A relaxed eddy accumulation (REA) LOPAP system for flux measurements of nitrous acid (HONO)

Abstract

Abstract. In the present study a relaxed eddy accumulation (REA) system for the quantification of vertical fluxes of nitrous acid (HONO) was developed and tested. The system is based on a three-channel long-path absorption photometer (LOPAP) instrument, for which two channels are used for the updrafts and downdrafts, respectively, and a third one for the correction of chemical interferences. The instrument is coupled to a REA gas inlet, for which an ultrasonic anemometer controls two fast magnetic valves to probe the two channels of the LOPAP instrument depending on the vertical wind direction. A software (PyREA) was developed, which controls the valves and measurement cycles, which regularly alternates between REA, zero and parallel ambient measurements. In addition, the assignment of the updrafts and downdrafts to the physical LOPAP channels is periodically alternated, to correct for differences in the interferences of the different air masses. During the study, only small differences of the interferences were identified for the updrafts and downdrafts excluding significant errors when using only one interference channel. In laboratory experiments, high precision of the two channels and the independence of the dilution-corrected HONO concentrations on the length of the valve switching periods were demonstrated. A field campaign was performed in order to test the new REA-LOPAP system at the TROPOS monitoring station in Melpitz, Germany. HONO fluxes in the range of -4×1013 molecules m−2 s−1 (deposition) to +1.0×1014 molecules m−2 s−1 (emission) were obtained. A typical diurnal variation of the HONO fluxes was observed with low, partly negative fluxes during night-time and higher positive fluxes around noon. After an intensive rain period the positive HONO emissions during daytime were continuously increasing, which was explained by the drying of the uppermost ground surfaces. Similar to other campaigns, the highest correlation of the HONO flux was observed with the product of the NO2 photolysis frequency and the NO2 concentration (J(NO2)⋅[NO2]), which implies a HONO formation by photosensitized conversion of NO2 on organic surfaces, such as humic acids. Other postulated HONO formation mechanisms are also discussed but are tentatively ranked being of minor importance for the present field campaign.

Large Contribution of Nitrous Acid to Soil-Emitted Reactive Oxidized Nitrogen and Its Effect on Air Quality

“…In that study, the soil HONO emissions were not measured but estimated on the basis of soil moisture and temperature. Another recent study presented similar findings of the dominance of the NO 2 production pathway in a rural grassland following a rainfall . Our model has considered HONO deposition and all known HONO sources, including the light-enhanced NO 2 heterogeneous reactions on surface (see Materials and Methods) and soil emissions based on the samples collected across China.…”

“…Another recent study presented similar findings of the dominance of the NO 2 production pathway in a rural grassland following a rainfall. 64 Our model has considered HONO deposition and all known HONO sources, including the light-enhanced NO 2 heterogeneous reactions on surface (see Materials and Methods) and soil emissions based on the samples collected across China. The positive contribution of soil to ambient HONO concentrations in our study may be partially explained by the high emissions of HONO from forest and cropland in China.…”

Measurement Report: Exchange Fluxes of HONO over Agricultural Fields in the North China Plain

“…In our recent study, elevated levels of HONO concentration and HONO-to-NO2 ratios were observed after fertilization at an agricultural site, implying that fertilized fields released a great amount of HONO (Xue et al, 2021). To study the characteristics and the corresponding Flux measurement can provide direct evidence about the production and/or deposition of HONO on the ground surface (Von Der Heyden et al, 2022;Xue et al, 2022a). There are two types of methods to measure HONO exchange flux between soil and atmosphere: micrometeorology and chamber methods.…”

“…Although the EC method has been widely adopted in measuring soil-atmosphere exchange fluxes of nitrogen gases (N2O, NO, etc.) (Soussana et al, 2007;Stella et al, 2012), it is difficult to be applied in HONO flux measurement due to the lack of highly sensitive and fast HONO measurement 85 techniques (Von Der Heyden et al, 2022;Xue et al, 2019a). The AG and REA methods have been developed and applied to HONO flux measurements in recent years, providing a good option to measure HONO flux.…”

“…However, available flux measurements are still limited and most of them were conducted over a short period of normally less than one month. Long-period measurements are still lacking (Laufs et al, 2017;Sörgel 90 et al, 2015;Von Der Heyden et al, 2022;Zhou et al, 2011). Chamber methods, including static chamber and dynamic chamber, were usually used in studying emission factors of nitrogen gases (EF, the ratio of nitrogen emission to application).…”