Field evaluations of newly available 'interference-free' monitors for nitrogen dioxide and ozone at near-road and conventional National Ambient Air Quality Standards compliance sites

摘要

Long-standing measurement techniques for determining ground-level ozone (O_3) and nitrogen dioxide (NO_2) are known to be biased by interfering compounds that result in overestimates of high O_3 and NO_2 ambient concentrations under conducive conditions. An increasing near-ground O_3 gradient (NGOG) with increasing height above ground level is also known to exist. Both the interference bias and NGOG were investigated by comparing data from a conventional Federal Equivalent Method (FEM) O_3 photometer and an identical monitor upgraded with an "interference-free" nitric oxide O_3 scrubber that alternatively sampled at 2 m and 6.2 m inlet heights above ground level (AGL). Intercomparison was also made between a conventional nitrogen oxide (NO_x) chemiluminescence Federal Reference Method (FRM) monitor and a new "direct-measure" NO_2 NO_x 405 nm photometer at a near-road air quality measurement site. Results indicate that the O_3 monitor with the upgraded scrubber recorded lower regulatory-oriented concentrations than the deployed conventional metal oxide-scrubbed monitor and that O_3 concentrations 6.2 m AGL were higher than concentrations 2.0 m AGL, the nominal nose height of outdoor populations. Also, a new direct-measure NO_2 photometer recorded generally lower NO_2 regulatory-oriented concentrations than the conventional FRM chemiluminescence monitor, reporting lower daily maximum hourly average concentrations than the conventional monitor about 3 of every 5 days. Implications: Employing bias-prone instruments for measurement of ambient ozone or nitrogen dioxide from inlets at inappropriate heights above ground level may result in collection of positively biased data. This paper discusses tests of new regulatory instruments, recent developments in bias-free ozone and nitrogen dioxide measurement technology, and the presence/extent of a near-ground O_3 gradient (NGOG). Collection of unbiased monitor inlet height-appropriate data is crucial for determining accurate design values and meeting National Ambient Air Quality Standards.