Measurements of ambient HONO concentrations and vertical HONO flux above a northern Michigan forest canopy

Abstract. Systems have been developed and deployed at a North Michigan forested site to measure ambient HONO and vertical HONO flux. The modified HONO measurement technique is based on aqueous scrubbing of HONO using a coil sampler, followed by azo dye derivatization and detection using a long-path absorption photometer (LPAP). A Na₂CO₃-coated denuder is used to generate "zero HONO" air for background correction. The lower detection limit of the method, defined by 3 times of the standard deviation of the signal, is 1 pptv for 1-min averages, with an overall uncertainty of ±(1 + 0.05 [HONO]) pptv. The HONO flux measurement technique has been developed based on the relaxed eddy accumulation approach, deploying a 3-D sonic anemometer and two HONO measurement systems. The overall uncertainty is estimated to be within ±(8 × 10⁻⁸ + 0.15 F_HONO) mol m⁻² h⁻¹, with a 20-min averaged data point per 30 min. Ambient HONO and vertical HONO flux were measured simultaneously at the PROPHET site from 17 July to 7 August 2008. The forest canopy was found to be a net HONO source, with a mean upward flux of 0.37 × 10⁻⁶ moles m⁻² h⁻¹. The HONO flux reached a maximal mean of ~0.7 × 10⁻⁶ moles m⁻² h⁻¹ around solar noon, contributing a major fraction to the HONO source strength required to sustain the observed ambient concentration of ~70 pptv. There were no significant correlations between [NO_x] and daytime HONO flux and between J_NO2 × [NO₂] and HONO flux, suggesting that NO_x was not an important precursor responsible for HONO daytime production on the forest canopy surface in this low-NO_x rural environment. Evidence supports the hypothesis that photolysis of HNO₃ deposited on the forest canopy surface is a major daytime HONO source.