The outdoor horizontal and vertical variations of respirable suspendedparticulate concentrations within a densely urban environment in HongKong: application of a box and plumedispersion model (airGIS/OSPM)

Abstract

Background

There have been many multicity studies assessing health effects of the

population’s exposure to PM10. They assume that there is homogeneous exposure to PM10

and the monitoring locations are representative of the population’s exposure. In a

densely urban environment, like Hong Kong, street canyons are common and could alter

PM10 exposure. The horizontal and vertical dispersion of PM10 in the urban environment

is complex due to the interactions of street geometric, metrological and pollution source

conditions. The airGIS/OSPM is a box and plume mathematical model which was

designed to model this complex dispersion of PM10.

Objectives

This study aims to utilize the airGIS/OSPM to assess how the health impact is

affected by the misclassifications of within city air pollution exposure. This helps

assessing the feasibility of using the airGIS/OSPM in health studies.

Methods

The airGIS/OSPM was used for an urban area of Hong Kong, in the north west of

Kowloon Peninsular for a period from 1998 to spring 2011. PM10 concentration’s were

estimated at field measurement points, an EPD roadside monitoring site, and subjects in a

subset of the elderly healthcare center cohort and for all building addresses in the

modeled area.

The airGIS/OSPM was validated by conducting a PM10 measurement campaign

over the winter of 2010 to 2011. Also validation was conducted using the measurements

of the Mong Kok EPD roadside monitoring station.

A pilot study using the time stratified case-crossover analysis was conducted to

explore the effects of using the airGIS/OSPM to express the subjects’ short-term outdoor

residential exposure to PM10 on all cause mortality.

AirGIS/OSPM PM10 estimates were compared with ambient concentrations

obtained from the EPD monitoring network. The population mean exposure was

calculated using the airGIS/OSPM estimated PM10 concentration at all building address

points from 2007 to 2009 at the ground level and at middle building height level.

The airGIS/OSPM estimate was used to identify spatial variation of PM10 within

the study area.

Results

The airGIS/OSPM estimated well the measured PM10 concentration from the field

measurement campaign and the EPD Mong Kok station.

The airGIS/OSPM estimate and the ambient measure for EHC subject exposure

both found an odds ratio for all cause mortality there was no difference from unity

between case and control times.

The airGIS/OSPM derived mean PM10 concentration at the middle height of each

building was 54.8μg/m3 while that at the all EPD was 54.4μg/m3. At ground level the

airGIS/OSPM PM10 estimate was 58.35μg/m3 while that at the EPD was 54.41μg/m3.

Conclusion

The misclassification of PM10 was negligible at the middle of buildings, but for

people regularly working in ground level microenvironments are often exposed to PM10

concentrations that are higher than those measured at EPD monitors.

It is feasible to use the airGIS/OSPM model to estimate PM10 exposure. The

small spatial variation in exposure means the airGIS/OSPM may not be appropriate in

assessing the short-term PM10 exposure, but due to the larger effect size it might be

important in long-term exposure assessment.