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. The atmosphere contains a rich variety of reactive organic compounds,including gaseous volatile organic carbon (VOCs), carbonaceous aerosols, andother organic compounds at varying volatility. Here we present a novel andsimple approach to measure atmospheric non-methane total organic carbon (TOC)based on catalytic oxidation of organics in bulk air to carbon dioxide. Thismethod shows little sensitivity towards humidity and near 100 % oxidationefficiencies for all VOCs tested. We estimate a best-case hourly precision of8 ppb C during times of low ambient variability in carbon dioxide, methane,and carbon monoxide (CO). As proof of concept of this approach, we showmeasurements of TOC+CO during August–September 2016 from a coastalcity in the southwest United Kingdom. TOC+CO was substantiallyelevated during the day on weekdays (occasionally over 2 ppm C) as a resultof local anthropogenic activity. On weekends and holidays, with a mean(standard error) of 102 (8) ppb C, TOC+CO was lower and showed muchless diurnal variability. TOC+CO was significantly lower when windswere coming off the Atlantic Ocean than when winds were coming off land if weexclude the weekday daytime. By subtracting the estimated CO from TOC+CO , we constrain the mean (uncertainty) TOC in Atlantic-dominatedair masses to be around 23 ( ± ≥ 8 ) ppb C during this period. Aproton-transfer-reaction mass spectrometer (PTR-MS) was deployed at the sametime, detecting a large range of organic compounds (oxygenated VOCs, biogenicVOCs, aromatics, dimethyl sulfide). The total speciated VOCs from the PTR-MS,denoted here as Sum(VOC), amounted to a mean (uncertainty) of 12 ( ± ≤ 3 ) ppb C in marine air. Possible contributions from a number of knownorganic compounds present in marine air that were not detected by the PTR-MSare assessed within the context of the TOC budget. Finally, we note that theuse of a short, heated sample tube can improve the transmission of organicsto the analyzer, while operating our system alternately with and without aparticle filter should enable a better separation of semi-volatile andparticulate organics from the VOCs within the TOC budget. Future concurrentmeasurements of TOC, CO, and a more comprehensive range of speciated VOCswould enable a better characterization and understanding ofthe atmospheric organic carbon budget.

Estimation of atmospheric total organic carbon (TOC) – paving the path towards carbon budget closure