Abstract. 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:
γNO2→HONO=2.4×10-7 to 3.5×10-6, γHONO,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.
Supplementary material to "Role of the dew water on the ground surface in HONO distribution: a case measurement in Melpitz"
