J. G. Murphy

Publications (10)5.89 Total impact

• Source

  Available from: J. M. Roberts

  Article: Total Peroxy Nitrates (ΣPNs) in the atmosphere: the Thermal Dissociation-Laser Induced Fluorescence (TD-LIF) technique and comparisons to speciated PAN measurements

  P. J. Wooldridge, A. E. Perring, T. H. Bertram, F. M. Flocke, J. M. Roberts, H. B. Singh, L. G. Huey, J. A. Thornton, G. M. Wolfe, J. G. Murphy, J. L. Fry, A. W. Rollins, B. W. LaFranchi, R. C. Cohen
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  ABSTRACT: Peroxyacetyl nitrate (PAN) and its chemical analogues are increasingly being quantified in the ambient atmosphere by thermal dissociation (TD) followed by detection of either the peroxyacyl radical or the NO₂ product. Here we present details of the technique developed at University of California, Berkeley which detects the sum of all peroxynitrates (ΣPNs) via laser-induced fluorescence (LIF) of the NO₂ product. We review the various deployments and compare the Berkeley ΣPNs measurements with the sums of PAN and its homologue species detected individually by other instruments. The observed TD-LIF ΣPNs usually agree to within 10% with the summed individual species, thus arguing against the presence of significant concentrations of unmeasured PAN-type compounds in the atmosphere, as suggested by some photochemical mechanisms. Examples of poorer agreement are attributed to a sampling inlet design that is shown to be inappropriate for high NO_x conditions. Interferences to the TD-LIF measurements are described along with strategies to minimize their effects.
  Atmospheric Measurement Techniques. 01/2010;
• Article: Total peroxy nitrates (ΣPNs) in the atmosphere: the thermal dissociation-laser induced fluorescence (TD-LIF) technique and comparisons to speciated PAN measurements

  P. J. Wooldridge, A. E. Perring, T. H. Bertram, F. M. Flocke, J. M. Roberts, H. B. Singh, L. G. Huey, J. A. Thornton, J. G. Murphy, J. L. Fry, A. W. Rollins, B. W. LaFranchi, R. C. Cohen
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  ABSTRACT: Peroxyacetyl nitrate (PAN) and its chemical analogues are increasingly being quantified in the ambient atmosphere by thermal dissociation (TD) followed by detection of either the peroxyacyl radical or the NO₂ product. Here we present details of the technique developed at University of California, Berkeley which detects the sum of all peroxynitrates (ΣPNs) via laser-induced fluorescence (LIF) of the NO₂ product. We review the various deployments and compare the Berkeley ΣPNs measurements with the sums of PAN and its homologue species detected individually by other instruments. The observed TD-LIF ΣPNs usually agree to within 10% with the summed individual species, thus arguing against the presence of significant concentrations of unmeasured PAN-type compounds in the atmosphere, as suggested by some photochemical mechanisms. Examples of poorer agreement are attributed to a sampling inlet design that is shown to be inappropriate for high NO_x conditions. Interferences to the TD-LIF measurements are described along with strategies to minimize their effects.
  Atmospheric Measurement Techniques Discussions. 01/2009;
• Article: Day-of-week Effects in Ozone, Nitrogen Oxides, and VOC Reactivity Within and Downwind of Sacramento

  J. G. Murphy, D. A. Day, P. A. Cleary, P. J. Wooldridge, D. B. Millet, A. H. Goldstein, R. C. Cohen
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  ABSTRACT: Day-of-week patterns in human activities can be used to examine the ways in which differences in primary emissions result in changes in the rates of photochemical reactions, and the production of secondary pollutants. Data from twelve California Air Resources Board monitoring sites in Sacramento, CA, and the downwind Mountain Counties air basin are analyzed to reveal day of week patterns in ozone and its precursors in the summers of 1998-2002. On the weekends, NOx concentrations drop by 35%, whereas VOC reactivity changes by less than 15%, likely due to significant reductions in diesel truck traffic. The main driver for day-of- week differences in ozone abundance at urban sites is the reduced titration of O3 by fresh NO emissions on weekends. By analyzing day-of-week patterns in odd oxygen (O3+NO2), we can eliminate this complication and focus on the NOx-dependence of ozone production and the related photochemistry. In the downwind Mountain Counties, ozone production is found to be NOx-limited, whereas at the urban and suburban sites of Sacramento, ozone production is NOx-saturated. An analysis of day-of-week cycles in Sacramento reveals that boundary layer chemistry and mixing in of residual air from above both contribute to the observed accumulation of ozone at surface sites. This interpretation is consistent with the day-of-week patterns in a range of VOC with different emission sources and lifetimes and in higher nitrogen oxides that are co-products of ozone. We use isoprene as a probe of local oxidation rates, and infer that on the weekend OH is higher in the urban area and lower at one of the forested Mountain Counties sites, as expected from the NOx- dependence. Our analysis also demonstrates that in NOx-saturated environments, gas phase HNO3, and by extension particulate nitrate, concentrations are relatively insensitive to changes in NOx emissions.
  AGU Fall Meeting Abstracts. 11/2007; -1:03.
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  Available from: themurphygroup.ca

  Article: Kinetics of NO and NO2 evolution from illuminated frozen nitrate solutions.

  C S Boxe, A J Colussi, M R Hoffmann, I M Perez, J G Murphy, R C Cohen
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  ABSTRACT: The release of NO and NO2 from frozen aqueous NaNO3 irradiated at 313 nm was studied using time-resolved spectroscopic techniques. The kinetic behavior of NO and NO2 signals during on-and-off illumination cycles confirms that NO2 is a primary photoproduct evolving from the outermost ice layers and reveals that NO is a secondary species generated deeper in the ice, whence it eventually emerges due to its inertness and larger diffusivity. NO is shown to be more weakly held than NO2 by ice in thermal desorption experiments on preirradiated samples. The partial control of gaseous emissions by mass transfer, and hence by the morphology and metamorphisms of polycrystalline ice, is established by (1) the nonmonotonic temperature dependence of NO and NO2 signals upon stepwise warming under continuous illumination, (2) the fact that the NO, NO2 or NOx (NOx identical with NO + NO2) amounts released in bright thermograms performed under various heating ramps fail to scale with photon dose, due to irreversible losses in the adsorbed state. Because present NO/NO2 ratios are up to 10-fold smaller than those determined over sunlit snowpacks, we infer that the immediate precursors to NO mostly absorb at lambda > lambda(max) (NO3-) approximately 302 nm.
  The Journal of Physical Chemistry A 04/2006; 110(10):3578-83. · 2.95 Impact Factor
• Source

  Available from: Paul J. Wooldridge

  Article: Observations of the diurnal and seasonal trends in nitrogen oxides in the western Sierra Nevada

  J. G. Murphy, D. A. Day, P. A. Cleary, P. J. Wooldridge, R. C. Cohen
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  ABSTRACT: Observations of speciated nitrogen oxides, namely NO₂, total peroxy nitrates (ΣPNs), total alkyl nitrates (ΣANs), and HNO₃ by thermal dissociation laser induced fluorescence (TD-LIF), and supporting chemical and meteorological measurements at Big Hill (1860 m), a high elevation site in California's Sierra Nevada Mountains, are described. From May through October, terrain-driven winds in the region routinely bring air from Sacramento, 100 km southwest of the site, upslope over oak and pine forests to Big Hill during the day, while at night, the site often samples clean, dry air characteristic of the free troposphere. Winter differs mainly in that the meteorology does not favour the buildup of Sacramento's pollution over the Sierra Nevada range, and the urban-influenced air that is seen has been less affected by biogenic VOC emissions, resulting in longer lifetime for NO₂ and a predominance of the inorganic forms of nitrogen oxides. Summertime observations at Big Hill can be compared with those from Granite Bay, a Sacramento suburb, and from the University of California's Blodgett Forest Research Station to examine the evolution of nitrogen oxides and ozone within the urban plume. Nitrogen oxide radicals (NO and NO₂), which dominate total nitrogen oxides (NO_y) at Granite Bay, are rapidly converted into HNO₃, ΣPNs, and ΣANs, such that these compounds contribute 29, 30, and 21% respectively to the NO_y budget in the plume at Big Hill. Nevertheless, the decreasing concentrations of NO₂ as the plume is advected to Big Hill lead to decreases in the production rate of HNO₃ and ozone. The data also demonstrate the role that temperature plays in sequestering NO₂ into peroxy nitrates, effectively decreasing the rate of ozone production. The important contribution of ΣANs to NO_y in the region suggests that they should be considered with regards to export of NO_y from the boundary layer. Nocturnal observations of airmasses characteristic of the free troposphere showed lower NO_y concentrations, which were dominated by HNO₃ with a relatively small contribution from the organic nitrates.
  Atmospheric Chemistry and Physics. 01/2006;
• Source

  Available from: Paul J. Wooldridge

  Article: The weekend effect within and downwind of Sacramento: Part 2. Observational evidence for chemical and dynamical contributions

  J. G. Murphy, D. A. Day, P. A. Cleary, P. J. Wooldridge, D. B. Millet, A. H. Goldstein, R. C. Cohen
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  ABSTRACT: Observations of day-of-week patterns and diurnal profiles of ozone, volatile organic compounds and nitrogen oxides are examined to assess the chemical and dynamical factors governing the daytime ozone accumulation and the distribution of chemically related species in Central California. Isoprene observations show that urban OH concentrations are higher on the weekend whereas rural OH concentrations are lower on the weekend, confirming that NOx concentrations have a direct effect on the rate of photochemical ozone production and that the transition from NOx-saturated (VOC-limited) to NOx-limited chemistry occurs between the city and the downwind rural counties. We quantify the extent to which mixing of ozone and its precursors from aloft contributes to the daytime accumulation of ozone at the surface in Sacramento. Ozone production in the rural Mountain Counties is currently NOx-limited and will decrease in response to NOx emission reductions in the Sacramento Valley. However, NOx emissions reductions of at least 50% (from weekday levels) are necessary to bring about a significant decrease in accumulation of ozone at the surface in the Sacramento Valley. The impact of NOx emission reductions on the frequency of exceeding the federal 8-hour ozone standard at an individual site will depend on the balance between reduced titration and the sign and magnitude of production changes. We further show that HNO3 production, which depends on the product of OH and NO2 mixing ratios, is a constant at high NOx, suggesting that NOx must be reduced below a threshold before nitrate aerosol can be expected to decrease.
  Atmospheric Chemistry and Physics Discussions. 01/2006;
• Source

  Available from: Paul J. Wooldridge

  Article: The weekend effect within and downwind of Sacramento: Part 1. Observations of ozone, nitrogen oxides, and VOC reactivity

  J. G. Murphy, D. A. Day, P. A. Cleary, P. J. Wooldridge, D. B. Millet, A. H. Goldstein, R. C. Cohen
  [show abstract] [hide abstract]
  ABSTRACT: Day-of-week patterns in human activities can be used to examine the ways in which differences in primary emissions result in changes in the rates of photochemical reactions, and the production of secondary pollutants. Data from twelve California Air Resources Board monitoring sites in Sacramento, CA, and the downwind Mountain Counties air basin are analyzed to reveal day of week patterns in ozone and its precursors in the summers of 1998–2002. This routine monitoring data is complemented by comprehensive data sets of ozone and nitrogen oxide concentrations and VOC reactivity obtained in the summers of 2001 and 2003 at three sites in the region. Daytime concentrations of nitrogen oxides (NOx) are approximately 35% lower on weekends at all the sites, whereas the VOC reactivity changes by less than 10%. All six sites in the Sacramento Valley have higher 8-h maximum average ozone on the weekend and are more likely to exceed the national standard of 85 ppb on the weekend. In contrast, all the sites in the Mountain Counties are less likely to exceed the federal ozone standard on the weekend. Analysis of the day-of-week trends in odd oxygen show that the weekend effect of ozone within Sacramento is strongly influenced by NO sources close to the monitoring sites. This suggests that ozone measurements from monitoring sites close to highways, including two rural locations, may not be representative of the regional abundance, and lead to underestimates of long term exposure for humans and ecosystems.
  Atmospheric Chemistry and Physics Discussions. 01/2006;
• Source

  Available from: themurphygroup.ca

  Article: Photochemical production and release of gaseous NO2 from nitrate-doped water ice.

  C S Boxe, A J Colussi, M R Hoffmann, J G Murphy, P J Wooldridge, T H Bertram, R C Cohen
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  ABSTRACT: Temperature-programmed NO2 emissions from frozen aqueous NaNO3 solutions irradiated at 313 nm were monitored as function of nitrate concentration and heating rate, H, above -30 degrees C. Emissions increase nonmonotonically with temperature, displaying transitions suggestive of underlying metamorphic transformations. Thus, NO2 emissions surge at ca. -8 degrees C in frozen [NO3-] > 200 microM samples warmed at H = 0.70 degrees C min(-1) under continuous irradiation, and also in the dark from samples that had been photolyzed at -30 degrees C. The amounts of NO2 released in individual thermograms, SigmaN, increase less than linearly with [NO3-] or the duration of experiments, revealing the significant loss of photogenerated NO2. The actual SigmaN proportional, variant [NO3-]1/2 dependence (at constant H) is consistent with NO2 hydrolysis: 2NO2 + H2O --> NO3- + NO2- + 2H+, overtaking NO2 desorption, even below the eutectic point (-18 degrees C for aqueous NaNO3). The increasingly larger NO2 losses detected in longer experiments (at constant [NO3-]) are ascribed to secondary photolysis of trapped NO2. The relevance of present results to the interpretation of polar NO2 measurements is briefly analyzed.
  The Journal of Physical Chemistry A 09/2005; 109(38):8520-5. · 2.95 Impact Factor
• Article: Measurements of the sum of HO₂NO₂ and CH₃O₂NO₂ in the remote troposphere

  J. G. Murphy, J. A. Thornton, P. J. Wooldridge, D. A. Day, R. S. Rosen, Cantrell C, R. E. Shetter, Lefer B, R. C. Cohen
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  ABSTRACT: The chemistry of peroxynitric acid (HO₂NO₂) and methyl peroxynitrate (CH₃O₂NO₂)is predicted to be particularly important in the upper troposphere where temperatures are frequently low enough that these compounds do not rapidly decompose. At temperatures below 240K, we calculate that about 20% of NO_y in the mid- and high-latitude upper troposphere is HO₂NO₂. Under these conditions, the reaction of OH with HO₂NO₂ is estimated to account for as much as one third of the permanent loss of hydrogen radicals. During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign, we used thermal dissociation laser-induced fluorescence (TD-LIF) to measure the sum of peroxynitrates ( PNs HO₂NO₂+CH₃O₂NO₂+PAN+PPN+...) aboard the NCAR C-130 research aircraft. We infer the sum of HO₂NO₂ and CH₃O₂NO₂ as the difference between PN measurements and gas chromatographic measurements of the two major peroxy acyl nitrates, peroxy acetyl nitrate (PAN) and peroxy propionyl nitrate (PPN). Comparison with NO_y and other nitrogen oxide measurements confirms the importance of HO₂NO₂ and CH₃O₂NO₂ to the reactive nitrogen budget and shows that current thinking about the chemistry of these species is approximately correct. During the spring high latitude conditions sampled during the TOPSE experiment, the model predictions of the contribution of (HO₂NO₂+CH₃O₂NO₂) to NO_y are highly temperature dependent: on average 30% of NO_y at 230K, 15% of NO_y at 240K, and 5% of NO_y above 250K. The temperature dependence of the inferred concentrations corroborates the contribution of overtone photolysis to the photochemistry of peroxynitric acid. A model that includes IR photolysis (J=1x10^-5s^-1) agreed with the observed sum of HO₂NO₂+CH₃O₂NO₂ to better than 35% below 240K where the concentration of these species is largest.
  Atmospheric Chemistry and Physics. 01/2004;
• Source

  Available from: Paul J. Wooldridge

  Article: Observations of total alkyl nitrates within the Sacramento Urban Plume

  P. A. Cleary, J. G. Murphy, P. J. Wooldridge, D. A. Day, D. B. Millet, M. Mckay, A. H. Goldstein, R. C. Cohen
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  ABSTRACT: During the summer of 2001, NO₂, total peroxy nitrates (?PNs), total alkyl nitrates (?ANs), HNO₃, volatile organic compounds (VOC), CO₂, O₃, and meteorological variables were measured at Granite Bay, CA. The diurnal variation in ?PNs, ?ANs and HNO₃ were all strongly correlated with sunlight, indicating both that they are photochemically produced and that they have a lifetime of a few hours at this site. The mixing ratios of ?ANs ranged as high as 2 ppbv. Mixing ratios at night averaged 0.4 ppbv. Odd-oxygen (O_x=O₃+NO₂) and ?ANs were strongly correlated reflecting both the common chemical source terms and the similar lifetimes of both species. Several approaches to interpreting the simultaneous variations of O_x and ?ANs are described, and used to derive a best estimate of the ?AN yield from the VOC mixture at this site of 4.2% and an estimate of the range that is consistent with the observations of 3.9?5.8%. A yield of 4.2% implies termination of the HO_x catalytic cycle by ?AN formation once every 24 cycles. Analysis of the HNO₃ observations in combination with the ?AN and O₃ measurements suggests that NO_x terminations limit the HO_x chain length to between 4.7 and 6.3.