Abstract. The atmosphere contains a rich variety of reactive organic compounds,
including gaseous volatile organic carbon (VOCs), carbonaceous aerosols, and
other organic compounds at varying volatility. Here we present a novel and
simple approach to measure atmospheric non-methane total organic carbon (TOC)
based on catalytic oxidation of organics in bulk air to carbon dioxide. This
method shows little sensitivity towards humidity and near 100 % oxidation
efficiencies for all VOCs tested. We estimate a best-case hourly precision of
8 ppb C during times of low ambient variability in carbon dioxide, methane,
and carbon monoxide (CO). As proof of concept of this approach, we show
measurements of TOC+CO during August–September 2016 from a coastal
city in the southwest United Kingdom. TOC+CO was substantially
elevated during the day on weekdays (occasionally over 2 ppm C) as a result
of local anthropogenic activity. On weekends and holidays, with a mean
(standard error) of 102 (8) ppb C, TOC+CO was lower and showed much
less diurnal variability. TOC+CO was significantly lower when winds
were coming off the Atlantic Ocean than when winds were coming off land if we
exclude the weekday daytime. By subtracting the estimated CO from
TOC+CO, we constrain the mean (uncertainty) TOC in Atlantic-dominated
air masses to be around 23 (±≥8) ppb C during this period. A
proton-transfer-reaction mass spectrometer (PTR-MS) was deployed at the same
time, detecting a large range of organic compounds (oxygenated VOCs, biogenic
VOCs, 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 known
organic compounds present in marine air that were not detected by the PTR-MS
are assessed within the context of the TOC budget. Finally, we note that the
use of a short, heated sample tube can improve the transmission of organics
to the analyzer, while operating our system alternately with and without a
particle filter should enable a better separation of semi-volatile and
particulate organics from the VOCs within the TOC budget. Future concurrent
measurements of TOC, CO, and a more comprehensive range of speciated VOCs
would enable a better characterization and understanding of
the atmospheric organic carbon budget.
Estimation of atmospheric total organic carbon (TOC) – paving the path towards carbon budget closure
