A newly constructed DOAS is described. The instrument was specially designed for making automated
measurements over diurnal cycles in relatively clean environments, and has been employed since 1989 to measure NO3,
NO2, 03 3fld CH2O. A comparative study of NO3 chemistry in two very different environments is presented. The first
study took place in the marine boundary layer over Biscayne Bay in South Florida during 1989. Of particular interest
was to determine the importance of dimethyl sulfide (DMS) as a sink for NxOr through its rapid reaction with NO3.
Modelling the observations indicates that in the clean marine boundary layer, ([O2] 2 ppb), DM5 accounts for up to
about 50% removal of NxOr whereas in more polluted conditions, ([NO2] 25 ppb), DMS only removes about 10%.
The other significant sink is probably adsorption of N205 onto moist aerosol surfaces. The second study took place near
Edison, California, as part of the San Joaquin Valley Air Quality Study. At this dry and polluted location, NO3 exhibits
marked variability during the night, although rarely exceeding 80 ppt. The presence of very high levels of industrial and
agricultural hydrocarbons appears to limit both the maximum concentration and the duration of significant levels of