The nitric acid data obtained by tunable laser diode spectroscopy (TDLAS) in October 1993 in Azusa, California were reviewed to evaluate their accuracy and hence suitability as a reference standard for the measurements made by CADMP, SCAQS-type and two-week denuder samplers. The most consistent sets of HNO3 data for comparison were those for the October 13 to October 24 period, which showed that the TDLAS results were about 30% higher than the SCAQS values and approximately 45% greater than the CADMP values (the latter were average of measurements by the primary and secondary CADMP samplers). The two-week sampler measurement was 30% higher than the TDLAS integrated value for the same two-week period. Based on previous observations of the daytime nitric acid to ozone ratios in Azusa and other locations in the South Coast Air Basin, the TDLAS nitric acid measurements in Azusa during the 1993 study were significantly higher than expected. An HNO3 : O3 ratio of 0.1 with good correlation was shown by the secondary CADMP sampler, thus indicating a high probability that its measurements for the total period July-October 1993 were correct. However, the variability in performance by the CADMP design became a question due to the erratic measurements by the primary CADMP sampler during the same period. The TDLAS HNO3 time profiles, when examined with those of the concurrent O3 and PAN profiles, showed no evidence of a nitric acid adsorption / desorption process occurring along the sampling train nor evidence of nitric acid vaporization from particulates on the Teflon front-filter. No errors in the overall TDLAS calibration was found. The possibility of interferences is raised by the presence of anomalous peaks on a number of the TDLAS HNO3 time profiles and by measurements preceding the study period which found elevated levels of carbonyl group-bearing solvents (e.g., methyl ethyl ketone) which could be traced to one or more of several industrial operations surrounding the Azusa site.
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