To characterize the role of dew water for the ground surface HONO distribution, nitrous acid (HONO) measurements with a Monitor for AeRosols and Gases in ambient Air (MARGA) and a LOng Path Absorption Photometer (LOPAP) instrument were performed at the Leibniz Institute for Tropospheric Research (TROPOS) research site in Melpitz, Germany, from 19 to 29 April 2018. The dew water was also collected and analyzed from 8 to 14 May 2019 using a glass sampler. The high time resolution of HONO measurements showed characteristic diurnal variations that revealed that (i) vehicle emissions are a minor source of HONO at Melpitz station; (ii) the heterogeneous conversion of NO₂ to HONO on the ground surface dominates HONO production at night; (iii) there is significant nighttime loss of HONO with a sink strength of 0.16 ± 0.12 ppbv h⁻¹; and (iv) dew water with mean NO₂^- of 7.91 ± 2.14 μg m⁻² could serve as a temporary HONO source in the morning when the dew droplets evaporate. The nocturnal observations of HONO and NO₂ allowed the direct evaluation of the ground uptake coefficients for these species at night: γ_NO2→HONO = 2.4 × 10⁻⁷ to 3.5 × 10⁻⁶, γ_HONO,ground = 1.7 × 10⁻⁵ to 2.8 × 10⁻⁴. A chemical model demonstrated that HONO deposition to the ground surface at night was 90 %–100 % of the calculated unknown HONO source in the morning. These results suggest that dew water on the ground surface was controlling the temporal HONO distribution rather than straightforward NO₂–HONO conversion. This can strongly enhance the OH reactivity throughout the morning time or in other planted areas that provide a large amount of ground surface based on the OH production rate calculation.

HONO,Dew water (露水）