Improving the accuracy and precision of broadband optical cavity measurements

Intercomparison of Ambient Nitrous Acid Measurements in a Shanghai Urban Site

Nitrous acid (HONO) is the major source of OH radicals in polluted regions and plays a key role in the nitrogen cycle of the atmosphere. Therefore, accurate measurements of HONO in the atmosphere is important. Long Path Absorption Photometer (LOPAP) is a common and highly sensitive method used for ambient HONO measurements. Incoherent Broadband Cavity Enhanced Absorption Spectroscopy (IBBCEAS) is a recent alternative for the detection of HONO with high temporal and spatial resolutions, which has shown a detection limit of 0.76 ppbv at a sampling average of 180 s. In this study, LOPAP and IBBCEAS-HONO instruments were deployed in a Shanghai Urban Site (Shanghai Academy of Environmental Sciences) and simultaneously recorded the data from both instruments for a quantitative intercomparison of the measured atmospheric HONO for four days from 30 December 2017–2 January 2018. The HONO concentration measured by IBBCEAS and LOPAP were well matched. The campaign average concentrations measured by IBBCEAS and LOPAP were 1.28 and 1.20 ppbv, respectively. The intercomparison results demonstrated that both the IBBCEAS-HONO instrument and LOPAP-HONO instrument are suitable for ambient monitoring of HONO in a polluted urban environment.

Monitoring Ambient Nitrate Radical by Open-Path Cavity-Enhanced Absorption Spectroscopy

We describe an open-path cavity-enhanced absorption spectroscopy (OP-CEAS) technique for the ambient measurement of nitrate radicals (NO₃) near 662 nm. Compared with the closed type of CEAS system with a sampling line, the OP-CEAS features high accuracy due to the lack of NO₃ loss in the sampling line and cavity. On the basis of a 0.84 m long open-path cavity, the effective absorption length of ∼5 km is achieved by mirrors with a reflectivity of 0.99985 at 662 nm. The detection limit of OP-CEAS for the measurement of NO₃ is 3.0 pptv (2σ) in 30 s, and the uncertainty is 11-15%. The instrument was successfully applied in a field measurement under low particulate matter (PM) loading conditions. As the sensitivity would be decreased due to strong PM extinction under heavy PM pollution conditions, we highlight the feasibility of this OP-CEAS configuration for field application in clean and moderate PM condition, such as forested regions affected by anthropogenic emissions. This technique is also appropriate for the field detection of other reactive trace gases in future studies.