This case study reports the investigation of a Christchurch company who needed to have discharge consent in their operation of drying wood chips with the primary source of power coming from a diesel generator.
AERMOD, a key tool in this assessment, is a steady-state dispersion model designed for short-range dispersion of air pollutant emissions from stationary industrial sources.
The need for this consent application came about with the implementation of the Canterbury Air Regional Plan (CARP) by the Canterbury Regional Council (CRC) in late 2017. To meet requirements for a discharge consent it needed to be proven that offsite concentrations of would not exceed any relevant criteria within policy. This included emissions of volatile organic compounds, notably formaldehyde, as well as other pollutants such as NO2, SO2, CO and Particulate Matter (PM10). As such NZ Air was commissioned to model emissions from the plant to provide evidence to meet CARP regulations. AERMOD was primarily used in this modelling process.
AERMOD modelling was based on basic details about the drying plant (i.e. flow rate of air throughout the system, concentrations of pollutants in the air flow), and also included meteorological data taken from the local area. Background concentrations of the air pollutants in question were also considered and compared to the final outputs.
The modelling approach for this case study was intentionally conservative, with continuous emissions modelled for a full 3 year period using maximum rated airflow/operating capabilities for the woodchip drying system. While this was not a realistic representation of the actual operations on the plant, it provided a high level of confidence in the results that were received.
The study was undertaken largely to assess the exposure to discharges derived from the drying plant on nearby discrete receptors that may be sensitive to changes in air quality. For this component a desktop study was completed identifying these neighbouring sites. This aspect of the study was then compared to modelled results with the outputs from AERMOD spatially comparable to the key sites.
To estimate the likely background concentrations for NO2, CO, PM10 and SO2 surrounding the Maugers site, NZ Air collated data collected from the Christchurch Burnside monitoring station (located approximately 8.5 km northeast of the Maugers site). It was considered that the upper quartile monitoring data from Burnside was representative of background concentrations surrounding the site.
The background concentrations of formaldehyde surrounding the site are considered to be very low given the lack of processes discharging formaldehyde in the vicinity of the site. However, the predicted concentrations of formaldehyde at the nearest residential receptors were so low that even if a conservative background was to be added to the conservatively predicted maximum concentrations the resulting cumulative concentration would still be below the relevant regulatory criteria.
Example of AERMOD outputs - Predicted formaldehyde 1 hour concentration (µg/m3)
In the case of this Christchurch companies air quality issues AERMOD was able to model results effectively and showed that even when highly conservative modelling was used the emissions from the drying plant did not exceed any of the relevant policy or conservative criteria, this statement hold true even when also conservative background figures are added.
AERMOD is a steady-state plume model that incorporates air dispersion based on planetary boundary layer (PBL) turbulence structure and scaling concepts, including treatment of both surface and elevated sources, and both simple and complex terrain.
Air Quality Assessment of Environmental Effects. (2018): Report prepared by NZ Air Ltd