## Development of an instrument for direct ozone production rate measurements: measurement reliability and current limitations

Ground-level ozone (O$_3$) is an important pollutant that affects both global climate change and regional air quality, with the latter linked to detrimental effects on both human health and ecosystems. Ozone is not directly emitted in the atmosphere but is formed from chemical reactions in- volving volatile organic compounds (VOCs), nitrogen oxides (NO$_x$ = NO + NO$_2$) and sunlight. The photochemical nature of ozone makes the implementation of reduction strategies challenging and a good understanding of its formation chemistry is fundamental in order to develop efficient strategies of ozone reduction from mitigation measures of primary VOCs and NO$_x$ emissions. An instrument for direct measurements of ozone production rates (OPRs) was developed and deployed in the field as part of the IRRONIC (Indiana Radical, Reactivity and Ozone Production Intercomparison) field campaign. The OPR in- strument is based on the principle of the previously published MOPS instrument (Measurement of Ozone Production Sensor) but using a different sampling design made of quartz flow tubes and a different O$_x$ (O$_3$ and NO$_2$) conversion– detection scheme composed of an O$_3$-to-NO$_2$ conversion unit and a cavity attenuated phase shift spectroscopy (CAPS) NO$_2$ monitor. Tests performed in the laboratory and in the field, together with model simulations of the radical chemistry occurring inside the flow tubes, were used to assess (i) the reliability of the measurement principle and (ii) potential biases associated with OPR measurements. This publication reports the first field measurements made using this instrument to illustrate its performance. The results showed that a photo-enhanced loss of ozone inside the sampling flow tubes disturbs the measurements. This issue needs to be solved to be able to perform accurate ambient measurements of ozone production rates with the instrument described in this study. However, an attempt was made to investigate the OPR sensitivity to NO$_x$ by adding NO in- side the instrument. This type of investigations allows checking whether our understanding of the turnover point between NO$_x$ -limited and NO$_x$ -saturated regimes of ozone production is well understood and does not require measuring ambient OPR but instead only probing the change in ozone production when NO is added. During IRRONIC, changes in ozone production rates ranging from the limit of detection (3σ ) of 6.2 ppbv h$^{−1}$ up to 20 ppbv h$^{−1}$ were observed when 6 ppbv of NO was added into the flow tubes.

Publication Date:
Feb 07 2018
Date Submitted:
Mar 03 2019
Citation:
Atmospheric Measurement Techniques, 11