Opacity standards were set for visual emissions by U.S. Environmental Protection Agency (USEPA) to protect the ambient visual air quality. Method 9 is the standard method to quantify plume opacity visually by certified observers during daytime. However, the use of observations by humans to quantify plume opacity introduces subjectivity, and is expensive due to semi-annual certification requirements of the observers. In addition, sources may emit visible plumes during nighttime that also need to be monitored for compliance. Digital Opacity Method (DOM) was developed to quantify plume opacity from digital photos for both daytime and nighttime conditions. Past daytime field campaigns have demonstrated that DOM has advantages over Method 9 by its lower cost, improved objectivity, and availability of graphical records of the visible emissions. A nighttime field campaign was completed with the same Illinois Environmental Protection Agency smoke generator that was used during past daytime field campaigns at Springfield, IL. Once again, the individual and average opacity errors of DOM, when compared to results from a reference in-stack transmissometer, were within USEPA's error limits for Method 9. These successful results are encouraging and suggest that DOM can be used as an alternative to Method 9 during nighttime conditions.
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