Abstract
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.
Highlights
Nitrous acid (HONO) is important in atmospheric chemistry as its photolysis (Reaction R1) is an important source of OH radicals
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
To investigate the possible sampling inlet and denuder artifacts of the MARGA, two different positions were selected for LOPAP during the measurement period: (M1) sampling unit of LOPAP was connected to the MARGA inlet in the back of the 2 m sampling tube and the PM10 inlet of MARGA as shown in Fig. S1a in the Supplement (18 April 2018, 13:00 UTC, to 20 April 2018, 08:00 UTC), and (M2) the sampling unit of LOPAP was settled in the same level as the sampling head of MARGA (Fig. S1b) (20 April 2018, 15:00 UTC, to 29 April 2018, 07:00 UTC)
Summary
Nitrous acid (HONO) is important in atmospheric chemistry as its photolysis (Reaction R1) is an important source of OH radicals. The probable reason was the off-line analysis of the batch denuder sample since the resulting longer interaction of gas and liquid phase during the transport led to further heterogenous reactions As both instruments are based on the same sampling technique, the present study could be a good starting point for an intercomparison between MARGA and LOPAP for HONO measurements to find possible reasons in the denuder deviations. Our observations provide a direct intercomparison between LOPAP and MARGA for HONO field measurements, offer additional insights into HONO chemical formation processes, and examine the relative importance of dew as a sink and source of HONO
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