Appendix 4.7    Conversion of ISCST3 Dispersion Model Outputs to 5-second Average Concentration



In accordance with Annex 4 of the Technical Memorandum on Environmental Impact Assessment Process, the assessment criterion for odour impact assessment is 5 odour units per cubic metre (OU m-3) based on an averaging time of 5 seconds. The results of the ISCST3 dispersion model are the maximum odour concentration predicted at the ASRs based on an averaging time of 1 hour. The modelled hourly-average results are converted to 5-seconds average values to enable direct comparison against the odour assessment criterion. The conversion process is described as follows.


The ISCST3 model outputs of hourly-average concentration correspond closely to a maximum 3-minute average. D. Bruce Turner of Trinity Consultants has given a detailed account on this discrepancy which is related to the Pasquill-Gifford vertical dispersion parameter used in the ISCST model[1]. In order to provide an additional margin of safety, the concentration predicted with the ISCST3 model is conservatively assumed to be equivalent to a 15-minute average concentration in the study. This approach was also adopted in the odour impact assessment of the EIA study for the Northshore Lantau Development Feasibility Study[2].  This EIA report was approved by EPD on 28th April 2000.


Conversion of the ISCST3 model outputs to 5-second average concentration is conducted with reference to the Guidelines[3] provided by the Environmental Protection Department.  The model output (15-minute average) is first converted to 3-minute average value using the power law formula proposed by Duffee et al[4] given below:


        Cl = Cs(ts/tl)p


where       Cl = concentration for the longer time-averaging period;

                Cs = concentration for the shorter time-averaging period;

                ts = shorter averaging time;

                tl = longer averaging time; and

p = power law exponent which is dependent on the Pasquill stability class, 0.5 for Class A & B, 0.333 for Class C, 0.2 for Class D, and 0.167 for Class E & F.


A second conversion factor (10 for unstable conditions and 5 for neutral to stable conditions) is then applied to convert the 3-minute average to 5-second average as suggested by Keddie[5].  It should be noted that this conversion factor refers to the ratio of the peak to mean concentrations within any 3-minute period for stack emissions; and the study by Warren Spring Laboratory found that the larger ratios of 10:1 will tend to occur within a few chimney heights of the source. With the emissions from area source on ground level, which is the case for odour emission from WFM, the peak to mean ratios will be further reduced. Nevertheless, this conversion factor is incorporated into the model outputs as a conservative approach.


The overall factors adopted in converting the ISCST3 model outputs to maximum 5-second average odour concentrations are given in Table 1 below.


Table 1    Factors for converting ISCST3 Model Outputs to Maximum

5-second Average Odour Concentration


Pasquill Stability Class







Conversion Factors












[1] D. Bruce Turner, Workbook of Atmospheric Dispersion Estimates, Lewis Publishers

[2] Construction of an International Theme Park in Penny’s Bay of North Lantau together with its Essential Associated Infrastructures – Environmental Impact Assessment, Final Environmental Impact Assessment, Scott Wilson (Hong Kong) Ltd.

[3] Modelling Section, Air Policy Group, Environmental Protection Department, HKSAR, Guidelines for Local-Scale Air Quality Assessment Using Models

[4] Richard A. Duffee, Martha A. O'Brien and Ned Ostojic, 'Odor Modeling - Why and How', Recent Developments and Current Practices in Odor Regulations, Controls and Technology, Air & Waste Management Association, 1991.

[5] A.W.C. Keddie, 'Dispersion of Odours', Odour Control - A Concise Guide, Warren Spring Laboratory, 1980.