Development of an incoherent broadband cavity-enhanced absorption spectrometer for in situ measurements of HONO and NO<sub>2</sub>

Abstract

Abstract. Gaseous nitrous acid (HONO) is an important source of OH radicals in the troposphere. However, its source, especially that during daytime hours remains unclear. We present an instrument for simultaneous unambiguous measurements of HONO and NO2 with high time resolution based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). To achieve robust performance and system stability under different environment conditions, the current IBBCEAS instrument has been developed with significant improvements in terms of efficient sampling as well as resistance against vibration and temperature change, and the IBBCEAS instrument also has low power consumption and a compact design that can be easily deployed on different platforms powered by a high-capacity lithium ion battery. The effective cavity length of the IBBCEAS was determined using the absorption of O2-O2 to account for the “shortening” effect caused by the mirror purge flows. The wall loss for HONO was estimated to be 2.0 % via a HONO standard generator. Measurement precisions (2σ) for HONO and NO2 are about 180 and 340 ppt in 30 s, respectively. A field inter-comparison was carried out at a rural suburban site in Wangdu, Hebei Province, China. The concentrations of HONO and NO2 measured by IBBCEAS were compared with a long optical path absorption photometer (LOPAP) and a NOx analyzer (Thermo Fisher Electron Model 42i), and the results showed very good agreement, with correlation coefficients (R2) of HONO and NO2 being ∼0.89 and ∼0.95, respectively; in addition, vehicle deployments were also tested to enable mobile measurements of HONO and NO2, demonstrating the promising potential of using IBBCEAS for in situ, sensitive, accurate and fast simultaneous measurements of HONO and NO2 in the future.

Intercomparison of nitrous acid (HONO) measurement techniques in a megacity (Beijing)

“…However, we did not observe such a change in relationship at 20 high and low concentrations between the AIOFM-BHAM/ICCAS, suggesting that this result was not related to instrument type. Furthermore, as the measurements from Duan et al (2018) were performed in a rural site, conditions may also be more homogenous mix compared to an urban location and this may explain why there was better agreement between the LOPAP and BBCEAS in their study compared to the current work.…”

“…With respect to photolysis, the lifetime of HONO at midday ranged from 17-300 and 9-33 min for winter and summer, respectively (depending upon weather / cloud cover / aerosol loading) and may have contributed to spatial heterogeneity in HONO concentrations. Duan et al (2018) presented results of an inter-comparison at a rural site in China between a BBCEAS and a LOPAP, with 15 good agreement observed (slope of 0.94 and r 2 0.89). The slope appeared to deviate from linearity above approximately 2 ppb suggesting that at higher concentrations the relationship was changing, as observed here between CAM and wet chemical techniques (BHAM and ICCAS) (Tables 2 and 4).…”

“…The custom-built BBCEAS instrument from the Anhui Institute of Optics and Fine Mechanics (AIOFM), Chinese Academy of Sciences has been described in detail in Duan et al (2018); therefore only a brief description is given here. Light is emitted by a single light emitting diode with peak wavelength of 365 nm, FWHM of 13 nm and is introduced into the resonant cavity, 25…”

“…LOng Path Absorption Photometer (LOPAP), (Heland et al, 2001)), broadband cavity enhanced absorption spectroscopy (e.g. (Duan et al, 2018)), soft chemical ionization mass spectrometry (CIMS, e.g. (Veres et al, 2015)), on-line ion chromatography (e.g.…”

Simultaneous measurement of NO and NO<sub>2</sub> by a dual-channel cavity ring-down spectroscopy technique

“…A sensitivity of 38 ppt was achieved using an integration time of 128 ms for single-shot detection in their work. To evaluate the uncertainty and the accuracy of the aforementioned individual instruments, a number of intercomparison studies have been carried out (Xu et al, 2013;Dunlea et al, 2007;Villena et al, 2012). The comparison results show that the method based on Mo converters can be affected by significant interferences such as N 2 O 5 , HONO, HNO 3 , PAN, etc., whereas the method based on optical absorption is relatively immune to interferences.…”