Diurnal and seasonal patterns in light scattering, extinction, and relative humidity

Since 1988, several federal and state governmental agencies in the US have coordinated efforts to operate the interagency monitoring of protected visual environments (IMPROVE) network at sites in remote areas. Most IMPROVE sites are equipped with either a transmissometer to measure light extinction (Bext) or a nephelometer to measure particle scattering (Bsp). Optical, temperature, and relative humidity (RH) measurements are made hourly at these sites. The diurnal and seasonal patterns in these data are examined and discussed here. At many IMPROVE sites the diurnal patterns in RH and therefore Bext or Bsp are as expected based on average temperature. On average, RH is higher at night and during the winter than during warmer times of the day and year. Also as expected, based on RH alone, at many sites hourly mean Bext or Bsp values are either in phase with RH or weakly dependent on time of day. Usually, the diurnal differences are not as large as the seasonal differences. Another group of IMPROVE sites have mean RH patterns similar to those described above but have a different diurnal pattern in measured scattering or extinction. At these sites, the highest mean Bsp or Bext occurs during mid-day rather than at night. At several of these sites, especially those on ridge tops, it is hypothesized that this is because the diurnal shifts in mixing height only allow the surface layer of the atmosphere to reach the monitor during mid-day. Several other sites have unique diurnal or seasonal patterns in average Bsp or Bext that can usually be linked to emissions in nearby source regions or local meteorology and terrain.