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

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 Na ₂CO ₃-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 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 JNO ₂ × [NO ₂] 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 HNO ₃ deposited on the forest canopy surface is a major daytime HONO source.