atmospheric science

Minor Constituent Measurement Laboratory (MCML)

Global scientific concerns like climate change, its anthropogenic causes and mitigation methods to contain them are on the rise. They affect the entire life on the globe, through equity and economics of nations. Next to space essential for the life forms to manifest themselves, the air we breathe is the fundamental need for life to survive on this planet. Air quality is decided by the contaminants ambient air carries with it temporally and spatially. Green house gases and their role in modulating regional climate are well established. Earth’s atmosphere is no longer indolent to the capricious dump of pollutants into it by any nation as its brunt is borne by all, mainly by its neighbours. Natural green house gases like water vapour readjust global rainfall patterns while carbon dioxide sinks less over the diminishing green cover of the globe. More lethal and noxious gases like carbon monoxide (CO) and surface ozone has been on the increase in the last century. Both have anthropogenic components from the industries, transport and forest sectors. Therefore a close audit and stocktaking their abundance and estimating residence time of these pollutants is vital to the global green economy. CO, the total ozone, surface ozone and water vapour along with solar UV radiation were monitored in CESS. The ongoing programs are listed below.

Carbon Monoxide in Ambient Ai

Ambient atmospheric carbon monoxide (CO) at Thiruvananthapuram, clean coastal site, was monitored continuously from 2003 using a non-dispersive IR analyzer (Monitor Europe Model 9830 B). CO at Thiruvananthapuram shows a double peak (FN peak at around 08:00 h and AN peak at around 20:00 h) diurnal pattern as shown in Fig. 1.

Diurnal variation

Fig. 1. Diurnal variation of CO at Thiruvananthapuram (std. deviation (SD) of 123 ppb)

Annual variation of CO has U–shape, with a high during winter, low in summer and the lowest in wet season as shown in Fig. 2, typical of carbonaceous constituents in air. Annual baseline was 180- ppb for this site, indicative of clean air quality. This measurement matches with the 850 hPa level CO measurements by satellites.

Annual variation

Figure 2 Annual variation of CO in 2004 2007 with polynomial fit (degree=4)

Solar UV Radiation, Total Ozone & Water vapour

Ozone is the only minor constituent of the Earth’s atmosphere that does not have its provenance at the surface. Photochemical interaction of solar ultraviolet (UV) radiation with oxygen in the stratosphere leads to the maximum production of ozone there. To the lower layers UV does not penetrate to dissociate molecular oxygen and in the upper layers of the stratosphere, there is insufficient molecular oxygen for ozone production even in the copious UV regime. Ozone keeps the stratosphere warm by exothermic reactions in production as well as loss. Ozone being heavy, about 1.5 times heavier than dry air, trickles lower down through the tropopouse discontinuities and is well documented for its highs over latitudes of 30, 45 & 60 zones. We established a relation between the reduction in surface ozone and daytime rain and an enhancement observed during nighttime rain. Dual annual highs during winter and summer with a low during wet months are in confirmation with global measurements.

Direct component of the solar UV radiation in the 290-320 nm region is of significance to us, as they are actinic, induce erythema or reddening on the human skin and in certain types cause skin cancer. Quantitative measurements were rare and we monitored UV-B radiation during 1980-94 and established that the spectral dosages have a very low trend of increase in the 14-year period. Total ozone column was also derived from the UV data and was found to be consistent with other global tropical measurements.

More of solar UV trickling to lower altitudes on account of the damage to the ozone layer causes concern to the biosphere and the life that it supports on the planet. Photo-dissociation of tropospheric water vapour forming the OH radical selectively hushes the CO rather quickly, leaving methane, the more potent greenhouse gas, to build up. In order to study the simultaneous changes in the solar UV and water vapour Microtops II and UV Biometer (Solar Light Inc., USA) are deployed in Trivandrum since 2009.

Facilities available:

  1. CO Analyser (Monitor Europe Model 9830B) – Measurement rage: 0-200 ppm spot and 0-50 ppb normal mode.
  2. Microtops II (solar Light Co., USA) – Measures UV-B at 305, 312 & 320 nm; Ozone column thickness, water vapour at 936 µm and aerosol optical depth (AOD).
  3. Solar UV-Biometer (Solar Light Co., USA) – Measures intensity in MED/hr in the 300-320 nm range.