MEASUREMENT OF O-3 AND RELATED-COMPOUNDS OVER SOUTHERN NOVA-SCOTIA .2. PHOTOCHEMICAL AGE AND VERTICAL TRANSPORT

Chemical measurements of O-3 and other trace substances in the atmosph ere over southern Nova Scotia, obtained during 48 flights of the Natio nal Research Council of Canada Twin Otter aircraft, provide evidence f or a variable degree of processing associated with transport times ran ging from less than 1 day to 5 days or more. Effects of chemical aging and dilution are determined using photochemical age estimates derived from the ratios. In (n-butane/propane) and In (i-butane/propane). Age estimates are used in a qualitative sense to divide the data set into four age groups. Vertical profiles and relations between O-3 and othe r trace substances are examined as a function of age group. We find th at high-O-3 events occurring in dry air are in the oldest age category . Moist high-O-3 events have photochemical ages that span all four age categories, but the most polluted episodes are only observed in relat ively ''new air.'' A geographically wide range of emission regions is suggested by the hydrocarbon measurements, which is consistent with ba ck trajectory results. The relative depletion of soluble substance (i. e., aerosol particles and NOy) in dry, relative to moist, high-O-3 air masses is discussed with respect to the dilution or precipitation sca venging that must accompany the transport of pollutants from the bound ary layer to the dryer middle or upper troposphere. Four case studies are presented that span a wide range of conditions associated with hig h O-3 concentrations. In a pair of these cases, samples were obtained in air masses having about the same photochemical age, altitude, and C 2H2 and O-3 concentration. One of the pair was from dry air and the ot her from moist air. The dry air mass had much lower concentrations of NOy and aerosol particles, which was interpreted as evidence for the s elective removal of soluble constituents during vertical lifting. The other case studies illustrate a biomass burning plume and the effects of a stable layer over the ocean on transport.