Decadal Regional Trends in Trace Gases and Reflectance As Measured with the Ozone Monitoring Instrument (OMI) on Eos Aura

Abstract:

The Dutch-Finnish Ozone Monitoring Instrument (OMI) is a UV-visible spectrometer on board of the NASA EOS Aura mission. Due to its innovative design, OMI combines a high spatial resolution (13x24km² at nadir) with a wide swath of 2600 km that enables daily global coverage. The OMI science data record started in October 2004 and already spans a decade. The instrument shows very low optical degradation: after 10 years in orbit the throughput at its shortest UV wavelengths has only been reduced by a few percent and at longer wavelengths this degradation is about 1%. This stability makes the instrument extremely valuable for trend analysis, although due to the so-called “row anomaly” part of the swath is no longer providing science-quality data since 2009. Both the optical degradation and the row anomaly are well characterized.

The OMI data record shows that in the past decade the emissions of trace gases have changed considerably. Over most of the industrialized countries in Europe, North America and Asia emissions of NO_x and SO₂ have been reduced, whereas in the developing countries the emissions have generally increased. These changes in emissions directly affect the air quality, including the concentration of secondary aerosol particles. Due to the direct and indirect effect of aerosols, it is expected that the radiation balance is also affected, resulting in changes in shortwave radiance at the surface and at the top of the atmosphere.

In this contribution we will present time series analysis of tropospheric NO₂ and formaldehyde columns from OMI, in combination with aerosol optical depth time series from MODIS on EOS Aqua. We concentrate on mega-cities in India, China and the U.S.A., because in these densely populated regions the effects of air quality are the largest. To quantify the local effects of aerosols on the radiation balance, we combine the trends in the aerosol optical depth with trends of the reflectance at the top of the atmosphere, as measured by OMI.