atmospheric CH3Cl loss rate could account for the net budget discrepancy but fails to reproduce the observed latitudinal distribution. We find that observed mixing ratios and latitudinal distributions can be reproduced by addition of a tropical terrestrial CH3Cl source of 2330–2430 Gg yr−1 combined with a 50% reduction in the southeastern Asian biomass burning contribution. This is equivalent to a net source of 3800–3900 Gg yr−1, slightly higher than previously estimated. The magnitude of additional emissions required to match observations is sensitive to their latitudinal distribution. We successfully simulate tropical land-based observations best when the added source is increased at the coasts relative to inland areas. Mixing ratios at remote sites are relatively insensitive to the finer details of the source parameterization.

Impact of chamber wall loss of gaseous organic compounds on secondary organic aerosol formation: explicit modeling of SOA formation from alkane and alkene oxidation

Abstract. Recent studies have shown that low volatility gas-phase species can be lost onto the smog chamber wall surfaces. Although this loss of organic vapors to walls could be substantial during experiments, its effect on secondary organic aerosol (SOA) formation has not been well characterized and quantified yet. Here the potential impact of chamber walls on the loss of gaseous organic species and SOA formation has been explored using the Generator for Explicit Chemistry and Kinetics of the Organics in the Atmosphere (GECKO-A) modeling tool, which explicitly represents SOA formation and gas–wall partitioning. The model was compared with 41 smog chamber experiments of SOA formation under OH oxidation of alkane and alkene series (linear, cyclic and C12-branched alkanes and terminal, internal and 2-methyl alkenes with 7 to 17 carbon atoms) under high NOx conditions. Simulated trends match observed trends within and between homologous series. The loss of organic vapors to the chamber walls is found to affect SOA yields as well as the composition of the gas and the particle phases. Simulated distributions of the species in various phases suggest that nitrates, hydroxynitrates and carbonylesters could substantially be lost onto walls. The extent of this process depends on the rate of gas–wall mass transfer, the vapor pressure of the species and the duration of the experiments. This work suggests that SOA yields inferred from chamber experiments could be underestimated up a factor of 2 due to the loss of organic vapors to chamber walls.

A global three-dimensional atmosphere-ocean model of methyl bromide distributions

A three-dimensional ocean-atmosphere model of the global methyl bromide budget is presented including an analysis of the temporally and spatially varying source and sink distributions. No correlation is found between oceanic net biological CH3Br production, implied by surface water saturation anomaly observations, and oceanic variables such as surface chlorophyll and primary production; therefore model ocean fluxes are constrained directly by the saturation anomaly observations. The resulting diagnosed biological production rates imply net production in the tropics and subtropics and net consumption at high latitudes. Results from this semicoupled ocean-atmosphere model show substantial longitudinal variability in the atmospheric boundary layer CH3Br concentrations with land-ocean contrasts of 1–6 ppt due to regional industrial and agricultural emissions on land and net fluxes into the ocean. Owing to an imbalance in current understanding of the global budget, our simulated mixing ratios of 3.5 to 6.5 ppt for the southern and northern hemispheres, respectively, are significantly lower than available measurements. Sensitivity studies reducing the ocean and soil surface sinks slightly improve the global mean CH3Br concentration but increase the interhemispheric ratio further beyond that supported by observations. Accordingly, an apparent terrestrial missing source of 89–104 kT yr−l is derived and applied to the model. This is of the same order as the sum of all other sources in the model (85 kT yr−1). The hemispheric distribution of the missing source is explored, indicating that 50–70% of this source occurs in the southern hemisphere and is likely to be biased toward tropical regions. Modeled seasonal variability in the interhemispheric ratio at specific monitoring sites agrees well with observations. The model-predicted vertical gradient of CH3Br through the troposphere and lower stratosphere is also presented.

Volatility dependence of Henry's law constants of condensable organics: Application to estimate depositional loss of secondary organic aerosols

The water solubility of oxidation intermediates of volatile organic compounds that can condense to form secondary organic aerosol (SOA) is largely unconstrained in current chemistry-climate models. We apply the Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere to calculate Henrys law constants for these intermediate species. Results show a strong negative correlation between Henrys law constants and saturation vapor pressures. Details depend on precursor species, extent of photochemical processing, and NOx levels. Henrys law constants as a function of volatility are made available over a wide range of vapor pressures for use in 3-D models. In an application using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) over the U.S. in summer, we find that dry (and wet) deposition of condensable organic vapors leads to major reductions in SOA, decreasing surface concentrations by ~50% (10%) for biogenic and ~40% (6%) for short chain anthropogenic precursors under the considered volatility conditions.

Multidecadal global cooling and unprecedented ozone loss following a regional nuclear conflict

We present the first study of the global impacts of a regional nuclear war with an Earth system model including atmospheric chemistry, ocean dynamics, and interactive sea ice and land components. A limited, regional nuclear war between India and Pakistan in which each side detonates 50 15 kt weapons could produce about 5 Tg of black carbon (BC). This would self-loft to the stratosphere, where it would spread globally, producing a sudden drop in surface temperatures and intense heating of the stratosphere. Using the Community Earth System Model with the Whole Atmosphere Community Climate Model, we calculate an e-folding time of 8.7 years for stratospheric BC compared to 4–6.5 years for previous studies. Our calculations show that global ozone losses of 20%–50% over populated areas, levels unprecedented in human history, would accompany the coldest average surface temperatures in the last 1000 years. We calculate summer enhancements in UV indices of 30%–80% over midlatitudes, suggesting widespread damage to human health, agriculture, and terrestrial and aquatic ecosystems. Killing frosts would reduce growing seasons by 10–40 days per year for 5 years. Surface temperatures would be reduced for more than 25 years due to thermal inertia and albedo effects in the ocean and expanded sea ice. The combined cooling and enhanced UV would put significant pressures on global food supplies and could trigger a global nuclear famine. Knowledge of the impacts of 100 small nuclear weapons should motivate the elimination of more than 17,000 nuclear weapons that exist today.

Associations of Cumulative Sun Exposure and Phenotypic Characteristics with Histologic Solar Elastosis

Background: Solar elastosis adjacent to melanomas in histologic sections is regarded as an indicator of sun exposure, although the associations of UV exposure and phenotype with solar elastosis are yet to be fully explored. Methods: The study included 2,589 incident primary melanoma patients with assessment of histologic solar elastosis in the population-based Genes, Environment, and Melanoma study. Ambient erythemal UV (UVE) at places of residence and sun exposure hours, including body site–specific exposure, were collected. We examined the association of cumulative site-specific and non–site-specific sun exposure hours and ambient UVE with solar elastosis in multivariable models adjusted for age, sex, center, pigmentary characteristics, nevi, and, where relevant, body site. Results: Solar elastosis was associated most strongly with site-specific UVE [odds ratio (OR) for top exposure quartile, 5.20; 95% confidence interval (95% CI), 3.40-7.96; P for trend <0.001] and also with site-specific sun exposure (OR for top quartile, 5.12; 95% CI, 3.35-7.83; P for trend <0.001). Older age (OR at >70 years, 7.69; 95% CI, 5.14-11.52; P for trend < 0.001) and having more than 10 back nevi (OR, 0.77; 95% CI, 0.61-0.97; P = 0.03) were independently associated with solar elastosis. Conclusion: Solar elastosis had a strong association with higher site-specific UVE dose, older age, and fewer nevi. Impact: Solar elastosis could be a useful biomarker of lifetime site-specific UV. Future research is needed to explore whether age represents more than simple accumulation of sun exposure and to determine why people with more nevi may be less prone to solar elastosis. Cancer Epidemiol Biomarkers Prev; 19(11); 2932–41. ©2010 AACR.

Litter decomposition as a potential natural source of methyl bromide

We assessed the potential significance of global litter decay as a new source of atmospheric methyl bromide. We combined information on the global distribution and quantity of litter decay, litter bromine content, and the halogen-methylating ability of wood-rotting fungi to produce a spatially explicit estimate of CH3Br emission from litter decay. The uncertainties are large and the potential methyl bromide source varies greatly in response to assumptions made, including those regarding the efficiency of bromine utilization and release. Our best estimate of the potential flux from woody litter, 0.5–5.2 kT yr−1, is unable to account for the entire “missing source.” Additional possible fluxes from regions of inadequate data and from nonwoody litter may raise this total. This proposed decomposition source is of potential interest to budget calculations and should be experimentally characterized and quantified.