Abstract. Most I−-CIMSs (iodide chemical ionization mass
spectrometers) for measurement of atmospheric trace gases utilize a
radioactive ion source with an initial activity of 10 or 20 mCi of
210Po. In this work, we characterize a 210Po ion source with an
initial activity of 1.5 mCi that can be easily constructed from commercially
available components. The low level of radioactive activity of this source
significantly reduces regulatory burden with storage and shipping relative
to higher-activity sources. We compare the sensitivity of the low-activity
source (LAS) to a standard 20 mCi source, as a function of carrier gas flow
and flow tube pressure, for peroxyacetyl nitrate (PAN), formic acid
(HCO2H), molecular chlorine (Cl2) and nitryl chloride
(ClNO2), using an I−-CIMS. The LAS provides 2 to 5 times less
sensitivity than that of the standard source even though the ratio of
activity is approximately 13. However, detection limits of less than 2 pptv
for the tested compounds are achieved for integration times on the order of
a minute. The sensitivity of the LAS is less dependent on the magnitude of
the carrier gas than a standard source. In addition, the LAS provides
maximum sensitivity at relatively low carrier gas flows. Finally, we
demonstrate that the LAS can be used to measure PAN in the remote atmosphere
from an aircraft by showing data obtained on the NASA DC-8 during the
Atmospheric Tomography (ATom) mission. In summary, the LAS may be an
excellent substitute for a standard ion source in short-term field
deployments.
Supplementary material to "A low activity ion source for measurement of atmospheric gases by CIMS"
