Evaluation of the use of a commercially available cavity ringdown absorption spectrometer for measuring NO2 in flight, and observations over the Mid-Atlantic States, during DISCOVER-AQ

Real time, atmospheric NO2 column profiles over the Mid-Atlantic states, during the July 2011 National Aeronautics and Space Administration (NASA) Deriving Information on Surface Conditions from Column and Vertically Resolved Observations to Air Quality (DISCOVER AQ) flight campaign, demonstrated that a cavity ring down spectrometer with a light emitting diode light source (LED-CRD) is a suitable technique for detecting NO2 in the boundary layer (BL) and lower free troposphere (LFT). Results from a side-by-side flight between a NASA P3 aircraft and a University of Maryland (UMD) Cessna 402B aircraft show that NO2 concentrations in ambient air from 0.08 nmol /mol (or ppbv) to 1.3 nmol/mol were consistent with NO2 measurements obtained via laser induced fluorescence (LIF) and photolysis followed by NO chemiluminescence (P-CL). The current LED-CRD, commercially available by Los Gatos Research (LGR), includes the modifications added by Castellanos et al. (Rev. Sci. Instrum. 80:113107, 2009) to compensate for baseline drift and humidity through built in zeroing and drying. Because of laser instability in the initial instrument, the laser light source in the Castellanos et al. (Rev. Sci. Instrum. 80:113107, 2009) instrument has been replaced with a light emitting diode. Six independent calibrations demonstrated the instrument’s linearity up through 150 nmol/mol NO2 and excellent stability in calibration coefficient of 1.26 (± 3.7 %). The instrument detection limit is 80 pmol/mol. Aircraft measurements over the Mid-Atlantic are included showing horizontal and vertical distributions of NO2 during air quality episodes. During 23 research flights, NO2 profiles were measured west and generally upwind of the Baltimore/Washington, D.C. area in the morning and east (generally downwind) of the metro- politan region in the afternoon. Column contents (surface to 2,500 m altitude) were remarkably similar (≈3×1015 molecules/cm2) indicating that NO2 is widely distributed over the eastern US contributing to the regional (spatial scales of approximately1000 km) nature of smog events.