It is important to recognize, however, that regional hazes are not necessarily caused by local emissions, nor do they depend on stagnant meteorological conditions. Studies have shown that regional hazes are often associated with transport from dis- tant sources. Airport visual range measurements are based on the identification by human observers of targets at known distances from the observation point, whereas standard visual range is calculated from light extinction measurements. However, airport observations have been calibrated against the results of instrumental studies for standard visual range, and the airport data presented in these figures have been adjusted accordingly. In the adjoining regions to the north and south, median standard visual range is also quite good, exceeding 100 km.
Observations show a distinct relationship between visual range and altitude.
The remaining mass and the nonsulfate hydrogen are believed to be qualita- tively related to the spatial distribution of organic aerosols. Do Cat . Livestock waste management operations are the dominant source. In the adjoining regions to the north and south, median standard visual range is also good, exceed- ing 100 km. Median visual range falls to less than 50 km along the northern Pacific coast, less than 30 km in the central valley of Califor- nia, and less than 15 km in the Los Angeles basin.