Abstract
Abstract. Due to the important contribution of nitrous acid (HONO) to OH radicals in the atmosphere, various technologies have been developed to measure HONO. Among them, wet-denuder–ion-chromatography (WD/IC) is a widely used measurement method. Here, we found interferences with HONO measurements by WD/IC based on a comparison study of concurrent observations of HONO concentrations using a WD/IC instrument (Monitor for AeRosols and Gases in ambient Air, MARGA) and long-path absorption photometer (LOPAP) at the Station for Observing Regional Processes of the Earth System (SORPES) in eastern China. The measurement deviation of the HONO concentration with the MARGA instrument, as a typical instrument for WD/IC, is affected by two factors. One is the change in denuder pH influenced by acidic and alkaline gases in the ambient atmosphere, which can affect the absorption efficiency of HONO by the wet denuder to underestimate the HONO concentration by up to 200 % at the lowest pH. The other is the reaction of NO2 oxidizing SO2 to form HONO in the denuder solution to overestimate the HONO concentration, which can be increased by to 400 % in denuder solutions with the highest pH values due to ambient NH3. These processes are in particularly important in polluted east China, which suffers from high concentrations of SO2, NH3, and NO2. The overestimation induced by the reaction of NO2 and SO2 is expected to be of growing importance with the potentially increased denuder pH due to the decrease in SO2. We further established a method to correct the HONO data measured by a WD/IC instrument such as the MARGA. In case a large amount WD/IC-technique-based instruments are deployed with the main target of monitoring the water-soluble composition of PM2.5, our study can help to obtain a long-term multi-sites database of HONO to assess the role of HONO in atmospheric chemistry and air pollution in east China.
Highlights
Since the first detection of nitrous acid (HONO) in the atmosphere in 1979 (Perner and Platt, 1979), HONO has attracted much attention due to its important contribution to OH radicals, which are the primary oxidants in the atmosphere (Kleffmann, 2007)
The HONO concentration measured by long-path absorption photometer (LOPAP) (HONOlopap) varied from 0.01 to 4.8 ppbv with an average value of 1.1 ± 0.77 ppbv, and the HONO concentration measured by the MARGA instrument (HONOmarga) was 0.01–9.6 ppbv, with an average value of 1.52 ± 1.21 ppbv
The average diurnal variations in HONOmarga and HONOlopap are shown Fig. 1b; HONOmarga / HONOlopap ratios were higher at night and especially in the morning, which were different from the results of Muller et al (1999), who found that a remarkable overestimation of HONO by WD/IC usually occurred during the daytime
Summary
Since the first detection of nitrous acid (HONO) in the atmosphere in 1979 (Perner and Platt, 1979), HONO has attracted much attention due to its important contribution to OH radicals, which are the primary oxidants in the atmosphere (Kleffmann, 2007). Unexpectedly high HONO concentrations have been observed in the daytime and are believed to be a major OH source even during the daytime (Kleffmann et al, 2005; Michoud et al, 2014; Sörgel et al, 2011). Because of the important role of HONO in atmospheric chemistry and the knowledge gap with regard to its sources, various techniques have been developed to detect the HONO concentration in ambient air or in a smog chamber.
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