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The impacts of personal mobility and diurnal concentration variability on exposure misclassification to ambient pollutants Rakefet Shafran-Nathan, -- Yuval, and David M Broday Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05656 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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Environmental Science & Technology

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The impacts of personal mobility and diurnal concentration

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variability on exposure misclassification to ambient pollutants

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Rakefet Shafran-Nathan, Yuval, David M. Broday*,

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Faculty of Civil and Environmental Engineering, Technion, Haifa, Israel

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All authors declare that they do not have any competing financial interests in the work

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and its results

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Key words: Air pollution; commute; exposure misclassification; integrated daily

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exposure; time-activity patterns

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Short running title: Spatiotemporal characteristics of exposure misclassification

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*Corresponding Author:

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David Broday

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Faculty of Civil & Environmental Engineering, Technion – I.I.T., Haifa, 32000, Israel

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Email: [email protected]

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Tel: +972-4-829-3468

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1 ACS Paragon Plus Environment


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Abstract

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Appreciating the uncertainty margins of exposure assessment to air pollution requires

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good understanding of its variability throughout the daily activities. This study

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describes a modeling framework for estimating exposure to air pollutants for a

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representative sample of working Israeli adults (N~168,000) for which both the

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residence and workplace addresses were available. Individual daily trajectories were

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simulated by accounting for five generic daily activities: at home, at work, while in

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commute from home to work and back, and during out-of-home leisure activities. The

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integrated daily exposure to nitrogen dioxide (NO2) was estimated for each individual

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by tracking the daily trajectory through an NO2 concentration map, obtained using a

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dynamic and highly resolved dispersion-like model (temporal resolution: half-hourly,

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spatial resolution: 500 m). Accounting for the subjects’ daily mobility was found to

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affect their exposure more significantly than accounting solely for the diurnal

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concentration variability, yet a synergistic effect was noted when accounting for both

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factors simultaneously. Exposure misclassification vary along the day, with the work

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microenvironment found to contribute the most to it. In particular, regardless of the

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high concentrations encountered during the commute, their contribution to the

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integrated daily exposure is small due to the relatively short time spent in this activity

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by most people.

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Introduction

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One of the challenges of a reliable exposure assessment is to integrate the pollutant

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concentrations over the time spent in each of the various microenvironments which

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the subjects probe along their daily trajectory 1. The most accurate way to do this is by

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direct measurement of the personal exposure. However, in practice this approach is

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limited to small study groups and for relatively short periods

2, 3

, which may not be

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representative of the whole population. Clearly this approach is not applicable to

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retrospective studies. Moreover, since tracking the individuals’ daily activities can

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usually be carried out only for relatively short periods, the data may not reflect

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seasonal variation in both space and in time 2. These shortcomings might prohibit

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achieving statistically significant results in environmental epidemiology studies

4-6

,

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since significant results require large number of cases/individuals. Thus, many studies

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resort to estimating the subjects’ exposure by models. In most cases, static

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concentration maps are used

4-6

. Namely, the short term (e.g. hourly) diurnal

concentration variability is smoothed out by long-term (e.g. daily) averaging.

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Reliable personal exposure estimates require detailed information on the time-

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location-duration activity patterns of the subjects throughout the day, which if

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combined with highly resolved pollutant concentrations in space and time can be used

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to derive accurate exposure estimates at the individual level. In particular, the former

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is normally related to the individuals’ commuting habits, e.g. travel mode, time of

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day, route, and the sequence of- and time spent in each microenvironment 7. A

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possible approach for deriving exposure estimates is to use the mean pollutant

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concentration at each microenvironment and the list of microenvironments visited

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throughout the day. Using this information, it is possible to approximate the integrated

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daily exposure 5, 7, 8. Yet, personal time-location-duration trajectories for large cohorts

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or on a population scale are generally missing. Therefore, in most epidemiological

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studies only the mean concentration at the residential address is used to represent the

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individual-specific exposure throughout the day 9. Recent studies proposed to account

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for different microenvironments in which the subjects were present when estimating

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exposure

7-12

. Few studies achieved this by directly modelling the subjects’ daily


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activity patterns 8 whereas other tracked the individuals’ mobility patterns via mobile

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telephone signals (either using GPS or triangulation) or by simulating commuting

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habits using, e.g., agent-based models 9, 11. In some cases, significant differences were

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found between the estimated exposures at the residence place and at the locations

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where the people were present according to the simulations

9, 12

. Moreover, although

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pollutant concentrations are known to vary throughout the day due to changes in the

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wind field, emission patterns, removal and fate processes, etc., the relative impact on

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exposure misclassification of the subject’s mobility and of the temporal concentration

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variation has been generally overlooked. Understanding the relative contributions of

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these factors to exposure misclassification is important, since accounting for each of

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them when estimating exposure may be time consuming and computationally

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demanding. This work studies in fairly detail and using a large real cohort exposure

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estimation differences that may occur when accounting for the persons’ mobility and

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the pollutant concentration variability throughout the day. Due to lack of complete

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information, as noted above, we tracked the cohort subjects’ daily trajectories while

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accounting for a limited set of typical microenvironments in which people tend to

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spend their time. The implication of our results for epidemiological studies is

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discussed.

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Methods

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Study area and population

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The study area covers Israel’s central coastal plain, from the city of Netanya in the

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north to the city of Ashdod in the south (70 x 25 km), including the Tel-Aviv

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metropolitan area. Geocoded home and work addresses of ~168,000 working adults

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(age 24-65) that live and work in the study area were obtained from the Israeli Central

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Bureau of Statistics (CBS). This database is a representative sample (both spatially

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and ethnically) of all the Israeli adult workers in this age group, and was generated by

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the Israel Central Bureau of Statistics (ICBS) based on the 2008 census. Due to

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privacy issues, the geocoding of the addresses was to the coordinates of the centers of

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the census tracts in which the home or workplace of each individual were located. To

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reduce the inherent inaccuracy of such a data aggregation, the centers of the census

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tracts were calculated while accounting only for the built area in each tract. Moreover,

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for subjects that reside or work in very dense urban areas, the coordinates of the

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centers of the buildings’ rooftops (home and workplace) were available.

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Pollutant concentrations

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The dynamic Optimized Dispersion Model (ODM)

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was used for calculating

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ambient nitrogen dioxide concentrations in the study area. The ODM accounts for

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temporal variation of the wind field at a half hourly resolution, and uses as a proxy of

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the spatiotemporal distribution of pollutant emissions traffic volumes in ~11,500 road

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network segments, obtained using an operative traffic assignment model and casted

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into structured grid (500 x 500 m). For every half-hour, the model optimizes the

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parameters of a nonlinear simple yet physically sound dispersion scheme, using the

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half-hourly monitoring records from 25 population monitoring stations as the

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dependent variables. The model then projects the concentrations to all the grid cells.

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The model was used for calculating half-hourly NO2 concentrations for the whole

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year of 2008, with its performance thoroughly evaluated

13

. In this work, the half-

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hourly concentrations at each grid cell were averaged over five time windows of

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typical activity periods, described in the next section, to produce a set of five

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concentration values assigned to each of the grid cells. The concentrations which we

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used, and the exposure derived from them for each subject based on the individual-

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specific simulated daily activity patterns, represent a typical day in 2008.

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Integrated daily exposure

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In general, personal exposure can be calculated as the integration of the pollutant

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concentrations to which an individual is exposed over contact-time increments. In

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practice, the exposure is usually estimated as a summation of the average

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concentrations that a subject encounters while spending time (over which the

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averaging is performed) at certain distinct microenvironments visited throughout the

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daily routine,

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E= ∙ ≈ ∑ ∙ = ∙ ̅ ,

(1)

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where E is the personal exposure to a given pollutant, ci is the average pollutant

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concentration in the ith microenvironment during the time the individual is present in

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the microenvironment, ti is the time spent by the individual in the ith

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microenvironment, T is the total exposure time, e.g. 24 h when the exposure is

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assessed over the entire day, and ̅ is the time-location-duration weighted average

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concentration to which the individual has been exposed during time-period T.

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Naturally, dividing Eq. (1) by T results in the daily average concentration, weighted

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according to the time spent in each microenvironment. The daily average pollutant

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concentration is oftentimes referred to as the daily exposure, with the common

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understanding that the actual exposure (in terms of dose) is the product of ̅ times T.

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For both simplicity and generalizability, we accounted for five typical daily activity-

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periods: at home, morning commute to work, at work, evening commute back home,

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and leisure. These activities are uniquely linked to the individual-specific location and

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time of the day, i.e. where, when and for how long these activities take place. It is

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noteworthy that since concentrations inside the homes, workplaces or transportation

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microenvironments were unavailable to us, the exposure of each individual is

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estimated based on the outdoor concentrations at these locations, similarly to the

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common practice of taking the outdoor residential concentration as a proxy of

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exposure.

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For simplicity, we consider only working days. Clearly, a true long-term

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exposure estimation should account also for the time-location-duration activities and

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concentration levels during weekends. Yet, this requires individual-level trajectory

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data and preferences that we did not have. The individual-specific workday daily

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trajectory was estimated based on general behavioral patterns in Israel (which may be

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relevant also for other places). All the study subjects were assumed to be at home for

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10 hours, from 21:00 until 07:00, and at work for 9 hours, from about 08:00 until

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about 17:00. The time at which each individual starts working varies slightly,

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depending on the individuals’ commute. As an initial guess, the morning commute

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was assumed to start at 07:00 and the evening commute was assumed start at 17:00,

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with both periods lasting for one hour. The leisure time completes the individual daily

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activity pattern, and was initially assumed to extend from 18:00 to 21:00. The

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individual-specific commute and leisure times were then modified as follows. An ad-

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hoc Python code was used to find the shortest route that connects the individual-

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specific home and work addresses (in practice, the centroids of the home and work

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census tracts), using the network analysis layer of Israel and the Network Analyst

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extension of ArcGIS 10.1 (ESRI, USA). The code outputs the shortest route for each

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person, accounting for the driving direction in one way roads and the average traffic

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conditions. The mean speed at each hour of the day along each segment of the road

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network has been obtained from on-board vehicle’s GPS signals, collected and

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processed by Decell Ltd, Tel Aviv, Israel. Consequently, the morning and evening

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commuting times of each individual were calculated based on the length and the mean

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driving speed at each road segment along the individual’s commuting route. Each

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route was divided into segments and the route mean traveling velocity or the

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concentrations were assigned to each segment vertices. The average pollutant

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concentration during the commute was calculated as the weighted average

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concentration, or velocity, in each road segment along the route during the commuting

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time.

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Since specific information on the leisure habits of the Israeli adult population is

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not available, the venue and duration of leisure had to be estimated for each

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individual. Based on subjective judgment, we assumed that Israeli adults normally

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spend their leisure time in areas close to their residences, either in commercial centers

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or in designated areas of cultural, recreational or sport activities. These types of

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leisure-activity areas were located in a GIS layer of Israel's land use map, and the

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nearest area within a distance of 3 km to the residence place of each individual was

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considered as the leisure time venue. For each individual, the leisure time was set

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such that it completes the 24 h daily cycle, with the time spent at home and at work

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fixed. Thus, when the total daily commute (home to work and back) was less (more)

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than two hours we added (subtracted) the extra time to (from) the daily leisure time.

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Exposure misclassification

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To assess the possible magnitude of exposure differences arising due to accounting

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for the daily mobility, temporal concentrations variability, or both, we compared four

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different exposure scenarios. In scenario (1), our benchmark, the 24 h exposure is

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estimated as the equally-weighted 24 h mean ambient pollutant concentrations at the

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subject’s home address. This scenario does not account for the subject’s mobility

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(designated here as static mobility, SM), and the concentration to which the subject is

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exposed is the daily average concentration (designated here static concentration, SC).

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Hence, this scenario is termed SMSC. The three other scenarios strive to better

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approximate the exposure E (Eq. 1) by considering mobility of the study subjects

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throughout the day in between the five

generic microenvironments (designated

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dynamic mobility, DM), or/and considering a dynamic concentration variability

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throughout the day (designated dynamic concentrations, DC). In particular, the

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alternative scenarios are: (2) static mobility and dynamic concentrations (SMDC), (3)

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dynamic mobility and static concentration (DMSC), and (4) dynamic mobility and

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dynamic concentrations (DMDC). Comparisons between the four scenarios are used

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for elucidating the effect of accounting for the individuals’ mobility and for the

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dynamic pollutant concentration field on exposure misclassification.

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The differences in exposure estimates based on the three more complex

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scenarios relative to the benchmark scenario are calculated for each person separately

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and presented as a frequency distribution. After normalization, the distributions

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represent the probability density functions (PDF) of the exposure estimation

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differences. To obtain a better insight into the specific contributions to the total

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differences between the exposure scenarios, we examined for each individual also the

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differences per microenvironment. Statistics used for studying the exposure

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differences include the root mean squared error (RMSE) and the coefficient of

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determination (R2).

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Sensitivity analysis

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A major shortcoming of the methodology we used to calculate the integrated exposure

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is the lack of actual information on the time spent by each individual in every

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microenvironment. To assess the uncertainty associated with our rather arbitrary daily

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trajectory assumptions (five general activities and durations), we carried out an in-

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depth sensitivity analysis. A random sample of 10,000 individuals was selected out of

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the total study population and for each of them we calculated the daily exposure based

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on 17 different time-activity patterns. Each of these patterns is a variation of the basic

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one (10 h at home and 9 h at work), with the time spent at home ranging from 8 to 16

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h at a half-hourly resolution, and the time at work varying, correspondingly, between

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11 and 3 h. Namely, in all these daily-activity patterns the time spent by any

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individual at both home and work always sums up to 19 h. The commute time was set

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based on the shortest route and average vehicle speed per road segment, as explained

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above. Hence, the commute time as well as the leisure time were both assumed to be

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independent of the sum of the two time periods spent at home and at work. The model

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sensitivity to the uncertainty in the time-activity patterns is reported in terms of the

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distribution of the coefficients of variation (CV) of the exposure estimates that were

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calculated for the randomly selected 10,000 individuals, based on the 17 time-activity

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patterns specified for each of them.

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Results

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Figure 1 depicts the sensitivity of the integrated daily exposure estimates to the time-

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activity pattern, based on the DMDC (i.e. most complex) scenario. The range of CV

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values (0-10%) suggests a relatively small sensitivity of the exposure estimates to the

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rather large uncertainty in the individual’s daily time-location-duration trajectory. We

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can thus expect that our exposure estimates under the DMSC scenario will also

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contain no more than 10% uncertainty due to the somewhat arbitrary choice of time

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periods spent at home and at work.

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Figure 1. Distribution of the coefficient of variation (CV) of the integrated daily

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exposure of a random sample of 10,000 individuals based on the DMDC scenario,

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with the individual-specific CV obtained by varying the home occupancy hours (at the

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expense of the working hours away from home), as part of the sensitivity analysis

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exploration (see Methods: Sensitivity analysis for more details).

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Figure 2a demonstrates the distribution of NO2 concentrations in the various

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microenvironments during the time-of-day spent in each of them by each subject. It is

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evident that the differences between the mean concentrations among the different

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microenvironments are small compared to the concentration variability within each of

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them. The wide concentration distribution in each of the microenvironments suggests

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that while the average concentrations in the different microenvironments are quite

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similar, some subjects may experience large concentration variation when moving

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between them. Figure 2b depicts the distributions of the exposures at the different

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microenvironments based on the DMDC scenario. The contributions of the exposure

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at each microenvironment (i.e. during different activities) to the integrated daily

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exposure can be clearly appreciated. Large and statistically significant (p

Impacts of Personal Mobility and Diurnal ... - ACS Publications

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