An absorption spectrometer, comprised of two tunable infrared quantum cascade lasers to simultaneously detect HONO and NO2 (a proposed chemical precursor to HONO production), was recently developed to measure concentrations and fluxes by eddy covariance [Lee et al., 2011]. The sampling system consists of a filter-free inlet followed by 180 feet (55 m) of temperature-controlled, shielded Teflon tubing to draw above-canopy air down to the ground-based instrument. Routine in-field additions of generated HONO and zero-air demonstrate the absence of positive and negative artifacts. Calibrations were periodically performed by catalytic conversion of generated HONO to nitric oxide, which was measured by a second co-located instrument. Supplemented by continuously monitoring the laser stability the main source of drift with spectroscopic techniques we confirm the absence of long-term drift in measurements. Short-term variability, driven by thermal perturbations to the instrument, were corrected by frequent spectral background subtractions with additions of zero-air. We report detection limits (3σ) of 30 and 3 ppt for HONO and NO2, respectively, with one-hour spectral averaging.
Continuous measurements made at the Harvard Forest Environmental Measurement Station from November 2010 to December 2011 show daytime HONO levels below detection limit, which is much lower than what previous studies have typically observed in similar environments [Sorgel et al., 2011; Zhou et al., 2007b]. The eddy covariance flux detection limit establishes an upper limit on the rate of atmosphere-biosphere exchange of HONO, indicating that HONO production is a negligible contributor to the hydroxyl radical budget at Harvard Forest. Nighttime heterogeneous HONO formation is observed under sufficiently high NOx levels.
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