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Charles Nicholas Hewitt  - - - 
Top co-authors See all
Chris D.F. Rogers

85 shared publications

Department of Civil Engineering, University of Birmingham, Birmingham, UK

Rachel Cooper

83 shared publications

Imagination, Lancaster University, Lancaster, UK

John R. Bryson

72 shared publications

School of Geography, Earth and Environmental Science, University of Birmingham, Birmingham, B152TT, UK

I.F. Jefferson

70 shared publications

Department of Civil Engineering, University of Birmingham, Birmingham, UK

A.R. MacKenzie

56 shared publications

Birmingham Institute for Forest Research and School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK

Publication Record
Distribution of Articles published per year 
(2001 - 2019)
Total number of journals
published in
Publications See all
Article 0 Reads 0 Citations Observations of Highly Oxidised Molecules and Particle Nucleation in the Atmosphere of Beijing James Brean, Roy M. Harrison, ZongBo Shi, David C. S. Beddow... Published: 27 March 2019
Atmospheric Chemistry and Physics Discussions, doi: 10.5194/acp-2019-156
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Particle nucleation is one of the main sources of atmospheric particulate matter by number, with new particles having great relevance for human health and climate. Highly oxidised multifunctional organic molecules (HOMs) have been recently identified as key constituents in the growth, and, sometimes, in initial formation of new particles. While there have been many studies of HOMs in atmospheric chambers, flow tubes and clean environments, analyses of data from polluted environments are scarce. Here, measurements of HOMs and particle size distributions down to small molecular clusters are presented alongside VOC and trace gas data from a campaign in Beijing. Many gas phase HOMs have been characterised and their temporal trends and behaviours analysed in the context of new particle formation. The HOMs identified have a comparable degree of oxidation to those seen in other, cleaner, environments, likely due to an interplay between the higher temperatures facilitating rapid hydrogen abstractions and the higher concentrations of NOx and other RO2. terminators ending the autoxidation sequence more rapidly. Our data indicate that alkylbenzenes, monoterpenes, and isoprene are important precursor VOCs for HOMs in Beijing. Many of the C5 and C10 compounds derived from isoprene and monoterpenes have a slightly greater degree of average oxidation state of carbon compared to those from other precursors. Most HOMs except for large dimers have daytime peak concentrations, indicating the importance of OH. chemistry in the formation of HOMs, as O3 is lower on the days with higher HOM concentrations; similarly, VOC concentrations are lower on the days with higher HOM concentrations. The daytime peaks of HOMs coincide with the growth of freshly formed new particles, and their initial formation coincides with the peak in sulphuric acid vapours, suggesting that the nucleation process is sulphuric acid-dependent, with HOMs contributing to subsequent particle growth.
Article 0 Reads 0 Citations Using green infrastructure to improve urban air quality (GI4AQ) C. Nick Hewitt, Kirsti Ashworth, A. Rob MacKenzie Published: 16 March 2019
Ambio, doi: 10.1007/s13280-019-01164-3
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As evidence for the devastating impacts of air pollution on human health continues to increase, improving urban air quality has become one of the most pressing tasks facing policy makers world-wide. Increasingly, and very often on the basis of conflicting and/or weak evidence, the introduction of green infrastructure (GI) is seen as a win–win solution to urban air pollution, reducing ground-level concentrations without imposing restrictions on traffic and other polluting activities. The impact of GI on air quality is highly context dependent, with models suggesting that GI can improve urban air quality in some situations, but be ineffective or even detrimental in others. Here we set out a novel conceptual framework explaining how and where GI can improve air quality, and offer six specific policy interventions, underpinned by research, that will always allow GI to improve air quality. We call GI with unambiguous benefits for air quality GI4AQ. However, GI4AQ will always be a third-order option for mitigating air pollution, after reducing emissions and extending the distance between sources and receptors.
Article 0 Reads 1 Citation Hybrid life-cycle assessment for robust, best-practice carbon accounting C. Kennelly, M. Berners-Lee, C.N. Hewitt Published: 01 January 2019
Journal of Cleaner Production, doi: 10.1016/j.jclepro.2018.09.231
DOI See at publisher website
Article 0 Reads 0 Citations Introduction to Special Issue – In-depth study of air pollution sources and processes within Beijing and its surrounding... ZongBo Shi, Tuan Vu, Simone Kotthaus, Sue Grimmond, Roy M. H... Published: 15 October 2018
Atmospheric Chemistry and Physics Discussions, doi: 10.5194/acp-2018-922
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APHH-Beijing (Atmospheric Pollution and Human Health in a Chinese Megacity) is an international collaborative project to examine the emissions, processes and health effects of air pollution in Beijing. The four research themes of APHH-China are: (1) sources and emissions of urban atmospheric pollution; (2) processes affecting urban atmospheric pollution; (3) exposure science and impacts on health; and (4) interventions and solutions to reduce health impacts. Themes 1 and 2 are closely integrated and support Theme 3, while Themes 1–3 provide scientific data for Theme 4 on the development of cost-effective solutions. A key activity within APHH-Beijing was the two month-long intensive field campaigns at two sites: (i) central Beijing, and (ii) rural Pinggu. The coordinated campaigns provided observations of the atmospheric chemistry and physics in and around Beijing during November–December 2016 and May–June 2017. The campaigns were complemented by numerical air quality modelling and air quality and meteorology data at the 12 national monitoring stations in Beijing. This introduction paper provides an overview of (i) APHH-Beijing programme, (ii) the measurement and modelling activities performed as part of it in Beijing, and (iii) the air quality and meteorological conditions during the two field campaigns. The winter campaign was characterized by high PM2.5 pollution events whereas the summer experienced high ozone pollution events. Air quality was poor during the winter campaign, but less severe than in the same period in 2015 when there were a number of major pollution episodes. PM2.5 levels were relatively low during the summer period, matching the cleanest periods over the previous five years. Synoptic scale meteorological analysis suggests that the greater stagnation and weak southerly circulation in November/December 2016 may have contributed to the poor air quality.
Article 0 Reads 1 Citation Current global food production is sufficient to meet human nutritional needs in 2050 provided there is radical societal ... M. Berners-Lee, C. Kennelly, R. Watson, C. N. Hewitt Published: 18 July 2018
Elem Sci Anth, doi: 10.1525/elementa.310
DOI See at publisher website
Article 0 Reads 0 Citations Isoprene emission potentials from European oak forests derived from canopy flux measurements: An assessment of uncertain... Ben Langford, James Cash, W. Joe F. Acton, Amy C. Valach, Ch... Published: 22 May 2017
Biogeosciences Discussions, doi: 10.5194/bg-2017-196
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Biogenic emission algorithms predict that oak forests account for ~ 70 % of the total European isoprene budget. Yet the isoprene emission potentials that underpin these model estimates are calculated from a very limited number of leaf-level observations and hence are highly uncertain. Increasingly, micrometeorological techniques such as eddy covariance are used to measure whole-canopy fluxes directly, from which isoprene emission potentials can be calculated. Here, we review five observational datasets of isoprene fluxes from a range of oak forests in the UK, Italy and France. We outline procedures to correct the measured net fluxes for losses from deposition and chemical flux divergence, which were found to be on the order of 5–8 % and 4–5 %, respectively. The corrected observational data were used to derive isoprene emission potentials at each site in a two-step process. Firstly, six commonly used emission algorithms were inverted to back out time series of isoprene emission potential, and then an average isoprene emission potential was calculated for each site with an associated uncertainty. We used these data to assess how the derived emission potentials change depending upon the specific emission algorithm used and importantly, on the particular approach adopted to derive an average site specific emission potential. Our results show that isoprene emission potentials can vary by up to a factor of four depending on the specific algorithm used and whether or not it is used in a big-leaf or canopy environment model format. When using the same algorithm, the calculated average isoprene emission potential was found to vary by as much as 34 % depending on how the average was derived. In order to best replicate the observed fluxes we propose a new weighted average method whereby the isoprene emission potential is calculated as the average of all flux observations divided by the average activity factor (γ) of the emission algorithm. This approach ensures that modelled fluxes always have the same average as the measurements. Using this new approach, with version 2.1 of the Model for Emissions of Gases and Aerosols from Nature (MEGAN), we derive new ecosystem-scale isoprene emission potentials for the five measurement sites, Alice Holt, UK (10 500 ± 2500 µg m−2 h−1), Bosco Fontana, Italy (1610 ± 420 µg m−2 h−1), Castelporziano, Italy (43 ± 10 µg m−2 h−1), Ispra, Italy (7590 ± 1070 µg m−2 h−1) and the Observatoire de Haute Provence, France (7990 ± 1010 µg m−2 h−1). Ecosystem-scale isoprene emission potentials were then extrapolated to the leaf-level and compared to previous leaf-level measurements for Quercus robur and Quercus pubescens, two species thought to account for 50 % of the total European isoprene budget. The literature values agreed closely with emission potentials calculated using the G93 algorithm, which were 85 ± 75 µg g−1 h−1 and 78 ± 25 µg g−1 h−1 for Q. robur and Q. pubescens respectively. By contrast, emission potentials calculated using the G06 algorithm, the same algorithm used in a previous study to derive the European budget, were significantly lower, which we attribute to the influence of past light and temperature conditions. Adopting these new G06 specific emission potentials for Q. robur (55 ± 24 µg g−1 h−1) and Q. pubescens (47 ± 16 µg g−1 h−1) reduced the projected European budget by ~ 17 %. Our findings demonstrate that calculated isoprene emission potentials vary considerably depending upon the specific approach used in their calculation. Therefore, it is our recommendation that the community now adopt a standardised approach to the way in which micrometeorological flux measurements are corrected and used to derive isoprene, and other biogenic VOC, emission potentials. Modellers who use derived emission potentials should pay particular attention to the way in which an emission potential was derived and ensure that the algorithm they are using, and the implementation thereof, is consistent with that used to derive the emission potential. Our results show that, in the worst cases, failure to account for this may result in modelled fluxes that differ from observations by up to a factor of four.