Anomalous low tropospheric column ozone over eastern India during the severe drought event of monsoon 2002: a case study

BACKGROUND, AIM, AND SCOPE

The present study is an attempt to examine some of the probable causes of the unusually low tropospheric column ozone observed over eastern India during the exceptional drought event in July 2002.

METHOD

We examined horizontal wind and omega (vertical velocity) anomalies over the Indian region to understand the large-scale dynamical processes which prevailed in July 2002. We also examined anomalies in tropospheric carbon monoxide (CO), an important ozone precursor, and observed low CO mixing ratio in the free troposphere in 2002 over eastern India.

RESULTS AND DISCUSSION

It was found that instead of a normal large-scale ascent, the air was descending in the middle and lower troposphere over a vast part of India. This configuration was apparently responsible for the less convective upwelling of precursors and likely caused less photochemical ozone formation in the free troposphere over eastern India in July 2002.

CONCLUSION

The insight gained from this case study will hopefully provide a better understanding of the process controlling the distribution of the tropospheric ozone over the Indian region.

Biomass burning in the tropics: impact on atmospheric chemistry and biogeochemical cycles

Biomass burning is widespread, especially in the tropics. It serves to clear land for shifting cultivation, to convert forests to agricultural and pastoral lands, and to remove dry vegetation in order to promote agricultural productivity and the growth of higher yield grasses. Furthermore, much agricultural waste and fuel wood is being combusted, particularly in developing countries. Biomass containing 2 to 5 petagrams of carbon is burned annually (1 petagram = 10(15) grams), producing large amounts of trace gases and aerosol particles that play important roles in atmospheric chemistry and climate. Emissions of carbon monoxide and methane by biomass burning affect the oxidation efficiency of the atmosphere by reacting with hydroxyl radicals, and emissions of nitric oxide and hydrocarbons lead to high ozone concentrations in the tropics during the dry season. Large quantities of smoke particles are produced as well, and these can serve as cloud condensation nuclei. These particles may thus substantially influence cloud microphysical and optical properties, an effect that could have repercussions for the radiation budget and the hydrological cycle in the tropics. Widespread burning may also disturb biogeochemical cycles, especially that of nitrogen. About 50 percent of the nitrogen in the biomass fuel can be released as molecular nitrogen. This pyrdenitrification process causes a sizable loss of fixed nitrogen in tropical ecosystems, in the range of 10 to 20 teragrams per year (1 teragram = 10(12) grams).

Tropical tropospheric ozone and biomass burning

New methods for retrieving tropospheric ozone column depth and absorbing aerosol (smoke and dust) from the Earth Probe-Total Ozone Mapping Spectrometer (EP/TOMS) are used to follow pollution and to determine interannual variability and trends. During intense fires over Indonesia (August to November 1997), ozone plumes, decoupled from the smoke below, extended as far as India. This ozone overlay a regional ozone increase triggered by atmospheric responses to the El Niño and Indian Ocean Dipole. Tropospheric ozone and smoke aerosol measurements from the Nimbus 7 TOMS instrument show El Niño signals but no tropospheric ozone trend in the 1980s. Offsets between smoke and ozone seasonal maxima point to multiple factors determining tropical tropospheric ozone variability.