Application of natural radionuclides for determination of tropospheric ozone and aerosol transport.

Natural radionuclides have been proposed for use in assessing the transport of ozone and aerosols in the troposphere. For example, {sup 7}Be is known to be produced in the upper troposphere and lower stratosphere by interactions with cosmogenic particles. Beryllium-7 has a 53.28-day half-life and is a gamma emitter that attaches itself to fine particles in the atmosphere once it is formed. Indeed, in tropospheric aerosol samples TBe is typically found in association with aerosol particles that are 0.3 {micro}m in diameter. Some investigators have asserted that ozone from aloft can be transported into rural and urban regions during stratospheric/tropospheric folding events, leading to increased background levels of ozone. During the Texas 2000 Air Quality study, aerosol samples with a 2.5-{micro}m cutoff were collected during 12-hour cycles (day/night) for a 30-day period at the Deer Park, Texas, field site in August-September 2000. To monitor {sup 7}Be levels, high-volume samples were collected on glass fiber filters on Julian dates 225-259. Sample collection was at a field site near a city park, away from any nearby traffic. This site is under routine operation by the Texas Natural Resource Conservation Commission. Instruments operated at this same site during the study period included anmore » ozone monitor (Dasibi), a nitrogen oxides instrument (API), a CO instrument (API), a nephelometer, a UV-B meter (Richardson-Berger), and a multifilter rotating shadow band radiometer (MFRSR, Yankee Environmental Systems). In addition, we made modified fast-response NO{sub 2} and peroxyacetyl nitrate (PAN) measurements by using a fast gas chromatography with luminol detection, to be described at this meeting (3). The results for {sup 7}Be (mBq m{sup {minus}3})are compared in Figure 1 with the maximum and average ozone values (ppb) observed at the site to identify potential correlations. In Figure 2, all of the {sup 7}Be data are plotted against the maximum and average ozone concentrations observed for 12-hour intervals during the study period. The data show no strong correlation between observed {sup 7}Be at this site and 12-hour maximum or 12-hour average ozone levels. Further analysis of the daytime and nighttime data indicated a weak correlation during the daytime. This may be because the transport of dry air over the area increases photochemical activity and is associated with upper-air-mass events, as indicated by increased {sup 7}Be at the ground stations. The levels observed during the Texas Air Quality 2000 study at the Deer Park and nearby Houston sites are far more likely to reflect local photochemical formation of ozone. Indeed, the release of molecular chlorine from local sources can lead to rapid formation of peroxy radicals by the reaction of photochemically produced Cl radicals with alkanes. This reaction proceeds hundreds of times faster that the normal OH chemistry.« less