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Nguyen T.B.,California Institute of Technology | Nguyen T.B.,University of California at Davis | Tyndall G.S.,U.S. National Center for Atmospheric Research | Crounse J.D.,California Institute of Technology | And 25 more authors.
Physical Chemistry Chemical Physics | Year: 2016

We use a large laboratory, modeling, and field dataset to investigate the isoprene + O3 reaction, with the goal of better understanding the fates of the C1 and C4 Criegee intermediates in the atmosphere. Although ozonolysis can produce several distinct Criegee intermediates, the C1 stabilized Criegee (CH2OO, 61 ± 9%) is the only one observed to react bimolecularly. We suggest that the C4 Criegees have a low stabilization fraction and propose pathways for their decomposition. Both prompt and non-prompt reactions are important in the production of OH (28% ± 5%) and formaldehyde (81% ± 16%). The yields of unimolecular products (OH, formaldehyde, methacrolein (42 ± 6%) and methyl vinyl ketone (18 ± 6%)) are fairly insensitive to water, i.e., changes in yields in response to water vapor (≤4% absolute) are within the error of the analysis. We propose a comprehensive reaction mechanism that can be incorporated into atmospheric models, which reproduces laboratory data over a wide range of relative humidities. The mechanism proposes that CH2OO + H2O (k(H2O) ∼ 1 × 10-15 cm3 molec-1 s-1) yields 73% hydroxymethyl hydroperoxide (HMHP), 6% formaldehyde + H2O2, and 21% formic acid + H2O; and CH2OO + (H2O)2 (k(H2O)2 ∼ 1 × 10-12 cm3 molec-1 s-1) yields 40% HMHP, 6% formaldehyde + H2O2, and 54% formic acid + H2O. Competitive rate determinations (kSO2/k(H2O)n=1,2 ∼ 2.2 (±0.3) × 104) and field observations suggest that water vapor is a sink for greater than 98% of CH2OO in a Southeastern US forest, even during pollution episodes ([SO2] ∼ 10 ppb). The importance of the CH2OO + (H2O)n reaction is demonstrated by high HMHP mixing ratios observed over the forest canopy. We find that CH2OO does not substantially affect the lifetime of SO2 or HCOOH in the Southeast US, e.g., CH2OO + SO2 reaction is a minor contribution (<6%) to sulfate formation. Extrapolating, these results imply that sulfate production by stabilized Criegees is likely unimportant in regions dominated by the reactivity of ozone with isoprene. In contrast, hydroperoxide, organic acid, and formaldehyde formation from isoprene ozonolysis in those areas may be significant. © the Owner Societies 2016.

Bernard F.,Earth Systems Research Laboratory | Bernard F.,University of Colorado at Boulder | McGillen M.R.,Earth Systems Research Laboratory | McGillen M.R.,University of Colorado at Boulder | And 4 more authors.
Journal of Photochemistry and Photobiology A: Chemistry | Year: 2015

CBrF3 (Halon-1301) is a man-made ozone depleting substance that is a major source of bromine in the Earth's stratosphere. Halon-1301 is predominantly removed from the atmosphere by UV photolysis in the stratosphere at wavelengths between 200 and 225 nm. The existing level of uncertainty in the Halon-1301 UV absorption spectrum temperature-dependence directly impacts the ability to model stratospheric ozone chemistry and climate change. In this work, the UV absorption spectrum of Halon-1301 between 195 and 235 nm was measured over the temperature range 210-320 K. An empirical parameterization of the spectrum and its temperature dependence is presented. The present results are critically compared with results from previous studies and the current recommendation for use in atmospheric models. A global annually averaged lifetime for Halon-1301 of 74.6 (73.7-75.5) years was calculated using a 2-D atmospheric model and the present results. The range of lifetimes given in parenthesis represents the possible values due solely to the 2σ uncertainty in the Halon-1301 UV spectrum obtained in this work. In addition, the CBrF3 ozone depletion potential was calculated using the 2-D model to be 18.6 (±0.1) using the UV spectrum and 2σ uncertainty from this work.

Desai A.R.,University of Wisconsin - Madison | Xu K.,University of Wisconsin - Madison | Tian H.,Auburn University | Weishampel P.,National Ecological Observatory Network Inc. | And 6 more authors.
Agricultural and Forest Meteorology | Year: 2015

Simulating the magnitude and variability of terrestrial methane sources and sinks poses a challenge to ecosystem models because the biophysical and biogeochemical processes that lead to methane emissions from terrestrial and freshwater ecosystems are, by their nature, episodic and spatially disjunct. As a consequence, model predictions of regional methane emissions based on field campaigns from short eddy covariance towers or static chambers have large uncertainties, because measurements focused on a particular known source of methane emission will be biased compared to regional estimates with regards to magnitude, spatial scale, or frequency of these emissions. Given the relatively large importance of predicting future terrestrial methane fluxes for constraining future atmospheric methane growth rates, a clear need exists to reduce spatiotemporal uncertainties. In 2010, an Ameriflux tower (US-PFa) near Park Falls, WI, USA, was instrumented with closed-path methane flux measurements at 122m above ground in a mixed wetland-upland landscape representative of the Great Lakes region. Two years of flux observations revealed an average annual methane (CH4) efflux of 785±75mgCCH4m-2yr-1, compared to a mean CO2 sink of -80gCCO2m-2yr-1, a ratio of 1% in magnitude on a mole basis. Interannual variability in methane flux was 30% of the mean flux and driven by suppression of methane emissions during dry conditions in late summer 2012. Though relatively small, the magnitude of the methane source from the very tall tower measurements was mostly within the range previously measured using static chambers at nearby wetlands, but larger than a simple scaling of those fluxes to the tower footprint. Seasonal patterns in methane fluxes were similar to those simulated in the Dynamic Land Ecosystem Model (DLEM), but magnitude depends on model parameterization and input data, especially regarding wetland extent. The model was unable to simulate short-term (sub-weekly) variability. Temperature was found to be a stronger driver of regional CH4 flux than moisture availability or net ecosystem production at the daily to monthly scale. Taken together, these results emphasize the multi-timescale dependence of drivers of regional methane flux and the importance of long, continuous time series for their characterization. © 2014 Elsevier B.V.

Ten Hoeve J.E.,National Weather Service - NWS | Augustine J.A.,Earth Systems Research Laboratory
Geophysical Research Letters | Year: 2016

Previous studies of the second aerosol indirect (lifetime) effect on cloud cover have estimated the strength of the effect without correcting for near-cloud contamination and other confounding factors. Here we combine satellite-based observations with a multiyear ground-based data set across five rural locations in the United States to more accurately constrain the second indirect aerosol effect and quantify aerosol effects on radiative forcing. Results show that near-cloud contamination accounts for approximately 40% of the satellite-derived aerosol-cloud relationship. When contamination is removed and the effect of meteorological covariation is minimized, a strong physical aerosol effect on cloud cover remains. Averaged over all stations and after correcting for contamination, the daytime solar and total (solar + IR) radiative forcing is -52 W/m2 and -19 W/m2, respectively, due to both direct and indirect aerosol effects for aerosol optical depths (τ) between 0 and 0.3. Averaged diurnally, the average total radiative forcing is +16 W/m2. © 2016. American Geophysical Union. All Rights Reserved.

A new study from NOAA shows that, by building new high-tech transmission lines, the US could slash energy sector global warming emissions by 80 percent within 15 years, while keeping consumer costs low and meeting increased demand. Alexander MacDonald, a co-author of the study and the recently retired director of NOAA's Earth Systems Research Laboratory in Boulder, Colorado, says studying the national weather map gave him the idea.

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