Research Projects

Project at a Glance

Title: Southern California air quality study: peroxyacetyl nitrate (PAN) measurements 1989.

Principal Investigator / Author(s): Williams, Edwin L, II & Daniel Grosjean

Contractor: Daniel Grosjean and Associates

Contract Number: A6-099-32


Research Program Area: Atmospheric Processes

Topic Areas: Field Studies, Monitoring


Abstract:

As part of the Southern California Air Quality Study (SCAQS), Daniel Grosjean and Associates, Inc. (DGA) has carried out measurements of ambient levels of peroxyacetyl nitrate (PAN) in the South Coast Air Basin (SCAB). PAN is a mutagen, a strong eye irritant, and a phytotoxic compound. PAN has no known direct sources. Like ozone, PAN is formed in-situ in the atmosphere as a product of hydrocarbon-NOx reactions. PAN is a major "carrier" for transport of atmospheric nitrogen on a regional scale. It is now recognized that PAN is of major importance (along with ozone) when evaluating the performance of air quality computer kinetic models.

As part of the SCAQS field measurements, PAN was measured simultaneously at nine (June and July 1987), seven (August and September 1987) and five locations (November and December 1987) as well as on board the two SCAQS aircraft. Ground-level measurements of PAN were carried out by electron capture gas chromatography (EC-GC). Calibration involved two methods, NOx chemiluminescence and liquid chromatography analysis of the PAN alkaline decomposition products. The latter method was also employed to measure PAN aloft.

Short, informal comparisons were carried out in the field during SCAQS. These limited studies suggest substantial differences between the EC-GC method and two variations of the luminol method. A more comprehensive comparison was carried out at the EPA laboratory in North Carolina. This study indicated good agreement between three PAN calibration methods, i.e. infrared spectroscopy, NOx chemiluminescence, and alkaline hydrolysis. Examination of a limited subset of ambient PAN data indicated that the corresponding measurement methods, both involving EC-GC, yielded ambient PAN data that were within 25% of each other, with good tracking of the PAN diurnal variations. As PAN measurements and the corresponding calibrations are by no means of a routine nature and are performed only by a small number of research groups, the 25% difference in reported ambient PAN concentrations appears reasonable and probably reflects the current state of measurements capabilities among laboratories that are proficient at measuring PAN by EC-GC.

A search for methyl nitrate, CH3ONO2, was also carried out. If present in ambient air, methyl nitrate would be recorded along with PAN under our EC-GC conditions. Methyl nitrate was synthesized, and response vs. concentration calibration curves were constructed using two methods, EC-GC and NOx chemiluminescence. Retention times of methyl nitrate and PAN were established for all field instruments Interferences from a number of chlorinated hydrocarbons were examined in detail. A search of the some 3,500 chromatograms acquired during SCAQS yielded only seven possible observations of methyl nitrate at concentrations exceeding our detection limit of 0.2 ppb. Thus, methyl nitrate was not abundant in the SCAB atmosphere during SCAQS.

While PAN has long been observed in Southern California air, our study yielded for the first time information on the spatial and seasonal variations of PAN in the South Coast Air Basin. The highest PAN concentration during SCAQS, 30 ppb, was recorded at the Claremont site. Summertime levels of PAN exhibited a strong increase from coastal to inland locations (PAN levels at the "control" site, San Nicolas Island, were < 1 ppb), and a corresponding shift in daily maxima from mid-day at coastal sites to late afternoon inland. Thus, spatial variations of PAN during the summer were consistent with considerations of photochemical production during transport. In contrast, levels of PAN during the fall phase of SCAQS were high at the coastal locations, e.g. 13-19 ppb at all five winter SCAQS sites on December 3, 1987. PAN concentrations aloft were up to 30 ppb and were comparable to (and often higher than) those measurement at ground level.


 

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