Jennifer Salmond

A review of turbulence in the very stable nocturnal boundary layer and its implications for air quality

Turbulence in the very stable nocturnal boundary layer is weak and typically characterized by intermittent bursts of activity. It often exists in isolated layers or pockets generated primarily from localized shear instabilities. As a result, turbulence is rarely in equilibrium with the conditions of the underlying surface. Given the layered structure of the nocturnal boundary layer, the spatial and temporal characteristics of turbulent activity (and resulting vertical mixing) can have a significant affect on local air quality at hourly to diurnal scales. However, while there is a wealth of information concerning turbulent processes operating during daytime conditions, until recently comparatively few studies have focused on the nocturnal case. Nevertheless the three-dimensional distribution of pollutants in the nocturnal boundary layer may have a significant impact on local pollutant budgets at a variety of temporal and spatial scales. This paper reviews recent progress in our understanding of the structure of, and processes operating in, the very stable nocturnal boundary layer. Then, drawing upon case studies from the Lower Fraser Valley, of British Columbia, Canada, it considers the implications of these developments for pollutant transport and surface air quality.

The influence of vegetation on the horizontal and vertical distribution of pollutants in a street canyon.

Space constraints in cities mean that there are only limited opportunities for increasing tree density within existing urban fabric and it is unclear whether the net effect of increased vegetation in street canyons is beneficial or detrimental to urban air quality at local scales. This paper presents data from a field study undertaken in Auckland, New Zealand designed to determine the local impact of a deciduous tree canopy on the distribution of the oxides of nitrogen within a street canyon. The results showed that the presence of leaves on the trees had a marked impact on the transport of pollutants and led to a net accumulation of pollutants in the canyon below the tree tops. The incidence and magnitude of temporally localised spikes in pollutant concentration were reduced within the tree canopy itself. A significant difference in pollutant concentrations with height was not observed when leaves were absent. Analysis of the trends in concentration associated with different wind directions showed a smaller difference between windward and leeward sides when leaves were on the trees. A small relative increase in concentrations on the leeward side was observed during leaf-on relative to leaf-off conditions as predicted by previous modelling studies. However the expected reduction in concentrations on the windward side was not observed. The results suggest that the presence of leaves on the trees reduces the upwards transport of fresh vehicle emissions, increases the storage of pollutants within the canopy space and reduces the penetration of clean air downwards from aloft. Differences observed between NO and NO(2) concentrations could not be accounted for by dispersion processes alone, suggesting that there may also be some changes in the chemistry of the atmosphere associated with the presence of leaves on the trees.

Health and climate related ecosystem services provided by street trees in the urban environment

Urban tree planting initiatives are being actively promoted as a planning tool to enable urban areas to adapt to and mitigate against climate change, enhance urban sustainability and improve human health and well-being. However, opportunities for creating new areas of green space within cities are often limited and tree planting initiatives may be constrained to kerbside locations. At this scale, the net impact of trees on human health and the local environment is less clear, and generalised approaches for evaluating their impact are not well developed.In this review, we use an urban ecosystems services framework to evaluate the direct, and locally-generated, ecosystems services and disservices provided by street trees. We focus our review on the services of major importance to human health and well-being which include ‘climate regulation’, ‘air quality regulation’ and ‘aesthetics and cultural services’. These are themes that are commonly used to justify new street tree or street tree retention initiatives. We argue that current scientific understanding of the impact of street trees on human health and the urban environment has been limited by predominantly regional-scale reductionist approaches which consider vegetation generally and/or single out individual services or impacts without considering the wider synergistic impacts of street trees on urban ecosystems. This can lead planners and policymakers towards decision making based on single parameter optimisation strategies which may be problematic when a single intervention offers different outcomes and has multiple effects and potential trade-offs in different places.We suggest that a holistic approach is required to evaluate the services and disservices provided by street trees at different scales. We provide information to guide decision makers and planners in their attempts to evaluate the value of vegetation in their local setting. We show that by ensuring that the specific aim of the intervention, the scale of the desired biophysical effect and an awareness of a range of impacts guide the choice of i) tree species, ii) location and iii) density of tree placement, street trees can be an important tool for urban planners and designers in developing resilient and resourceful cities in an era of climatic change.

Validation of low-cost ozone measurement instruments suitable for use in an air-quality monitoring network

This paper presents a novel low-cost instrument that uses a sensor based on conductivity changes of heated tungstic oxide, which is capable of accurately measuring ambient concentrations of ozone. A combination of temperature steps and air flow-rate steps is used to continually reset and re-zero the sensor. A two-stage calibration procedure is presented, in which a nonlinear transformation converts sensor resistance to a signal linear in ozone concentration, then a linear correlation is used to align the calibration with a reference instrument. The required calibration functions specific for the sensor, and control system for air flow rate and sensor temperature, are housed with the sensor in a compact, simple-to-exchange assembly. The instrument can be operated on solar power and uses cell phone technology to enable monitoring in remote locations. Data from field trials are presented here to demonstrate that both the accuracy and the stability of the instrument over periods of months are within a few parts-per-billion by volume. We show that common failure modes can be detected through measurement of signals available from the instrument. The combination of long-term stability, self-diagnosis, and simple, inexpensive repair means that the cost of operation and calibration of the instruments is significantly reduced in comparison with traditional reference instrumentation. These instruments enable the economical construction and operation of ozone monitoring networks of accuracy, time resolution and spatial density sufficient to resolve the local gradients that are characteristic of urban air pollution.

Personal Exposure to Air Pollution for Various Modes of Transport in Auckland, New Zealand

This paper investigates the carbon monoxide (CO) doses received while commuting by different modes (car, bus, train, motorcycle, bicycle and running), taking into account the commute time as well as the level of physical activity required. While the participants were constrained to travel at specific peak traffic times and between designated start and end points, they were free to choose a route appropriate for their mode of transport. The results of this study suggest that the lowest exposures (concentrations of pollutants) are experienced by train commuters, largely a reflection of the routes being removed from any significant road traffic. Motorcyclists experienced significantly higher average concentrations as a result of high-concentration and very-short-duration peaks not seen in the traces of car and bus commuters travelling on the same road. Travel by bus along a dedicated busway was also found to be effective in reducing commuter air pollution exposure compared to travel by car on a congested stretch of motorway. The average concentrations to which cyclists and runners were exposed were found to be not significantly different for those travelling by car or bus (except when on dedicated pedestrian/cycleways). However, when the increased physical activity that is required is taken into account (leading to higher volumes of air breathed) along with the increased commuting time (especially in the case of runners), the air pollution doses (as estimated by the product of the concentration, commute time and breathing factor) were found to be significantly higher than for the motorised modes. The results suggest that separate pedestrian/cycleways go some way towards providing healthier options for cyclists and pedestrians.

Venting of Heat and Carbon Dioxide from Urban Canyons at Night

Turbulent fluxes of carbon dioxide and sensible heat were observed in the surface layer of the weakly convective nocturnal boundary layer over the center of the city of Marseille, France, during the Experience sur Sites pour Contraindre les Modeles de Pollution Atmospherique et de Transport d’Emission (ESCOMPTE) field experiment in the summer of 2001. The data reveal intermittent events or bursts in the time series of carbon dioxide (CO2) concentration and air temperature that are superimposed upon the background values. These features relate to intermittent structures in the fluxes of CO2 and sensible heat. In Marseille, CO2 is primarily emitted into the atmosphere at street level from vehicle exhausts. In a similar way, nocturnal sensible heat fluxes are most likely to originate in the deep street canyons that are warmer than adjacent roof surfaces. Wavelet analysis is used to examine the hypothesis that CO2 concentrations can be used as a tracer to identify characteristics of the venting of pollutants and heat from street canyons into the above-roof nocturnal urban boundary layer. Wavelet analysis is shown to be effective in the identification and analysis of significant events and coherent structures within the turbulent time series. Late in the evening, there is a strong correlation between the burst structures observed in the air temperature and CO2 time series. Evidence suggests that the localized increases of temperature and CO2 observed above roof level in the urban boundary layer (UBL) are related to intermittent venting of sensible heat from the warmer urban canopy layer (UCL). However, later in the night, local advection of CO2 in the UBL, combined with reduced traffic emissions in the UCL, limit the value of CO2 as a tracer of convective plumes in the UBL.

Development of an ANN-based air pollution forecasting system with explicit knowledge through sensitivity analysis

Abstract Little attention is given to applying the artificial neural network (ANN) modeling technique to understand site–specific air pollution dispersion mechanisms, the order of importance of meteorological variables in determining concentrations as well as the important time scales that influence emission patterns. In this paper, we propose a methodology for extracting the key information from routinely–available meteorological parameters and the emission pattern of sources present throughout the year (e.g. traffic emissions) to build a reliable and physically–based ANN air pollution forecasting tool. The methodology is tested by modeling NO 2 concentrations at a site near a major highway in Auckland, New Zealand. The basic model consists of an ANN model for predicting NO 2 concentrations using eight predictor variables: wind speed, wind direction, solar radiation, temperature, relative humidity, as well as “hour of the day”, “day of the week” and “month of the year” representing the time variations in emissions according to their corresponding time scales. Of the three input optimization techniques explored in this study, namely a genetic algorithm, forward selection, and backward elimination, the genetic algorithm technique gave predictions resulting in the smallest mean absolute error. The nature of the internal nonlinear function of the trained genetically–optimized neural network model was then extracted based on the response of the model to perturbations to individual predictor variables through sensitivity analyses. A simplified model, based on the successive removal of the least significant meteorological predictor variables, was then developed until subsequent removal resulted in a significant decrease in model performance. The developed ANN model was found to outperform a linear regression model based on the same input parameters. The proposed approach illustrates how the ANN modeling technique can be used to identify the key meteorological variables required to adequately capture the temporal variability in air pollution concentrations for a specific scenario.

Characteristics and applications of small, portable gaseous air pollution monitors

BACKGROUND Traditional approaches for measuring air quality based on fixed measurements are inadequate for personal exposure monitoring. To combat this issue, the use of small, portable gas-sensing air pollution monitoring technologies is increasing, with researchers and individuals employing portable and mobile methods to obtain more spatially and temporally representative air pollution data. However, many commercially available options are built for various applications and based on different technologies, assumptions, and limitations. A review of the monitor characteristics of small, gaseous monitors is missing from current scientific literature. PURPOSE A state-of-the-art review of small, portable monitors that measure ambient gaseous outdoor pollutants was developed to address broad trends during the last 5-10 years, and to help future experimenters interested in studying gaseous air pollutants choose monitors appropriate for their application and sampling needs. METHODS Trends in small, portable gaseous air pollution monitor uses and technologies were first identified and discussed in a review of literature. Next, searches of online databases were performed for articles containing specific information related to performance, characteristics, and use of such monitors that measure one or more of three criteria gaseous air pollutants: ozone, nitrogen dioxide, and carbon monoxide. All data were summarized into reference tables for comparison between applications, physical features, sensing capabilities, and costs of the devices. RESULTS Recent portable monitoring trends are strongly related to associated applications and audiences. Fundamental research requires monitors with the best individual performance, and thus the highest cost technology. Monitor networking favors real-time capabilities and moderate cost for greater reproduction. Citizen science and crowdsourcing applications allow for lower-cost components; however important strengths and limitations for each application must be addressed or acknowledged for the given use.

Can Surface-Cover Tiles Be Summed to Give Neighborhood Fluxes in Cities?

The paper addresses the question of whether the modeling practice of summing separate land-cover tiles to give urban fluxes at the neighborhood scale has merit. A central-city site in Basel, Switzerland, was instrumented to measure turbulent sensible heat fluxes QH from the two main land-cover types (roofs and canyons) separately and from the whole neighborhood. Path-averaged QH values were measured in the roughness sublayer (RSL) using scintillometry, and the spatially averaged QH neighborhood-scale flux was measured in the inertial sublayer (ISL) by an eddy-covariance system. The roof and canyon flux results are combined and weighted according to the respective plan-area abundance of each to give an estimated value of the neighborhood flux. The results show that this ‘‘bottom up’’ approach underestimates the measured ISL values by about 25% when averaged across all periods and wind directions. This finding led to consideration of possible errors from instrumentation, inappropriate turbulent source areas, failure to sample representative surfaces, and inability to fully capture RSL heat exchange. Sorting data by the two main wind directions revealed significant differences. The measured fluxes in the ISL and across the canyon top depend little upon wind direction, but daytime roof values show a marked sensitivity to wind direction. Qualitative analysis suggests this might be caused by systematic controls such as solar angle, site morphometry, and observational setup. The comparison of bottom up versus ISL is inconclusive; in some conditions agreement appears promising, and in others it does not. The question has not been proven or disproven. It may be too ambitious to test the concept at a real-world site.

Applied climatology Doing the relational work of climate

Applied climatology has long been a niche domain, straddling the intersection between the social and natural sciences and populated largely by geographers explicitly interested in reframing human activities around climate. As human-atmospheric relations become increasingly embedded within institutions of governance, new narratives of and for applied climatology are emerging to champion particular atmospheric objects, orientations, practices and institutions into positions of policy relevance and investment priority. This paper attempts to understand these intersecting politics of ‘climate and society’ research by situating their emergence through three lenses of inquiry. First, we explore the historical disciplinary work of ‘application’ in geographical climatology, paying particular attention to how ‘relevance’ has been understood and practised. Second, we reassemble a missed disciplinary conversation about ideology in applied geography, and link this to definitions and rationales for applied climatology. Third, we explore five recent thematic engagements in applied climatology, to generate thinking about the institutions and practices of assembling climate in new circles of ‘application’, policy and elsewhere. The ‘applications’ that climatologists choose to pursue – and the ways in which they pursue them – are deeply political questions that reproduce decision-making logics, funding rationalities, notions of expertise and problem framings. In conclusion, we argue that, rather than understanding ‘climate’ and ‘society’ as stable entities with standard (e.g. quantitative) practices or modes of association, we might instead concern ourselves with the practices of assembling human-atmospheric relations.