D. M. Brookes

University of Leicester, Leicester, ENG, United Kingdom

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Publications (12)33.13 Total impact

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    ATMOSPHERIC CHEMISTRY AND PHYSICS 08/2011; 11(16):8825-8826. · 5.51 Impact Factor
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    ABSTRACT: Forests are the dominant source of volatile organic compounds into the atmosphere, with isoprene being the most significant species. The oxidation chemistry of these compounds is a significant driver of local, regional and global atmospheric composition. Observations made over Borneo during the OP3 project in 2008, together with an observationally constrained box model are used to assess our understanding of this oxidation chemistry. In line with previous work in tropical forests, we find that the standard model based on MCM chemistry significantly underestimates the observed OH concentrations. Geometric mean observed to modelled ratios of OH and HO2 in airmasses impacted with isoprene are 5.32-4.43+3.68 and 1.18-0.30+0.30 respectively, with 68 % of the observations being within the specified variation. We implement a variety of mechanistic changes into the model, including epoxide formation and unimolecular decomposition of isoprene peroxy radicals, and assess their impact on the model success. We conclude that none of the current suggestions can simultaneously remove the bias from both OH and HO2 simulations and believe that detailed laboratory studies are now needed to resolve this issue.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 07/2011; 11(13):6749-6771. · 5.51 Impact Factor
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    ABSTRACT: During the Texas Air Quality Study II (Tex-AQS 2006) campaign, a PEroxy Radical Chemical Ampli-fier (PERCA) was deployed on the NOAA research vessel R/V Brown to measure total peroxy radicals (HO 2 + RO 2). Day-time mixing ratios of HO 2 + RO 2 between 25 and 110 ppt were observed throughout the study area – the Hous-ton/Galveston region and the Gulf coast of the US – and ana-lyzed in relation to measurements of nitrogen oxides, volatile organic compounds (VOC) and photolysis rates to assess rad-ical sources and sinks in the region. The measurements of HO 2 + RO 2 were used to calcu-late the in-situ net photochemical formation of ozone. Mea-sured median values ranged from 0.6 ppb/h in clean oceanic air masses up to several tens of ppb/h in the most polluted in-dustrial areas. The results are consistent with previous stud-ies and generally agree with observations made during the previous TexAQS 2000 field campaign. The net photochem-ical ozone formation rates determined at Barbours Cut, a site immediately south of the Houston Ship Channel, were ana-lyzed in relation to local wind direction and VOC reactivity Correspondence to: R. Sommariva (r.sommariva@uea.ac.uk) to understand the relationship between ozone formation and local VOC emissions. The measurements of HO 2 + RO 2 made during the R/V Brown TexAQS 2006 cruise indicate that ozone forma-tion is NO x -limited in the Houston/Galveston region and in-fluenced by highly reactive hydrocarbons, especially alkenes from urban and industrial sources and their photo-oxidation products, such as formaldehyde.
    Atmos. Chem. Phys. 01/2011; 11:2471-2485.
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    ABSTRACT: Ozone (O3) is a photochemical oxidant, an air pollutant and a greenhouse gas. As the main precursor of the hydroxyl radical (OH) it strongly affects the oxidation power of the atmosphere. The remote marine boundary layer (MBL) is considered an important region in terms of chemical O3 loss; however surface-based atmospheric observations are sparse and the photochemical processes are not well understood. To investigate the photochemistry under the clean background conditions of the Southern Atlantic Ocean, ship measurements of NO, NO2, O3, JNO2, J(O1D), HO2, OH, ROx and a range of meteorological parameters were carried out. The concentrations of NO and NO2 measured on board the French research vessel Marion-Dufresne (28° S-57° S, 46° W-34° E) in March 2007, are among the lowest yet observed. The data is evaluated for consistency with photochemical steady state (PSS) conditions, and the calculations indicate substantial deviations from PSS (Phi>1). The deviations observed under low NOx conditions (5-25 pptv) demonstrate a remarkable upward tendency in the Leighton ratio (used to characterize PSS) with increasing NOx mixing ratio and JNO2 intensity. It is a paradigm in atmospheric chemistry that OH largely controls the oxidation efficiency of the atmosphere. However, evidence is growing that for unpolluted low-NOx (NO + NO2) conditions the atmospheric oxidant budget is poorly understood. Nevertheless, for the very cleanest conditions, typical for the remote marine boundary layer, good model agreement with measured OH and HO2 radicals has been interpreted as accurate understanding of baseline photochemistry. Here we show that such agreement can be deceptive and that a yet unidentified oxidant is needed to explain the photochemical conditions observed at 40°-60° S over the Atlantic Ocean.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; 11(16):8497-8513. · 5.51 Impact Factor
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    ABSTRACT: Forests are the dominant source of volatile organic compounds into the atmosphere, with isoprene being the most significant species. The oxidation chemistry of these compounds is a significant driver of local, regional and global atmospheric composition. Observations made over Borneo during the OP3 project in 2008, together with an observationally constrained box model are used to assess our understanding of this oxidation chemistry. In line with previous work in tropical forests, we find that the standard model based on MCM chemistry significantly underestimates the observed OH concentrations. Geometric mean observed to modelled ratios of OH and HO2 in airmasses impacted with isoprene are 5.32-4.43+3.68 and 1.18-0.30+0.30 respectively, with 68% of the observations being within the specified variation. We implement a variety of mechanistic changes into the model, including epoxide formation and unimolecular decomposition of isoprene peroxy radicals, and assess their impact on the model success. We conclude that none of the current suggestions can simultaneously remove the bias from both OH and HO2 simulations and believe that detailed laboratory studies are now needed to resolve this issue.
    Atmospheric Chemistry and Physics. 01/2011; 11(13):6749-6771.
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    ABSTRACT: Measurements of total peroxy radicals (HO2 + RO2) and nitrate radical (NO3) were made on the NOAA research vessel R/V along the U.S. Gulf Coast during the TexAQS 2006 field campaign. The measurements were modelled using a constrained box-model based upon the Master Chemical Mechanism (MCM). The agreement between modelled and measured HO2 + RO2 was typically within similar to 40% and, in the unpolluted regions, within 30%. The analysis of the model results suggests that the MCM might underestimate the concentrations of some acyl peroxy radicals and other small peroxy radicals. The model underestimated the measurements of NO3 by 60-70%, possibly because of rapid heterogeneous uptake of N2O5. The MCM model results were used to estimate the composition of the peroxy radical pool and to quantify the role of DMS, isoprene and alkenes in the formation of RO2 in the different regions. The measurements of HO2 + RO2 and NO3 were also used to calculate the gas-phase budget of NO3 and quantify the importance of organic peroxy radicals as NO3 sinks. RO2 accounted, on average, for 12-28% of the total gas-phase NO3 losses in the unpolluted regions and for 1-2% of the total gas-phase NO3 losses in the polluted regions.
    Journal of Atmospheric Chemistry 01/2011; 68(4):331-362. · 1.33 Impact Factor
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    ABSTRACT: During the Texas Air Quality Study II (TexAQS 2006) campaign, a PEroxy Radical Chemical Amplifier (PERCA) was deployed on the NOAA research vessel R/V Brown to measure total peroxy radicals (HO2+ΣRO2). Day-time mixing ratios of HO2+ΣRO2 between 25 and 110 ppt were observed throughout the study area – the Houston/Galveston region and the Gulf coast of the U.S. – and analyzed in relation to measurements of nitrogen oxides, volatile organic compounds (VOC) and photolysis rates to assess radical sources and sinks in the region. The measurements of HO2+ΣRO2 were used to calculate the in-situ net photochemical formation of ozone. Measured median values ranged from 0.6 ppb/h in clean oceanic air masses up to several tens of ppb/h in the most polluted industrial areas. The results are consistent with previous studies and generally agree with observations made during the previous TexAQS 2000 field campaign. The net photochemical ozone formation rates determined at Barbours Cut, a site immediately south of the Houston Ship Channel, were analyzed in relation to local wind direction and VOC reactivity to understand the relationship between ozone formation and local VOC emissions. The measurements of HO2+ΣRO2 made during the R/V Brown TexAQS 2006 cruise indicate that ozone formation is NOx-limited in the Houston/Galveston region and influenced by highly reactive hydrocarbons, especially alkenes from urban and industrial sources and their photooxidation products, such as formaldehyde.
    Atmospheric Chemistry and Physics 01/2010; · 4.88 Impact Factor
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    ABSTRACT: OH and HO<sub>2</sub> aircraft measurements made over West Africa during the AMMA field campaign in summer 2006 have been investigated using a box model constrained to observations of long-lived species and physical parameters. "Good" agreement was found for HO<sub>2</sub> (modelled to observed gradient of 1.23±0.11). However, the model significantly overpredicts OH concentrations. The reasons for this are not clear, but may reflect instrumental instabilities affecting the OH measurements. Within the model HO<sub>x</sub> concentrations in West Africa are controlled by relatively simple photochemistry, with production dominated by ozone photolysis and reaction of O(<sup>1</sup>D) with water vapour, and loss processes dominated by HO<sub>2</sub>+HO<sub>2</sub> and HO<sub>2</sub>+RO<sub>2</sub>. Isoprene chemistry was found to influence forested regions. In contrast to several recent field studies in very low NO<sub>x</sub> and high isoprene environments we do not observe any dependence of model success for HO<sub>2</sub> on isoprene and attribute this to efficient recycling of HO<sub>x</sub> through RO<sub>2</sub>+NO reactions under the moderate NO<sub>x</sub> concentrations (5–300 ppt NO in the boundary layer) encountered during AMMA. This suggests that some of the problems with understanding the impact of isoprene on atmospheric composition may be limited to the extreme low range of NO<sub>x</sub> concentrations.
    Atmospheric Chemistry and Physics 01/2010; · 4.88 Impact Factor
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    ABSTRACT: Peroxy radicals were measured onboard two scientific aircrafts during the AMMA (African Monsoon Multidisciplinary Analysis) campaign in summer 2006. This paper reports results from the flight on 16 August 2006 during which measurements of HO2 by laser induced fluorescence spectroscopy at low pressure (LIF-FAGE) and total peroxy radicals (RO2* = HO2+ΣRO2, R = organic chain) by two similar instruments based on the peroxy radical chemical amplification (PeRCA) technique were subject of a blind intercomparison. The German DLR-Falcon and the British FAAM-BAe-146 flew wing tip to wing tip for about 30 min making concurrent measurements on 2 horizontal level runs at 697 and 485 hPa over the same geographical area in Burkina Faso. A full set of supporting measurements comprising photolysis frequencies, and relevant trace gases like CO, NO, NO2, NOy, O3 and a wider range of VOCs were collected simultaneously. Results are discussed on the basis of the characteristics and limitations of the different instruments used. Generally, no data bias are identified and the RO2* data available agree quite reasonably within the instrumental errors. The [RO2*]/[HO2] ratios, which vary between 1:1 and 3:1, as well as the peroxy radical variability, concur with variations in photolysis rates and in other potential radical precursors. Model results provide additional information about dominant radical formation and loss processes.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2010; · 5.51 Impact Factor
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    ABSTRACT: Aircraft OH and HO2 measurements made over West Africa during the AMMA field campaign in summer 2006 have been investigated using a box model constrained to observations of long-lived species and physical parameters. "Good" agreement was found for HO2 (modelled to observed gradient of 1.23 ± 0.11). However, the model significantly overpredicts OH concentrations. The reasons for this are not clear, but may reflect instrumental instabilities affecting the OH measurements. Within the model, HOx concentrations in West Africa are controlled by relatively simple photochemistry, with production dominated by ozone photolysis and reaction of O(1D) with water vapour, and loss processes dominated by HO2 + HO2 and HO2 + RO2. Isoprene chemistry was found to influence forested regions. In contrast to several recent field studies in very low NOx and high isoprene environments, we do not observe any dependence of model success for HO2 on isoprene and attribute this to efficient recycling of HOx through RO2 + NO reactions under the moderate NOx concentrations (5–300 ppt NO in the boundary layer, median 76 ppt) encountered during AMMA. This suggests that some of the problems with understanding the impact of isoprene on atmospheric composition may be limited to the extreme low range of NOx concentrations.
    Atmospheric Chemistry and Physics. 01/2010; 10:9415-9429.
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    ABSTRACT: Measurements of radicals (HO2+RO2, NO3) were taken on board of the NOAA ship R/V Brown during the Texas Air Quality Study/Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS) 2006 field campaign. HO2+RO2 was measured by Chemical Amplification and NO3 (with N2O5) was measured by Cavity Ring-Down Spectroscopy. The R/V Brown cruised for a month (August-September 2006) off the coast of Texas and inside Galveston Bay and Houston industrial and shipping area, sampling air masses coming from the highly industrialized region of Southern United States. The data collected during the cruise were analyzed using a zero-dimensional box-model based upon the Leeds Master Chemical Mechanism (MCM). The model was constrained to the measurements of long-lived species and physical parameters and used to calculate the concentrations of radicals (OH, HO2, RO2, NO3) during the cruise of the R/V Brown. The modelled concentrations of HO2+RO2 and NO3 were compared with the measurements. The model and the measurements were used to study ozone formation and photochemistry in one of the most polluted areas of the United States.
    AGU Fall Meeting Abstracts. 12/2007;