Laboratory and field evaluation of crystallized DOW 704 oil on the performance of the Well Impactor Ninety-Six fFine particulate matter fractionator.

Subsequent to the 1997 promulgation of the Federal Reference Method (FRM) for monitoring fine particulate matter (PM2.5) in ambient air, U.S. Environmental Protection Agency (EPA) received reports that the DOW 704 diffusion oil used in the method's Well Impactor Ninety-Six (WINS) fractionator would occasionally crystallize during field use, particularly under wintertime conditions. Although the frequency of occurrence on a nationwide basis was low, uncertainties existed as to whether crystallization of the DOW 704 oil may adversely affect a sampling event's data quality. In response to these concerns, EPA and the State of Connecticut Department of Environmental Protection jointly conducted a series of specialized tests to determine whether crystallized oil adversely affected the performance of the WINS fractionator. In the laboratory, an experimental setup used dry ice to artificially induce crystallization of the diffusion oil under controlled conditions. Using primary polystyrene latex calibration aerosols, standard size-selective performance tests of the WINS fractionator showed that neither the position nor the shape of the WINS particle size fractionation curve was substantially influenced by the crystallization of the DOW 704 oil. No large particle bounce from the crystallized impaction surface was observed. During wintertime field tests, crystallization of the DOW 704 oil did not adversely affect measured PM2.5 concentrations. Regression of measurements with crystallized DOW 704 versus liquid dioctyl sebacate (DOS) oil produced slope, intercept, and R2 values of 0.98, 0.1, and 0.997 microg/m3, respectively. Additional field tests validated the use of DOS as an effective impaction substrate. As a result of these laboratory and field tests, DOS oil has been approved by EPA as a substitute for DOW 704 oil. Since the field deployment of DOS oil in 2001, users of this alternative oil have not reported any operational problems associated with its use in the PM2.5 FRM. Limited field evaluation of the BGI very sharp cut cyclone indicates that it provides a viable alternative to the WINS fractionator.

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.

Long-standing measurement techniques for determining ground-level ozone (O3) and nitrogen dioxide (NO2) are known to be biased by interfering compounds that result in overestimates of high O3 and NO2 ambient concentrations under conducive conditions. An increasing near-ground O3 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) O3 photometer and an identical monitor upgraded with an "interference-free" nitric oxide O3 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 (NOx) chemiluminescence Federal Reference Method (FRM) monitor and a new "direct-measure" NO2 NOx 405 nm photometer at a near-road air quality measurement site. Results indicate that the O3 monitor with the upgraded scrubber recorded lower regulatory-oriented concentrations than the deployed conventional metal oxide-scrubbed monitor and that O3 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 NO2 photometer recorded generally lower NO2 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 O3 gradient (NGOG). Collection of unbiased monitor inlet height-appropriate data is crucial for determining accurate design values and meeting National Ambient Air Quality Standards.

Potential interference bias in ozone standard compliance monitoring.

The U.S. Environmental Protection Agency has established a federal reference method (FRM) for ozone (O3) and allowed for designation of federal equivalent methods (FEMs). However, the ethylene-chemiluminescence FRM for O3 has been replaced by the UV photometric FEM by most state and local monitoring agencies because of its relative ease of operation. Accumulating evidence indicates that the FEM is prone to bias under the hot, humid, and stagnant conditions conducive to high O3 formation. This bias may lead to overreporting hourly O3 concentrations by as much as 20-40 ppb. Measurement bias is caused by contamination of the O3 scrubber, a problem that is not detected by dry air calibration. An adequate wet test has not been codified, although a procedure has been proposed for agency consideration. This paper includes documentation of laboratory tests quantifying specific interferant responses, collocated ambient FRM/FEM monitoring results, and smog chamber comparisons of the FRM and FEMs with alternative scrubber designs. As the numbers of reports on monitor interferences have grown, interested parties have called for agency recognition and correction of these biases.