High-time resolution NH3 and NOy

High-time resolution (∼5 s) measurements of NH₃ were made using a Picarro G1103 Analyzer, a cavity ring-down spectroscopy instrument, located in mobile laboratory. The inlet line (Teflon tubing) was located approximately 6 m above ground level (agl). The entire length of the inlet line was insulated and heated to 40 °C to minimize wall losses. A filter (Picarro P/N S1021) was placed on the end of the inlet to prevent particles from entering the instrument. Ammonium nitrate captured by the filter likely volatilized on the heated filter during sampling and is a potential source of bias in the NH₃ measurement. However, concurrent 24-h averaged measurements from an annular denuder/filter-pack sampler (see below) suggest that, assuming that all of the NH₄NO₃ sublimates and is measured as NH₃, the median bias in the measurements was 7.7%. A correction was not applied to the final dataset. Calibrations were performed at the beginning and end of the field deployment with a known concentration of ammonia, generated using a calibration gas diluted with a Teledyne Zero Air Generator (Model 701). A phosphorus acid (10% w/v) coated denuder operated in parallel confirmed the supplied NH₃ concentration. The mobile lab, which housed the instrument, had an air conditioning failure July 23–30 resulting in conditions that were too hot for proper operation of the Picarro. After A/C was restored, a section of the heated inlet overheated and this problem was not discovered until August 8. Thus NH₃ data for the period July 23–August 9 are omitted from this analysis.

CASTNET NO_y and NO data were downloaded from http://www.epa.gov/castnet (US Environmental Protection Agency Clean Air Markets Division, 2016a). A Teledyne API T200U∕NO_y instrument is operated at the ROMO CASTNET site (ROM206). This instrument measures both NO_y and NO by chemiluminescence. NO is directly measured and NO_y is converted using a thermal catalytic converter and measured as NO (US EPA, 2017). This method has an uncertainty of ≤ ±10% with the additional caveats that some reduced organic nitrogen compounds may be converted and the conversion of the different NO_y species is highly temperature dependent (Crosley, 1996; Kondo et al., 1997; Williams et al., 1998). The site is operated in accordance with the Quality Assurance Plan for Procuring, Installing, and Operating NCore Air Monitoring Equipment at CASTNET sites (Amec Foster Wheeler, 2016).