Continuous measurement of atmospheric Carbon Monoxide at Thiruvananthapuram
Carbon monoxide (CO) is a trace gas found
in ppm by volume in the Earth’s atmosphere. It gets there by various
burning processes at insufficient oxygen levels. Its primary provenance
is at the surface. CO has natural and anthropogenic sources. It has only
a few sinks at the surface, chiefly in respiration by warm-blooded animals.
Thus, atmospheric CO is of concern to the biosphere, particularly to animals.
In upper troposphere, CO is found to be low (30% of surface) by reactions
with OH in competition with CH4. This process inhibits the decrease of
CH4, a potent green house gas. Photo-dissociation of H2 O by solar UV
radiation produces OH radical. Global total ozone reduction and consequent
penetration of solar UV to lower layers produces more OH, that masks the
real atmospheric CO load. In order to establish CO variability by continuous
measurement using an instrument in a relatively clean tropical coastal
environment and to understand the relationship between CO and trace species
like O3, CH4 and to evaluate seasonal and long term CO variability, a
CO analyzer (Monitor Europe Model 9830B) that measures CO in 0-200 ppm
in 4 ranges (0-50 ppm default) with auto-ranging facility was installed
at CESS. Lowest CO level detected is ~1 ppb, with sampling period from
1 min to 1 hour The instrument was operational continuously for an year
at Akkulam. Diurnal features associated with sea and land breeze were
studied. Monthly mean CO and seasonal CO behaviour were obtained. The
annual baseline CO behaviour for this site was arrived at. Also, the behaviour
of CO during weekdays and weekends were differentiated.
Diurnal Variation
Diurnal CO shows two enhancements one in the morning and other in the
late evening. The morning peak is associated with the sea breeze and the
evening peak with the land breeze. The monthly average of the diurnal
behaviour is shown in Fig 1.4. Diurnal CO peaks are also affected by temperature,
wind speed, wind direction and boundary layer height.
Seasonal variation
Annual pattern depicts CO variation in different seasons (Fig. 1.5). During
winter, the surface temperature is low. So is the boundary layer height.
Consequently, CO is not transported vertically, leading to a high ambient
value. During summer, the surface temperature is high. So also, the boundary
layer height. This causes more CO to move vertically, explaining the summer
low. In wet months, we observed lowest annual CO, possibly associated
with large scale monsoon winds.
CO during Pongala festival
Concentration of CO was found to be high during a local religious festival,
Pongala. For this event massive firewood burning takes place on a day
within 2-3 hours. Fig. 1.6 shows the CO pattern during the Pongala. CO
decreased to background level after 3 hours. Midnight and pre-sunrise
high is solely from the traffic associated CO.
