Research Projects

Project at a Glance

Title: Study of temporal and vertical ozone patterns at selected locations in California

Principal Investigator / Author(s): Pankratz, David V

Contractor: AeroVironment, Inc.

Contract Number: a132-165


Research Program Area: Atmospheric Processes


Abstract:

The California Clean Air Act requires that the Air Resources Board (ARB) address the issue of the transport of pollutants into air basins from other areas. ozone is a secondary pollutant that cannot be traced to any specific source. However, one can track the transport of ozone and its precursors from an area in order to estimate the effect of such transport on air quality. In order to make an accurate assessment of ozone transport, it is necessary to know the vertical distribution of the ozone concentrations with time. In order to assess the transport of ozone into the Sacramento Valley, it is necessary to calculate the flux. This requires that the vertical ozone distribution be known. As a check of accuracy, it is also desirable to know the ozone concentration at several locations along the transport pathway.

The objective of the ARB-sponsored monitoring effort described in this report was to measure, in real time, the vertical distribution of ozone up to 1600 feet at Walnut Grove and the surface concentration at Sutter Buttes (at an elevation of 2,300 feet). A second goal of the study was to investigate the possibility of using a single "switching" analyzer to perform monitoring at all levels by sequentially obtaining samples from each level.

Continuous ozone measurements were performed at Sutter Buttes using conventional ambient ozone monitoring equipment and methods. Ozone measurements at Walnut Grove were performed using two methods. First, each monitoring height at Walnut Grove had an ozone analyzer dedicated to continuous monitoring of ozone at that level, or a total of five "dedicated" ozone analyzers. Secondly, a sixth analyzer continuously "switched" from one sampling height to the next at two-minute intervals.

The sample residence times in the Teflon sampling lines for the monitoring equipment at Walnut Grove ranged from 45 seconds for the ground-level sampler to 7.5 minutes for the 1600-foot Ievel. Following the conditioning of the sample lines, an assessment of ozone losses in these sample lines was made prior to field use of the lines. The lines appeared to lose a constant 12 ppb of ozone plus 1.8 % of the measured concentration per 1000 feet of line. After field set up, line losses were checked over the course of the study. This testing showed the losses to be a relatively constant 3.7 % loss at all concentrations, with virtually no constant loss (losses that were not a function of the input concentration). These latter data were considered more representative of actual ambient monitoring conditions and were used to make corrections to the "raw" monitoring results.

A second problem introduced by the long sample lines was a decrease in the sample pressure within the lines due to friction. Vacuums within each sample line at the Walnut Grove site were monitored. The "span number" for each analyzer was set at the ideal value used for analyzers at sea level and corrections for sample line pressure decreases were performed during data processing. The situation for the "switching" ozone analyzer was different. The "switching" ozone analyzer operated at different sample cell pressures as it switched from one sample line to the next. To avoid additional complicated monitoring and data processing procedures, a Dasibi Model 1008 ozone analyzer was used, which included an internal absolute pressure sensor and internal circuitry to automatically correct the analyzer readings to the appropriate value for the sample cell pressure.

During the approximately two months of monitoring performed at the Walnut Grove site, several instances of sustained high concentrations of ozone aloft were noted. On two occasions, the high concentrations aloft appear to have significantly contributed to ozone exceedances at the surface. In contrast, ozone concentrations at the lower three levels follow a strong diurnal trend, with low concentrations in the night and early morning hours rising to the concentrations at the upper levels in the late afternoon. These data reveal a layer of high ozone aloft which has the potential of persisting for many days. The spatial extent of this polluted layer is demonstrated by the data from the Sutter Buttes site for these periods. Concentrations at the 2300-foot level approximately 50 miles north of the Walnut Grove site had similar high, sustained ozone concentrations. The most likely explanation for this phenomena is that high concentrations of ozone are being held aloft by a strong elevated inversion, and fumigated to the surface during the afternoon.

The data from the "dedicated" and "switching" ozone analyzers were compared to each other using linear regression plots for each sampling location at Walnut Grove. These plots demonstrated remarkably good agreement between the two measurement approaches throughout the monitoring range. The slopes were all within 7 % of an ideal slope of 1.00, and the intercepts are all less than 6 ppb, falling within the project's agreement goal of 10 %. These results are even more remarkable when it is remembered that the "dedicated" analyzers all had extensive pressure corrections applied to their data. To further put these results into perspective, if the "switching" analyzer was treated as a reference standard, all results would have met EPA criteria for audits.

Monitoring ozone in order to obtain vertical profiles is required in order to quantify the role of transport on ozone concentrations within an area. Long lines on a vertical tower provide a potential means of obtaining these measurements. Despite several complications inherent with this method, including line losses and sampling pressure changes, results indicate that long line sampling on a tower accurately provides these vertical measurements in a cost-effective manner. In addition, results indicate that this monitoring can potentially be performed using a single "switching" analyzer that sequentially samples each line, rather than multiple analyzers dedicated to each line. This alternative method could not only greatly reduce operating costs, but also eliminate any biases that may be encountered if multiple analyzers are used.

The importance of these measurements is demonstrated by the data obtained during monitoring. The vertical ozone profiles on several days show an elevated layer of high ozone concentrations over the Sacramento Valley. These data provided a mechanism for explaining ozone exceedances at the surface that could not have been identified using only surface measurements. These data demonstrate the continued need for these types of measurements.


 

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