Tropospheric ozone (O 3 ) continues to be a threat to human health and agricultural productivity. While O 3 control is challenging, tracking underlying formation mechanisms provides insights for regulatory directions.Here, we describe a comprehensive analysis of the effects of changing emissions on O 3 formation mechanisms with observational evidence. We present a new approach that provides a quantitative metric for the ozone production rate (OPR) and its sensitivity to precursor levels by interpreting two decades of in situ observations of the six criteria air pollutants (2001−2018). Applying to the South Coast Air Basin (SoCAB), California, we show that by 2016−2018, the basin was at the transition region between nitrogen oxide (NO x )-limited and volatile organic compound (VOC)-limited chemical regimes. Assuming future weather conditions are similar to 2016−2018, we predict that NO x -focused reduction is required to reduce the number of summer days the SoCAB is in violation of the National Ambient Air Quality Standard (70 ppbv) for O 3 . Roughly, ∼40% (∼60%) NO x reductions are required to reduce the OPR by ∼1.8 ppb/h (∼3.3 ppb/h). This change would reduce the number of violation days from 28 to 20% (10%) in a year, mostly in summertime. Concurrent VOC reductions which reduce the production rate of HO x radicals would also be beneficial.

Abstract. We describe the characteristics and performances of our newly built broadband cavity-enhanced absorption spectrometer for measurements of nitrate radical (NO3), nitrogen dioxide (NO2), and water vapor (H2O). A customized vibration-resistance cavity layout incorporated with N2 purging on high-reflection mirror surfaces was implemented with a red light-emitting diode (LED) as a light source. In general, this system achieved over 40 km (up to 101.5 km) of effective light path length at 662 nm from a 0.52 m long cavity. For accurate NO3 measurement, the measured absorption spectrum of H2O was used for simultaneous concentration retrievals with the other species instead of being treated as interferences to be removed or corrected prior to NO3 detection. Synthesized N2O5 crystals under atmospheric pressure were used for performance tests of linear response and transmission efficiency. From the standard injection experiments of NO3, NO2, and H2O, high linearities were observed (R2≥0.9918). The total NO3 transmission efficiency through the system was determined to be 81.2 % (±2.9, 1σ) within the residence time of 2.59 s. The precisions (1σ) of NO3, NO2, and H2O in 1 Hz measurement from a single pixel on the charge-coupled device (CCD) were 1.41 pptv, 6.92 ppbv, and 35.0 ppmv with uncertainties of 10.8 %, 5.2 %, and ≥20.5 %, respectively, mainly from the errors in the literature absorption cross-section. The instrument was successfully deployed aboard the Korean icebreaker R/V Araon for an expedition conducted in the remote marine boundary layer in the Arctic Ocean during the summer of 2021.

Abstract. We describe the characteristics and performances of our newly built broadband cavity-enhanced absorption spectrometer for measurements of nitrate radical (NO3), nitrogen dioxide (NO2), and water vapor (H2O). A customized vibration-resistance cavity layout incorporated with N2 purging on high-reflection mirror surfaces was implemented with a red light-emitting diode (LED) as a light source. In general, this system achieved over 40 km (up to 101.5 km) of effective light path length at 662 nm from a 0.52 m long cavity. For the accurate NO3 measurement, the measured absorption spectrum of H2O was used for simultaneous concentration retrievals with the other species, instead of being treated as interferences to be removed or corrected prior to NO3 detection. Synthesized N2O5 crystals under atmospheric pressure were used for performance tests of linear response and transmission efficiency. From the standard injection experiments of NO3, NO2, and H2O, high linearities were observed (R2 ≥0.9918). The total NO3 transmission efficiency through the system was determined to be 81.2 % (±2.9, 1σ) within the residence time of 2.59 seconds. The precisions (1σ) of NO3, NO2, and H2O in 1 Hz measurement from a single pixel on the CCD were 1.41 pptv, 6.92 ppbv, and 35.0 ppmv with uncertainties of 10.8, 5.2, and ≥20.5 %, respectively, mainly from the errors in literature absorption cross-sections. The instrument was successfully deployed aboard the Korean icebreaker R/V Araon for an expedition conducted in remote marine boundary layer in the Arctic Ocean during the summer of 2021.

Abstract. Living in an era of government allocated carbon dioxide (CO2) emissions, knowing the accurate amount of human-induced CO2 becomes very critical. To this end, an in-depth understanding of CO2 emissions in urban areas where human activities are concentrated will be of practical help. With this motivation, we quantify CO2 emission strengths of individual urban activities (i.e. vehicle, industry, heat generation, etc.) based on direct observations of vertical CO2 exchanges at urban-atmosphere interface using Eddy-Covariance (EC) method at Gwangju, Korea (2017.11–2018.08). Day of week difference analysis, together with varying wind sector, grounded from carefully designed measurement set-up, enables us to assess CO2 emission factors (EFs) free from seasonal bias (i.e. heating and urban vegetation); evaluated EFs of traffic from day of week difference was 0.017(±0.011) μmol m-2 s-1 car-1 which is more than 10 times larger than that from simple relation (0.0012 ± 0.0011 μmol m-2 s-1 car-1) between CO2 flux and traffic counts. The CO2 emissions due to the car manufacturing industry within the fetch and heating when air temperatures were lower than 18 °C were quantified as 103.25(±42.18) μmol m-2 s-1 and 2.41(±1.71) μmol m-2 s-1 °C-1, respectively. Urban vegetation uptake was estimated as -1.72 kg C m-2 yr-1 only with EFs traffic inferred from day of week difference indicating possible erroneous estimation in simple relation unless it properly reflects representative seasonal changes in a year. Even though our estimations are conservative EFs due to limitations in corrections of horizontal seepage and vertical storage, we found that both EFs for traffic and heat in latest emission inventory were more than 2.5 times lower than our estimations which indicate the urgency in bottom-up inventory validations.

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