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TIME, TEMPERATURE, AND WIND SPEED: THE IMPORTANCE OF VARYING EMISSION RATES IN ODOR MODELING

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Dispersion model results, used to predict odor impacts, can vary dramatically depending on the emission rates input. Emission inputs are often based directly on measured values, assumed to remain constant for an entire model simulation, typically run for at least one full year of meteorological conditions. Actual emission rates, particularly from areal sources, are strongly correlated with meteorological parameters such as temperature and wind speed. Emissions may also vary with time of day, as wastewater strength varies. Thus, predicting odor impacts based on a constant measured emission rate may result in significantly underestimating or overestimating ambient concentrations.

In this paper, air dispersion modeling is presented for a wastewater treatment plant (WWTP), for which emission rates are varied according to wind speed, ambient temperature, and time of day. Measured emission rates at the WWTP were collected using a flux chamber device, and ambient concentrations and temperatures were recorded over a multi-day period using a datalogging monitor. Based on the field measurements, correlations of emissions with time of day and temperature were developed, and used to create an hourly emissions file for input to the model. Finally, flux chamber emission measurements were adjusted to account for the impact of wind speed variations on the actual emission rate according to a study conducted by Jiang and Kaye (2000).

Model results are presented for the following cases:



Base Case. Constant emission rates based directly on flux chamber measurements;


Peak Case 1: Time and temperature adjustments. Peak constant emission rates, with flux chamber measurements adjusted to the peak time on a day with 100°F temperatures;


Peak Case 2: Time, temperature, and wind speed adjustments. Peak emission rates, with flux chamber measurements adjusted to reflect not only the peak time on a day with 100°F temperatures but also to account for the impact of wind speed on emissions;


Realistic Case: Hourly time, temperature, and wind speed adjustments. Emission rates varied hourly according to temperature, time of day, and wind speed. This should provide the most realistic model scenario.


It was found that adjusting emissions by temperature and time of day to mid-afternoon on a hot day, and increasing emissions from area sources according to wind speed, represents a worstcase condition (Peak Case 2). It greatly overestimates emissions, however, compared with a more realistic scenario, which varies emissions by hour of day and temperature, as well as wind speed, over the course of a year (Realistic Case).

Using direct flux chamber measurements without accounting for wind speed effects greatly underestimates emissions compared with the Realistic Case, even if the flux chamber measurements are adjusted to afternoon on a hot day (Base Case without temperature adjustment and Peak Case 1 with temperature adjustment). Hence, accounting for wind effects on area sources is critical.
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Document Type: Research Article

Publication date: 01 January 2002

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