Increased biomass burning (e.g., forest fires, controlled burns, etc.) and anthropogenic emissions into the earth’s atmosphere in the past century have led to much debate with regard to greenhouse gases, atmospheric carbon buildup, and global warming. Atmospheric aerosols have been linked to reduced air quality and visibility (V) in many parts of the world. In this work, visibility data have been acquired from a few major stations along or near the U.S. East Coast (5 stations) and along or near a similar geographic area in East Asia (9 stations). Two time periods were chosen to contrast the atmospheric visibility reduction/increases for these periods for two widely separated parts of the world.

Data were taken for the U.S. stations from 1980 to 1983 and from 1990 to 1994. The first period represents a time before major changes were made to improve air quality and visibility, while the latter period was chosen to show how reduced aerosol emissions due to legislative influences might have improved air quality and visibility. A more recent time period for the US was avoided due to changes in visibility data collection (mainly due to automated weather stations). For East Asia, data were taken from 1980 to 1983 and from 1998 to 2002. The visibility data have been filtered to eliminate influences due mainly to precipitation.

The aim of this study was to examine differences in visibility in the two regions before and after the factors influencing aerosol emission changed. Averages on an annual basis over the 4 or 5 year period reveal an increase in high V ( 20 km) for the U.S. stations and a decrease for the East Asian stations. On the other hand, low visibility occurrences ( 5 km) decline in the U.S. and increase in East Asia. Improvements in the U.S. are not as dramatic as the declines in East Asia. The largest visibility declines in East Asia occurred along the southern part of the region.

Besides examining visibility for different seasons (as has been done in previous work), the results in this study were examined for six different times of the day in an effort to see diurnal patterns. Results are demonstrated using a ‘visibility rose’ that shows the frequency of the wind direction and the corresponding visibility. The coastal influence on visibility was found to be very important. For the U.S. stations the best visibility occurred for the early morning hours while the worst visibility occurred in the late evening, especially during summer. This is likely due to lower wind speeds thus limiting transport of aerosols in the atmosphere. For the East Asian stations, visibility tends to be enhanced most during the early evening hours of summer, a direct result of sea breeze scouring of the atmosphere. In contrast, the stations along the southern region (where industrialization is most predominant) show reduced visibility during the nocturnal period when winds tend to originate from land regions.

The influence of different climatic regimes on visibility, as indicated by conventional wind roses, was also explored using unfiltered data for the same time periods.