The determination of the identity of missing OH reactivity in the atmosphere
Wednesday, 17 December 2014: 1:55 PM
The hydroxyl radical is the dominant daytime oxidant responsible for the removal of many trace gases. Its short lifetime means it is an ideal target species for models in order to test the accuracy of tropospheric chemical mechanisms. Its steady-state concentration is controlled by the balance of its rate of production and destruction, but it is often difficult, particularly in more polluted conditions, to adequately characterise the nature of all of the sinks for OH. Measurements of OH reactivity and comparison with model calculations have revealed that OH sinks are often underestimated, sometimes by significant amounts. In this presentation we will describe a new instrument to identify the chemical nature of “missing” OH reactivity in the troposphere. A chemical flow reactor in which OH is generated artificially was interfaced to a gas chromatograph time-of-flight mass spectrometer (GC-TOF-MS). Ambient air is passed through the reactor and a fraction of the OH sinks are removed according to their reactivity towards OH. By measuring the change in the TOFMS signal in the presence and absence of OH in the reactor (the latter to establish the trapping efficiency for each species), the relative rate coefficient for that species reacting with OH can be measured. By comparison with the known values for identified species, the rate coefficient for reaction of OH with unidentified sinks can therefore be measured, and their retention time and mass/fragmentation pattern used to infer their identity and functionality. The new instrument was deployed in a field campaign at the University of York in May/June 2014 and identified OH sinks were found to decrease on exposure to OH according to their rate coefficient for reaction with OH. The resulting relationship was used to determine rate coefficients for unidentified species and from their estimated concentrations their contribution towards total OH reactivity was estimated. In addition, the total OH reactivity was measured independently, as well as concentrations of OH, HO2 and RO2 radicals, a very detailed suite of VOCs, HCHO, HONO, NOx, CO and O3.