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1. Background
A set of air quality objectives (AQO) which includes 10-minute averaged SO2 concentration is expected to be promulgated as reference for air quality impact assessment in Hong Kong.
Both the existing and the proposed AQO include a 1-hour averaged SO2 concentration. Current assessment practices are sufficient to address compliance with these standards. Addressing 10-minute averaged SO2 concentration will be treated as an extension of the current practice.
2. Calculating 10-minute averaged SO2 Concentrations
EPD's guidelines on air quality assessment recommend a three-tier approach to arrive at the total impact (Guidelines on Assessing the 'Total' Air Quality Impacts).
To achieve a conservative estimate of SO2 impacts, chemical conversion of SO2 will not be considered in the assessment.
Numerical models are usually used to estimate concentrations of pollutants based on emissions and meteorological data. Inputs to these models are usually averaged over 1 hour and the outputs are correspondingly hourly averages. In some assessments, the third-tier contribution is taken from measurements which are also hourly. As such, the proposed method still applies.
The commonly adopted practice is to add up the contributions of all three tiers to get the 'total' air quality. For hourly concentrations, these contributions are added up hour-by-hour, i.e. synchronised in time.
Winds fluctuate in time. The shorter the averaging time, the larger is the fluctuation. In the same way, concentrations of the pollutant carried by the fluctuating wind also fluctuate, resulting in higher peak concentrations with shorter averaging time. The fluctuation of the winds in turn varies with how conducive the atmosphere is to dispersion, i.e. atmospheric stability.
Conservative (erring on the high side) factors for converting 1-hour averaged concentration to shorter duration averaged concentration have been developed.
The following stability-dependent multiplicative factors from Duffee et al. (1991) have been widely used and are adopted here:
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Stability class
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A
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B
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C
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D
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E
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F
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Conversion Factor
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2.45
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2.45
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1.82
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1.43
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1.35
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1.35
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The three-tier total hourly SO2 concentration can be multiplied to the above factors according to the prevailing atmospheric stability class to obtain the 10-minute SO2 concentration.
Some Lagrangian models have built-in options to output 10-minute averaged concentrations from hourly meteorological and emission inputs. These 10-minute SO2 concentration outputs can be used directly as the first two tiers' contributions. In this case, the above stability-dependent factors only have to be applied to the third tier.
EPD will consider proposals of alternative methodologies or conversion factors for determining 10-minute SO2 concentration based on appropriate local measurements and sound scientific reasoning.
References
Richard A. Duffee, Martha A. O'Brien and Ned Ostojic (1991) Odor Modeling - Why and How. Page 295, Recent Developments and Current Practices in Odor Regulations, Controls and Technology. Air & Waste Management Association, 1991.
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