Detection of nitrous acid in the atmospheric simulation chamber SAPHIR using open-path incoherent broadband cavity-enhanced absorption spectroscopy and extractive long-path absorption photometry

An instrument based on 20 m open-path incoherent broadband cavity-enhanced absorption spectroscopy (IB-BCEAS) was established at the Julich SAPHIR chamber in spring 2011. The setup was optimized for the detection of HONO and NO2 in the near-UV region 352-386 nm, utilizing a bright hot-spot Xe-arc lamp and a UV-enhanced charge-coupled device (CCD) detector. A 2 sigma detection limit of 26 pptv for HONO and 76 pptv for NO2 was achieved for an integration time of 1 min. Methacrolein (MACR) was also detected at mixing ratios below 5 ppbv with an estimated 2 sigma detection limit of 340 pptv for the same integration time. The IBBCEAS instrument's performance for HONO and NO2 detection was compared to that of extractive wet techniques, long-path absorption photometry (LOPAP), and chemiluminescence spectrometry (CLS) NOx detection, respectively. For the combined data sets an overall good agreement for both trend and absolute mixing ratios was observed between IBBCEAS and these established instruments at SAPHIR. Correlation coefficients r for HONO range from 0.930 to 0.994 and for NO2 from 0.937 to 0.992. For the single measurement of MACR r=0.981 is found in comparison to proton-transfer-reaction mass spectrometry (PTRMS).