Continuous, year-long measurement of atmospheric HONO and NO2 by direct absorption spectroscopy above a rural New England forest: absence of daytime HONO production

Numerous field studies report daytime HONO concentrations well above those expected at photo-stationary state, suggesting a sustained, yet uncharacterized production pathway of HONO in unpolluted, forested environments [Kleffmann et al., 2005; Zhou et al., 2011]. The formation rate of HONO inferred from the imbalance between measured and steady state levels indicates that this source significantly influences the photochemistry of the boundary layer, as HONO readily photodissociates to yield nitric oxide and the hydroxyl radical. Depending on the mechanism, it may also represent a pathway by which deposited nitrogen oxides, which fertilize ecosystem production, are repartitioned back into the atmosphere in reactive form [Zhou et al., 2007a].

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 NO_x levels.

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