Development of accurate and stable primary standard gas mixtures for global atmospheric acetonitrile monitoring: evaluating adsorption loss and long-term stability

Abstract

Acetonitrile plays an important role in atmospheric processes and serves as a key tracer of biomass burning, the major emission source of primary carbonaceous particles and trace gases in the global atmosphere. Recognizing its significance, the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) Programme has designated acetonitrile as one of the target volatile organic compounds for global atmospheric monitoring, aiming for data quality better than 20 % accuracy and 15 % precision. Meeting the objectives of the WMO GAW Programme requires accurate and stable calibration standards with expanded uncertainties of less than 5 %. In this study, we explored the feasibility of gravimetry for developing primary standard gas mixtures (PSMs) in three different types of aluminium cylinders, each with distinct internal surface treatments, at nmolmol-1 and μmolmol-1 levels with a relative expanded uncertainty of less than 5 % and nitrogen as the matrix gas. We found that all three types of cylinders were inadequate without further passivation for developing acetonitrile PSMs below 10 nmolmol-1 due to significant adsorption losses (6 %-49 %) onto the cylinder's inner surface. To overcome this challenge, we prepared acetonitrile gas mixtures at 100 nmolmol-1 using a modified gravimetric method and at 10 μmolmol-1 using a conventional gravimetric method and then evaluated their long-term stability. Results showed that the effect of the adsorption loss at 100 nmolmol-1 and 10 μmolmol-1 was negated and negligible, respectively. Stability results show that we can disseminate acetonitrile calibration standards at both 100 nmolmol-1 and 10 μmolmol-1 with a relative expanded uncertainty of 3 % and 1 %, respectively (with an expiration period of 3 years), meeting the target uncertainty of the WMO GAW Programme. Further research is still needed to develop accurate and stable acetonitrile calibration standards below 10 nmolmol-1 that are closer to atmospheric levels.