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Smoke Science: Are Visual Estimates of PM2.5 Reliable?  

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Are human visual ratings a reliable estimate of PM2.5? 
 
Data Analysis and Literature Review with THREE featured documents  
FAST FACTS:
 

WHY IS THIS WORK IMPORTANT?

In situations where wildland fire has the potential to affect public health and air quality monitoring data is not available, some officials are turning to human observed visual range estimates (and associated particulate matter concentrations) to inform their public health warnings.  However this method may not provide reliable results when used in conditions different from the unique environment it was originally designed for.    

WHAT DID WE LEARN?

  • There are five uncertainties that come into play when trying to estimate a visual range and the associated particulate matter (PM) level.  When all of these are added together particulate matter estimates could be off by a factor of 2
 
  • Visual range/particulate concentration relationships could be improved by including location, and; PM estimates could be improved by incorporating a measure of relative humidity.  

 

  • If observed visual range is going to be used to estimate the short-term (1-3 hour) average PM concentration, the air quality warning system should consist of no more than two levels of warning instead of the five currently used. 

 

  • Visual range should not be estimated by a human observer.  There are more scientifically defensible procedures that could be used/developed. 

 

MANAGEMENT IMPLICATIONS

  • A simple alternative to the current method would be to develop “contrast cards” that can be visually compared to landscape features allowing for a better visibility estimate and in turn a better particulate estimate.
 
  • A smart phone app could be developed to directly measure landscape feature contrast, which would allow calculation of visual range and therefore more accurate assessment of particulate concentration. 
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Final Report Figure 2.3:  Diagram showing how path radiance and attenuated image-forming information combine to form the observed image at the eye of an observer. Direct, diffuse, and reflected radiance all contribute to illuminating the image, as well as the incremental volumes of atmosphere that sum to form sight-path radiance.

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