We plan to write an article about this soon, but in the interim, here are a few bullet points:
* this is also referred to as Method 9 (found in 40 CFR part 60)
* there are many, many criterium required to perform a ‘legal’ visual observation
– time of day
– relative humidity
– temperature
– wind direction
– sun position
– weather conditions
– status of your EPA VEO certification
– distance to emission
– angle of incidence
– background contrast used
– white smoke or black smoke
– (will look up the other criteria and post it here)