We plan to write an article about this soon, but in the interim, here are a few bullet points:
1. this is also referred to as Method 9 (found in 40 CFR part 60)
2. 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)