ORCID Profile
0000-0002-4075-3651
Current Organisations
University of York
,
National Centre for Atmospheric Science
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Publisher: Copernicus GmbH
Date: 07-02-2018
Abstract: Abstract. A fast-growing area of research is the development of low-cost sensors for measuring air pollutants. The affordability and size of low-cost particle sensors makes them an attractive option for use in experiments requiring a number of instruments such as high-density spatial mapping. However, for these low-cost sensors to be useful for these types of studies their accuracy and precision need to be quantified. We evaluated the Alphasense OPC-N2, a promising low-cost miniature optical particle counter, for monitoring ambient airborne particles at typical urban background sites in the UK. The precision of the OPC-N2 was assessed by co-locating 14 instruments at a site to investigate the variation in measured concentrations. Comparison to two different reference optical particle counters as well as a TEOM-FDMS enabled the accuracy of the OPC-N2 to be evaluated. Comparison of the OPC-N2 to the reference optical instruments shows some limitations for measuring mass concentrations of PM1, PM2.5 and PM10. The OPC-N2 demonstrated a significant positive artefact in measured particle mass during times of high ambient RH ( 85 %) and a calibration factor was developed based upon κ-Köhler theory, using average bulk particle aerosol hygroscopicity. Application of this RH correction factor resulted in the OPC-N2 measurements being within 33 % of the TEOM-FDMS, comparable to the agreement between a reference optical particle counter and the TEOM-FDMS (20 %). Inter-unit precision for the 14 OPC-N2 sensors of 22 ± 13 % for PM10 mass concentrations was observed. Overall, the OPC-N2 was found to accurately measure ambient airborne particle mass concentration provided they are (i) correctly calibrated and (ii) corrected for ambient RH. The level of precision demonstrated between multiple OPC-N2s suggests that they would be suitable devices for applications where the spatial variability in particle concentration was to be determined.
Publisher: Copernicus GmbH
Date: 19-12-2013
DOI: 10.5194/ACP-13-12451-2013
Abstract: Abstract. In situ airborne measurements were made over eastern Canada in summer 2011 as part of the BORTAS experiment (Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites). In this paper we present observations of greenhouse gases (CO2 and CH4) and other biomass burning tracers (CO, HCN and CH3CN), both climatologically and through case studies, as recorded on board the FAAM BAe-146 research aircraft. Vertical profiles of CO2 were generally characterised by depleted boundary layer concentrations relative to the free troposphere, consistent with terrestrial biospheric uptake. In contrast, CH4 concentrations were found to rise with decreasing altitude due to strong local and regional surface sources. BORTAS observations were found to be broadly comparable with both previous measurements in the region during the regional burning season and with reanalysed composition fields from the EU Monitoring Atmospheric Composition and Change (MACC) project. We use coincident tracer–tracer correlations and a Lagrangian trajectory model to characterise and differentiate air mass history of intercepted plumes. In particular, CO, HCN and CH3CN were used to identify air masses that have been recently influenced by biomass burning. Examining in idual cases we were able to quantify emissions from biomass burning. Using both near-field ( 1 day) and far-field ( 1 day) s ling, boreal forest fire plumes were identified throughout the troposphere. Fresh plumes from fires in northwestern Ontario yield emission factors for CH4 and CO2 of 8.5 ± 0.9 g (kg dry matter)−1 and 1512 ± 185 g (kg dry matter)−1, respectively. We have also investigated the efficacy of calculating emission factors from far-field s ling, in which there might be expected to be limited mixing with background and other characteristic air masses, and we provide guidance on best practice and limitations in such analysis. We have found that for measurements within plumes that originated from fires in northwestern Ontario 2–4 days upwind, emission factors can be calculated that range between 1618 ± 216 and 1702 ± 173 g (kg dry matter)−1 for CO2 and 1.8 ± 0.2 and 6.1 ± 1 g (kg dry matter)−1 for CH4.
Publisher: Royal Society of Chemistry (RSC)
Date: 2017
DOI: 10.1039/C7FD90041D
Publisher: American Meteorological Society
Date: 05-2015
DOI: 10.1175/BAMS-D-12-00245.1
Abstract: Air quality and heat are strong health drivers, and their accurate assessment and forecast are important in densely populated urban areas. However, the sources and processes leading to high concentrations of main pollutants, such as ozone, nitrogen dioxide, and fine and coarse particulate matter, in complex urban areas are not fully understood, limiting our ability to forecast air quality accurately. This paper introduces the Clean Air for London (ClearfLo www.clearflo.ac.uk) project’s interdisciplinary approach to investigate the processes leading to poor air quality and elevated temperatures. Within ClearfLo, a large multi-institutional project funded by the U.K. Natural Environment Research Council (NERC), integrated measurements of meteorology and gaseous, and particulate composition/loading within the atmosphere of London, United Kingdom, were undertaken to understand the processes underlying poor air quality. Long-term measurement infrastructure installed at multiple levels (street and elevated), and at urban background, curbside, and rural locations were complemented with high-resolution numerical atmospheric simulations. Combining these (measurement–modeling) enhances understanding of seasonal variations in meteorology and composition together with the controlling processes. Two intensive observation periods (winter 2012 and the Summer Olympics of 2012) focus upon the vertical structure and evolution of the urban boundary layer chemical controls on nitrogen dioxide and ozone production—in particular, the role of volatile organic compounds and processes controlling the evolution, size, distribution, and composition of particulate matter. The paper shows that mixing heights are deeper over London than in the rural surroundings and that the seasonality of the urban boundary layer evolution controls when concentrations peak. The composition also reflects the seasonality of sources such as domestic burning and biogenic emissions.
Publisher: Elsevier BV
Date: 12-2015
Publisher: Copernicus GmbH
Date: 22-03-2018
Abstract: Abstract. Gas-phase rate coefficients are fundamental to understanding atmospheric chemistry, yet experimental data are not available for the oxidation reactions of many of the thousands of volatile organic compounds (VOCs) observed in the troposphere. Here, a new experimental method is reported for the simultaneous study of reactions between multiple different VOCs and OH, the most important daytime atmospheric radical oxidant. This technique is based upon established relative rate concepts but has the advantage of a much higher throughput of target VOCs. By evaluating multiple VOCs in each experiment, and through measurement of the depletion in each VOC after reaction with OH, the OH + VOC reaction rate coefficients can be derived. Results from experiments conducted under controlled laboratory conditions were in good agreement with the available literature for the reaction of 19 VOCs, prepared in synthetic gas mixtures, with OH. This approach was used to determine a rate coefficient for the reaction of OH with 2,3-dimethylpent-1-ene for the first time k = 5.7 (±0.3) × 10−11 cm3 molecule−1 s−1. In addition, a further seven VOCs had only two, or fewer, in idual OH rate coefficient measurements available in the literature. The results from this work were in good agreement with those measurements. A similar dataset, at an elevated temperature of 323 (±10) K, was used to determine new OH rate coefficients for 12 aromatic, 5 alkane, 5 alkene and 3 monoterpene VOC + OH reactions. In OH relative reactivity experiments that used ambient air at the University of York, a large number of different VOCs were observed, of which 23 were positively identified. Due to difficulties with detection limits and fully resolving peaks, only 19 OH rate coefficients were derived from these ambient air s les, including 10 reactions for which data were previously unavailable at the elevated reaction temperature of T = 323 (±10) K.
Publisher: Copernicus GmbH
Date: 31-07-2208
Abstract: Abstract. Measurements of the radical species OH and HO2 were made using the fluorescence assay by gas expansion (FAGE) technique during a series of night-time and daytime flights over the UK in summer 2010 and winter 2011. OH was not detected above the instrument's 1σ limit of detection during any of the night-time flights or during the winter daytime flights, placing upper limits on [OH] of 1.8 × 106 molecule cm−3 and 6.4 × 105 molecule cm−3 for the summer and winter flights, respectively. HO2 reached a maximum concentration of 3.2 × 108 molecule cm−3 (13.6 pptv) during a night-time flight on 20 July 2010, when the highest concentrations of NO3 and O3 were also recorded. An analysis of the rates of reaction of OH, O3, and the NO3 radical with measured alkenes indicates that the summer night-time troposphere can be as important for the processing of volatile organic compounds (VOCs) as the winter daytime troposphere. An analysis of the instantaneous rate of production of HO2 from the reactions of O3 and NO3 with alkenes has shown that, on average, reactions of NO3 dominated the night-time production of HO2 during summer and reactions of O3 dominated the night-time HO2 production during winter.
Publisher: Copernicus GmbH
Date: 22-07-2011
Abstract: Abstract. The lifetime of methane is controlled to a very large extent by the abundance of the OH radical. The tropics are a key region for methane removal, with oxidation in the lower tropical troposphere dominating the global methane removal budget (Bloss et al., 2005). In tropical forested environments where biogenic VOC emissions are high and NOx concentrations are low, OH concentrations are assumed to be low due to rapid reactions with sink species such as isoprene. New, simultaneous measurements of OH concentrations and OH reactivity, k'OH, in a Borneo rainforest are reported and show much higher OH than predicted, with mean peak concentrations of ~2.5×106 molecule cm−3 (10 min average) observed around solar noon. Whilst j(O1D) and humidity were high, low O3 concentrations limited the OH production from O3 photolysis. Measured OH reactivity was very high, peaking at a diurnal average of 29.1±8.5 s−1, corresponding to an OH lifetime of only 34 ms. To maintain the observed OH concentration given the measured OH reactivity requires a rate of OH production approximately 10 times greater than calculated using all measured OH sources. A test of our current understanding of the chemistry within a tropical rainforest was made using a detailed zero-dimensional model to compare with measurements. The model over-predicted the observed HO2 concentrations and significantly under-predicted OH concentrations. Inclusion of an additional OH source formed as a recycled product of OH initiated isoprene oxidation improved the modelled OH agreement but only served to worsen the HO2 model/measurement agreement. To replicate levels of both OH and HO2, a process that recycles HO2 to OH is required equivalent to the OH recycling effect of 0.74 ppbv of NO. This recycling step increases OH concentrations by 88 % at noon and has wide implications, leading to much higher predicted OH over tropical forests, with a concomitant reduction in the CH4 lifetime and increase in the rate of VOC degradation.
Publisher: American Geophysical Union (AGU)
Date: 06-2015
DOI: 10.1002/2014JD022629
Publisher: Copernicus GmbH
Date: 14-07-2011
Abstract: 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.
Publisher: Copernicus GmbH
Date: 12-03-2018
Abstract: Abstract. The chemistry of the halogen species bromine and iodine has a range of impacts on tropospheric composition, and can affect oxidising capacity in a number of ways. However, recent studies disagree on the overall sign of the impacts of halogens on the oxidising capacity of the troposphere. We present simulations of OH and HO2 radicals for comparison with observations made in the remote tropical ocean boundary layer during the Seasonal Oxidant Study at the Cape Verde Atmospheric Observatory in 2009. We use both a constrained box model, using detailed chemistry derived from the Master Chemical Mechanism (v3.2), and the three-dimensional global chemistry transport model GEOS-Chem. Both model approaches reproduce the diurnal trends in OH and HO2. Absolute observed concentrations are well reproduced by the box model but are overpredicted by the global model, potentially owing to incomplete consideration of oceanic sourced radical sinks. The two models, however, differ in the impacts of halogen chemistry. In the box model, halogen chemistry acts to increase OH concentrations (by 9.8 % at midday at the Cape Verde Atmospheric Observatory), while the global model exhibits a small increase in OH at the Cape Verde Atmospheric Observatory (by 0.6 % at midday) but overall shows a decrease in the global annual mass-weighted mean OH of 4.5 %. These differences reflect the variety of timescales through which the halogens impact the chemical system. On short timescales, photolysis of HOBr and HOI, produced by reactions of HO2 with BrO and IO, respectively, increases the OH concentration. On longer timescales, halogen-catalysed ozone destruction cycles lead to lower primary production of OH radicals through ozone photolysis, and thus to lower OH concentrations. The global model includes more of the longer timescale responses than the constrained box model, and overall the global impact of the longer timescale response (reduced primary production due to lower O3 concentrations) overwhelms the shorter timescale response (enhanced cycling from HO2 to OH), and thus the global OH concentration decreases. The Earth system contains many such responses on a large range of timescales. This work highlights the care that needs to be taken to understand the full impact of any one process on the system as a whole.
Publisher: Elsevier BV
Date: 12-2006
Publisher: Royal Society of Chemistry (RSC)
Date: 2005
DOI: 10.1039/B417403H
Abstract: A photochemical trajectory model containing speciated emissions of 124 non-methane volatile organic compounds (VOC), and a comprehensive description of the chemistry of VOC degradation, has been used to simulate the chemical evolution of boundary layer air masses arriving at a field c aign site in the southern UK during a widespread and prolonged photochemical pollution event in August 2003. The simulated concentrations and distributions of organic compounds at the arrival location are compared with observations of a series of hydrocarbons and carbonyl compounds, which were measured using GC-FID and multidimensional GC methods. The comparison of the simulated and observed distributions of 34 emitted hydrocarbons provides some support for the magnitude and applied emissions speciation of anthropogenic hydrocarbons, but is indicative of an under representation of the input of biogenic hydrocarbons, particularly at elevated temperatures. Simulations of the detailed distribution of ca. 1250 carbonyl compounds, formed primarily from the degradation of the 124 emitted VOC, focus on 61 aldehydes, ketones, dicarbonyls, hydroxycarbonyls and aromatic aldehydes which collectively account for ca. 90% of the simulated total molar concentration of carbonyls. The simulated distributions indicate that the photolysis of formaldehyde and alpha-dicarbonyls make major contributions to free radical production for the arrival conditions of five case study trajectories. The simulated concentrations of hydroxycarbonyls demonstrate preferential formation of the 1,4-substituted isomers (compared with 1,2- and 1,3-isomers of the same carbon number), which are formed during the initial oxidation sequence of longer chain alkanes.
Publisher: Copernicus GmbH
Date: 05-06-2019
Abstract: Abstract. The Atmospheric Pollution and Human Health in a Chinese Megacity (APHH-Beijing) programme is an international collaborative project focusing on understanding the sources, processes and health effects of air pollution in the Beijing megacity. APHH-Beijing brings together leading China and UK research groups, state-of-the-art infrastructure and air quality models to work on four research themes: (1) sources and emissions of air pollutants (2) atmospheric processes affecting urban air pollution (3) air pollution exposure and health impacts and (4) interventions and solutions. Themes 1 and 2 are closely integrated and support Theme 3, while Themes 1–3 provide scientific data for Theme 4 to develop cost-effective air pollution mitigation solutions. This paper provides an introduction to (i) the rationale of the APHH-Beijing programme and (ii) the measurement and modelling activities performed as part of it. In addition, this paper introduces the meteorology and air quality conditions during two joint intensive field c aigns – a core integration activity in APHH-Beijing. The coordinated c aigns provided observations of the atmospheric chemistry and physics at two sites: (i) the Institute of Atmospheric Physics in central Beijing and (ii) Pinggu in rural Beijing during 10 November–10 December 2016 (winter) and 21 May–22 June 2017 (summer). The c aigns were complemented by numerical modelling and automatic air quality and low-cost sensor observations in the Beijing megacity. In summary, the paper provides background information on the APHH-Beijing programme and sets the scene for more focused papers addressing specific aspects, processes and effects of air pollution in Beijing.
Publisher: Wiley
Date: 03-2008
DOI: 10.1897/07-347.1
Abstract: Crude oils are complex mixtures of many thousands of compounds, both resolved and unresolved by conventional gas chromatography (GC). Recent research using comprehensive two-dimensional gas chromatography-time-of-flight-mass-spectrometry (GC x GC-ToF-MS) identified branched alkylbenzenes (BABs) as a major component of some unresolved complex mixtures of hydrocarbons (UCMs) bioaccumulated in the tissues of North Sea mussels, Mytilus edulis, previously found to have poor health status. Here the effect of long-term exposure to low aqueous concentration of BABs and mussels' ability to recover, was determined. Mussels were exposed to 5 microg/L of a complex mixture of C(12-14) BABs for 14 d. Feeding rates and the viability of hemocytes were measured immediately after exposure and again after 5 d depuration. Tissues were extracted, analyzed and alkylbenzenes quantified by both GC-MS and GC x GC-ToF-MS. Mussel extracts from previous acute tests were also reanalyzed and quantified using GC x GC-ToF-MS. Mussels exposed to 5 microg/L BABs for 14 d accumulated 46 to 47 microg/g dry weight alkylbenzenes this was similar to tissue concentrations of mussels exposed to 41microg/L for 72 h. Feeding rates were significantly reduced (p < or = 0.05) and were dependent upon tissue concentration. Cellular viability was not significantly affected. Following 5 d in clean seawater, the BABs were only partially depurated and feeding rates failed to fully recover. The use of GC x GC-ToF-MS in the present study has shown that mussel tissue concentrations of complex mixtures of alkylbenzenes, and their corresponding effects, are consistent with reported concentrations within UCM-contaminated wild mussel populations with poor health status.
Publisher: Copernicus GmbH
Date: 05-02-2014
Abstract: Abstract. The RONOCO (ROle of Nighttime chemistry in controlling the Oxidising Capacity of the AtmOsphere) aircraft c aign during July 2010 and January 2011 made observations of OH, HO2, NO3, N2O5 and a number of supporting measurements at night over the UK, and reflects the first simultaneous airborne measurements of these species. We compare the observed concentrations of these short-lived species with those calculated by a box model constrained by the concentrations of the longer lived species using a detailed chemical scheme. OH concentrations were below the limit of detection, consistent with model predictions. The model systematically underpredicts HO2 by ~200% and overpredicts NO3 and N2O5 by around 80 and 50%, respectively. Cycling between NO3 and N2O5 is fast and thus we define the NO3x (NO3x=NO3+N2O5) family. Production of NO3x is overwhelmingly dominated by the reaction of NO2 with O3, whereas its loss is dominated by aerosol uptake of N2O5, with NO3+VOCs (volatile organic compounds) and NO3+RO2 playing smaller roles. The production of HOx and ROx radicals is mainly due to the reaction of NO3 with VOCs. The loss of these radicals occurs through a combination of HO2+RO2 reactions, heterogeneous processes and production of HNO3 from OH+NO2, with radical propagation primarily achieved through reactions of NO3 with peroxy radicals. Thus NO3 at night plays a similar role to both OH and NO during the day in that it both initiates ROx radical production and acts to propagate the tropospheric oxidation chain. Model sensitivity to the N2O5 aerosol uptake coefficient (γN2O5) is discussed and we find that a value of γN2O5=0.05 improves model simulations for NO3 and N2O5, but that these improvements are at the expense of model success for HO2. Improvements to model simulations for HO2, NO3 and N2O5 can be realised simultaneously on inclusion of additional unsaturated volatile organic compounds, however the nature of these compounds is extremely uncertain.
Publisher: Elsevier BV
Date: 02-2010
Publisher: Copernicus GmbH
Date: 24-02-2016
Abstract: Abstract. Near-continuous measurements of hydroxyl radical (OH) reactivity in the urban background atmosphere of central London during the summer of 2012 are presented. OH reactivity behaviour is seen to be broadly dependent on air mass origin, with the highest reactivity and the most pronounced diurnal profile observed when air had passed over central London to the east, prior to measurement. Averaged over the entire observation period of 26 days, OH reactivity peaked at ∼ 27 s−1 in the morning, with a minimum of ∼ 15 s−1 during the afternoon. A maximum OH reactivity of 116 s−1 was recorded on one day during morning rush hour. A detailed box model using the Master Chemical Mechanism was used to calculate OH reactivity, and was constrained with an extended measurement data set of volatile organic compounds (VOCs) derived from a gas chromatography flame ionisation detector (GC-FID) and a two-dimensional GC instrument which included heavier molecular weight (up to C12) aliphatic VOCs, oxygenated VOCs and the biogenic VOCs α-pinene and limonene. Comparison was made between observed OH reactivity and modelled OH reactivity using (i) a standard suite of VOC measurements (C2–C8 hydrocarbons and a small selection of oxygenated VOCs) and (ii) a more comprehensive inventory including species up to C12. Modelled reactivities were lower than those measured (by 33 %) when only the reactivity of the standard VOC suite was considered. The difference between measured and modelled reactivity was improved, to within 15 %, if the reactivity of the higher VOCs (⩾ C9) was also considered, with the reactivity of the biogenic compounds of α-pinene and limonene and their oxidation products almost entirely responsible for this improvement. Further improvements in the model's ability to reproduce OH reactivity (to within 6 %) could be achieved if the reactivity and degradation mechanism of unassigned two-dimensional GC peaks were estimated. Neglecting the contribution of the higher VOCs (⩾ C9) (particularly α-pinene and limonene) and model-generated intermediates increases the modelled OH concentrations by 41 %, and the magnitude of in situ ozone production calculated from the production of RO2 was significantly lower (60 %). This work highlights that any future ozone abatement strategies should consider the role that biogenic emissions play alongside anthropogenic emissions in influencing London's air quality.
Publisher: Copernicus GmbH
Date: 21-02-2018
Abstract: Abstract. Measurements of OH, HO2, RO2i (alkene and aromatic-related RO2) and total RO2 radicals taken during the ClearfLo c aign in central London in the summer of 2012 are presented. A photostationary steady-state calculation of OH which considered measured OH reactivity as the OH sink term and the measured OH sources (of which HO2+ NO reaction and HONO photolysis dominated) compared well with the observed levels of OH. Comparison with calculations from a detailed box model utilising the Master Chemical Mechanism v3.2, however, highlighted a substantial discrepancy between radical observations under lower NOx conditions ([NO] 1 ppbv), typically experienced during the afternoon hours, and indicated that the model was missing a significant peroxy radical sink the model overpredicted HO2 by up to a factor of 10 at these times. Known radical termination steps, such as HO2 uptake on aerosols, were not sufficient to reconcile the model–measurement discrepancies alone, suggesting other missing termination processes. This missing sink was most evident when the air reaching the site had previously passed over central London to the east and when elevated temperatures were experienced and, hence, contained higher concentrations of VOCs. Uncertainties in the degradation mechanism at low NOx of complex biogenic and diesel related VOC species, which were particularly elevated and dominated OH reactivity under these easterly flows, may account for some of the model–measurement disagreement. Under higher [NO] ( 3 ppbv) the box model increasingly underpredicted total [RO2]. The modelled and observed HO2 were in agreement, however, under elevated NO concentrations ranging from 7 to 15 ppbv. The model uncertainty under low NO conditions leads to more ozone production predicted using modelled peroxy radical concentrations (∼ 3 ppbv h−1) versus ozone production from peroxy radicals measured (∼ 1 ppbv h−1). Conversely, ozone production derived from the predicted peroxy radicals is up to an order of magnitude lower than from the observed peroxy radicals as [NO] increases beyond 7 ppbv due to the model underprediction of RO2 under these conditions.
Publisher: The Royal Society
Date: 27-11-2011
Abstract: We report measurements of atmospheric composition over a tropical rainforest and over a nearby oil palm plantation in Sabah, Borneo. The primary vegetation in each of the two landscapes emits very different amounts and kinds of volatile organic compounds (VOCs), resulting in distinctive VOC fingerprints in the atmospheric boundary layer for both landscapes. VOCs over the Borneo rainforest are dominated by isoprene and its oxidation products, with a significant additional contribution from monoterpenes. Rather than consuming the main atmospheric oxidant, OH, these high concentrations of VOCs appear to maintain OH, as has been observed previously over Amazonia. The boundary-layer characteristics and mixing ratios of VOCs observed over the Borneo rainforest are different to those measured previously over Amazonia. Compared with the Bornean rainforest, air over the oil palm plantation contains much more isoprene, monoterpenes are relatively less important, and the flower scent, estragole, is prominent. Concentrations of nitrogen oxides are greater above the agro-industrial oil palm landscape than over the rainforest, and this leads to changes in some secondary pollutant mixing ratios (but not, currently, differences in ozone). Secondary organic aerosol over both landscapes shows a significant contribution from isoprene. Primary biological aerosol dominates the super-micrometre aerosol over the rainforest and is likely to be sensitive to land-use change, since the fungal source of the bioaerosol is closely linked to above-ground bio ersity.
Publisher: Copernicus GmbH
Date: 26-09-2013
Abstract: Abstract. OH (hydroxyl radical) reactivity, the inverse of the chemical lifetime of the hydroxyl radical, was measured for 12 days in April 2008 within a tropical rainforest on Borneo as part of the OP3 (Oxidant and Particle Photochemical Processes) project. The maximum observed value was 83.8 ± 26.0 s−1 with the c aign averaged noontime maximum being 29.1 ± 8.5 s−1. The maximum OH reactivity calculated using the diurnally averaged concentrations of observed sinks was ~ 18 s−1, significantly less than the observations, consistent with other studies in similar environments. OH reactivity was dominated by reaction with isoprene (~ 30%). Numerical simulations of isoprene oxidation using the Master Chemical Mechanism (v3.2) in a highly simplified physical and chemical environment show that the steady state OH reactivity is a linear function of the OH reactivity due to isoprene alone, with a maximum multiplier, to account for the OH reactivity of the isoprene oxidation products, being equal to the number of isoprene OH attackable bonds (10). Thus the emission of isoprene constitutes a significantly larger emission of reactivity than is offered by the primary reaction with isoprene alone, with significant scope for the secondary oxidation products of isoprene to constitute the observed missing OH reactivity. A physically and chemically more sophisticated simulation (including physical loss, photolysis, and other oxidants) showed that the calculated OH reactivity is reduced by the removal of the OH attackable bonds by other oxidants and photolysis, and by physical loss (mixing and deposition). The calculated OH reactivity is increased by peroxide cycling, and by the OH concentration itself. Notable in these calculations is that the accumulated OH reactivity from isoprene, defined as the total OH reactivity of an emitted isoprene molecule and all of its oxidation products, is significantly larger than the reactivity due to isoprene itself and critically depends on the chemical and physical lifetimes of intermediate species. When constrained to the observed diurnally averaged concentrations of primary VOCs (volatile organic compounds), O3, NOx and other parameters, the model underestimated the observed diurnal mean OH reactivity by 30%. However, it was found that (1) the short lifetimes of isoprene and OH, compared to those of the isoprene oxidation products, lead to a large variability in their concentrations and so significant variation in the calculated OH reactivity (2) uncertainties in the OH chemistry in these high isoprene environments can lead to an underestimate of the OH reactivity (3) the physical loss of species that react with OH plays a significant role in the calculated OH reactivity and (4) a missing primary source of reactive carbon would have to be emitted at a rate equivalent to 50% that of isoprene to account for the missing OH sink. Although the presence of unmeasured primary emitted VOCs contributing to the measured OH reactivity is likely, evidence that these primary species account for a significant fraction of the unmeasured reactivity is not found. Thus the development of techniques for the measurement of secondary multifunctional carbon compounds is needed to close the OH reactivity budget.
Publisher: Elsevier BV
Date: 05-2006
DOI: 10.1016/J.SCITOTENV.2005.08.053
Abstract: The PUMA (Pollution of the Urban Midlands Atmosphere) Consortium project involved intensive measurement c aigns in the Summer of 1999 and Winter of 1999/2000, respectively, in which a wide variety of air pollutants were measured in the UK West Midlands conurbation including detailed speciation of VOCs and major component analysis of aerosol. Measurements of the OH and HO2 free radicals by the FAGE technique demonstrated that winter concentrations of OH were approximately half of those measured during the summer despite a factor of 15 reduction in production through the photolysis of ozone. Detailed box modelling of the fast reaction chemistry revealed the decomposition of Criegee intermediates formed from ozone-alkene reactions to be responsible for the majority of the formation of hydroxyl in both the summer and winter c aigns, in contrast to earlier rural measurements in which ozone photolysis was predominant. The main sinks for hydroxyl are reactions with NO2, alkenes and oxygenates. Concentrations of the more stable hydrocarbons were found to be relatively invariant across the conurbation, but the impacts of photochemistry were evident through analyses of formaldehyde which showed the majority to be photochemical in origin as opposed to emitted from road traffic. Measurements on the upwind and downwind boundaries of the conurbation revealed substantial enhancements in NOx as a result of emissions within the conurbation, especially during westerly winds which carried relatively clean air. Using calcium as a tracer for crustal particles, it proved possible to reconstruct aerosol mass from the major chemical components with a fairly high degree of success. The organic to elemental carbon ratios showed a far greater influence of photochemistry in summer than winter, presumably resulting mainly from the greater availability of biogenic precursors during the summer c aign. Two urban airshed models were developed and applied to the conurbation, one Eulerian, the other Lagrangian. Both were able to give a good simulation of concentrations of both primary and secondary pollutants at urban background locations.
Publisher: Copernicus GmbH
Date: 06-10-2010
Abstract: Abstract. Aircraft OH and HO2 measurements made over West Africa during the AMMA field c aign 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.
Publisher: Elsevier BV
Date: 08-2010
Publisher: Copernicus GmbH
Date: 09-12-2019
Abstract: Abstract. Nitrous acid (HONO) is a key determinant of the daytime radical budget in the daytime boundary layer, with quantitative measurement required to understand OH radical abundance. Accurate and precise measurements of HONO are therefore needed however HONO is a challenging compound to measure in the field, in particular in a chemically complex and highly polluted environment. Here we report an intercomparison exercise between HONO measurements performed by two wet chemical techniques (the commercially available a long-path absorption photometer (LOPAP) and a custom-built instrument) and two broadband cavity-enhanced absorption spectrophotometer (BBCEAS) instruments at an urban location in Beijing. In addition, we report a comparison of HONO measurements performed by a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) and a selected ion flow tube mass spectrometer (SIFT-MS) to the more established techniques (wet chemical and BBCEAS). The key finding from the current work was that all instruments agree on the temporal trends and variability in HONO (r2 0.97), yet they displayed some ergence in absolute concentrations, with the wet chemical methods consistently higher overall than the BBCEAS systems by between 12 % and 39 %. We found no evidence for any systematic bias in any of the instruments, with the exception of measurements near instrument detection limits. The causes of the ergence in absolute HONO concentrations were unclear, and may in part have been due to spatial variability, i.e. differences in instrument location and/or inlet position, but this observation may have been more associative than casual.
Publisher: American Geophysical Union (AGU)
Date: 13-02-2007
DOI: 10.1029/2006JD007584
Publisher: Springer Science and Business Media LLC
Date: 06-2000
DOI: 10.1038/35015540
Abstract: Volatile organic compounds play a central role in the processes that generate both urban photochemical smog and tropospheric ozone. For successful and accurate prediction of these pollution episodes, identification of the dominant reactive species within the volatile organic carbon pool is needed. At present, lack of resolution inherent in single-column chromatographic analysis limits such a detailed chemical characterization of the complex urban atmosphere. Here we present an improved method of peak deconvolution from double-column (orthogonal) gas chromatography. This has enabled us to isolate and classify more than 500 chemical species of volatile organic compounds in urban air, including over 100 multi-substituted monoaromatic and volatile oxygenated hydrocarbons. We suggest that previous assessments of reactive carbon species may therefore have underestimated the contribution made by volatile organic compounds to urban pollution, particularly for compounds with more than six carbon atoms. Incorporating these species in predictive models should greatly improve our understanding of photochemical ozone yields and the formation of harmful secondary organic aerosols.
Publisher: Elsevier BV
Date: 09-2001
Publisher: American Chemical Society (ACS)
Date: 07-12-2007
DOI: 10.1021/ES0615829
Publisher: Wiley
Date: 16-08-2010
DOI: 10.1002/ASL.289
Publisher: American Geophysical Union (AGU)
Date: 18-07-2002
DOI: 10.1029/2001JD001568
Publisher: American Geophysical Union (AGU)
Date: 09-2004
DOI: 10.1029/2004GL020544
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Alastair Lewis.