The Development of an Airborne Gas Chromatograph
BACKGROUND
The Leeds Atmospheric Field Studies Group is currently developing an airborne gas
chromatographic (GC) system, which will be used for the measurement of selected
non-methane hydrocarbons in the troposphere.
Reactive non-methane hydrocarbon (NMHC) species in the troposphere provide substantial
perturbations from background clean air chemistry. Over the North Atlantic and Europe,
NMHC species released from anthropogenic sources are removed predominantly via reaction
with the hydroxyl radical (OH) generated through the photolysis of ozone at wavelenghths of
300-320 nm (Wayne 1991).
(1)O3 + hv -> O(1D) + O2
(2)O(1D) + M -> O(3P) + M
(3)O(1D) + H2O -> 2OH
Typical NMHC lifetimes in Northern hemisphere regions range from less than an hour to
several days depending on a combination of NMHC reactivity, atmospheric conditions and
solar flux. The reactions of NMHC (RH) species with OH are known to lead to the
formation of the peroxy radical (RO2) which, in the presence of NOx, leads
to the net formation of tropospheric ozone via the photolysis of NO2 produced
through reaction (6).
(4)RH + OH -> R + H2O
(5)R + O2 -> RO2
(6)RO2 + NO -> RO + NO2
(7)NO2 + hv -> O + NO
(8)O + O2 + M -> O3 + M
Hydrocarbons in the troposphere, therefore, have the potential to create significant
quantities of ozone, known to be a potent greenhouse gas in this region of the atmosphere.
In addition, the reaction of peroxy radical species in the presence of NO2 may be
important in ‘locking up’ NOx as peroxyacetylnitrate (PAN), a relatively unreactive
temporary reservoir of NOx (Singh 1981) The advection of these species to higher
altitudes provides an upper troposphere NOx source, significant due to the potential
for creating ozone.
The unique rates of oxidation for individual hydrocarbon species can
allow for integrated estimates of atmospheric radical species concentrations.
Through changing ratios of branched to straight chain alkane isomers much information
on hydroxyl and nitrate radical concentrations may be inferred.
References
- Singh H.B. and Hanst P.L. (1981) Peroxyacetyl Nitrate (PAN) in the unpolluted atmosphere:
An important reservoir for nitrogen oxides. Geophys. Res. Lett., 8, 941-944.
- Wayne R.P. (1991) Chemistry of Atmospheres. 2nd Ed. Clarendon Press, Oxford.
This page was originally constructed by Jude Davies
Last modified by Jim McQuaid
on Sat Nov 02 12:49:36 2002