Research Projects

Project at a Glance

Project Status: complete

Title: Evaluation of NOy and nitric acid measurement methods and collection of ambient data.

Principal Investigator / Author(s): Fitz, Dennis R.

Contractor: UC Riverside

Contract Number: 98-341


Research Program Area: Atmospheric Processes

Topic Areas: Chemistry & Reactivity, Modeling


Abstract:

California has an ambient air quality standard for nitrogen dioxide (NO2), and oxides of nitrogen are also critical participants in atmospheric photochemistry to form ozone. Ambient air quality monitoring networks use chemiluminescent analyzers that alternately measure NO and all nitrogenous species that are converted to NO by a heated catalyst. The difference is defined as NO2. In the atmosphere these nitrogenous species (NOy) consist of, in addition to NO and NO2, nitrogenous acids (nitric and nitrous), ammonia (NH3), and organic nitrates. Dual converter NOx analyzers were used during SCOS97-NARSTO to measure NOy and nitric acid. These instruments were refurbished by replacing or reactivating their reduction converters and thoroughly evaluating their efficiency to convert nitrogen-containing species. These instruments are equipped with dual converters with one channel fitted with a NaCl-coated fabric denuder to selectively remove nitric acid (NOy*). Since the instrument automatically cycles between the two channels, the difference is a measure of nitric acid. A pair of collocated analyzers was set up at the CE-CERT laboratory in Riverside and a single analyzer was installed in Azusa at the monitoring site operated by the South Coast Air Quality Management District (SCAQMD). Data collection started April and October 2000, respectively, for the Riverside and Azusa measurement sites. Data were compiled and validated until April 2001. NOy values at Riverside ranged up to 300 ppb, and values at Azusa reached up to 500 ppb. The Azusa NOy and NOx reported by the SCAQMD compared quite closely. Since it was not possible to have both analyzers sample the same span gas, the significance of differences between the two approaches could not be evaluated. The diurnal pattern of nitric acid was observable nearly every day during the smog season. The concentrations at Riverside ranged up to 6 ppb in the summer. Despite the measurement period at Azusa not including the summer smog season, nitric acid concentrations as high as 8 ppb were observed. A thermoevolution denuder (TED) to measure nitric acid was evaluated under laboratory conditions. This device uses a special ceramic fabric denuder coated with sodium chloride to selectively remove nitric acid. Since this fabric strongly adsorbs microwave radiation at the same
wavelength as that for cooking, the fabric can be heated rapidly and controllably to high temperatures using a standard microwave oven. This heating will decompose the collected sodium nitrate to NOx. While this design allowed the denuder to adsorb nitric acid, the amount recovered was low and variable. We found that the denuder was not evenly heated, and the amount recovered when drops of dilute nitric acid were placed on the denuder depended on the location on the denuder that was used. It was likely that the microwave oven, which was a consumer product, was not designed to produce a uniform microwave field. We concluded that additional research was needed before the system was ready to evaluate with other methods of measuring nitric acid. This report included the initial year of NOy and nitric acid data. We continued to maintain the NOy analyzers and added NO2/PAN gas chromatographs at both sites. These chromatographs separate NO2 and quantify the amount using luminol detection. This selective method of measuring NO2 will be used to quantify the amount of NO2 contained in the NOy. Under additional funding these instruments will be maintained until October 2002. The results from this period will be the subject of a final report for the second period of funding.


 

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