Aerosols in the Atmosphere: Sources, Transport, and Multi-decadal Trends

Abstract

We present our recent studies with global modeling and analysis of atmospheric aerosols. We have used the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model and satellite and in situ data to investigate (1) long-term variations of aerosols over polluted and dust source regions and downwind ocean areas in the past three decades and the cause of the changes and (2) anthropogenic and volcanic contributions to the sulfate aerosol in the upper troposphere/lower stratosphere.

Question and Answer

Questioner: Ivanka Stajner

Question: In the comparison for early years AVHRR and TOMS data are used giving different trends over oceans. Are these data of trend-quality? What are the particular retrieval data sets? Can different sensitivity of AVHRR and TOMS to aerosol composition explain different trends?

Answer: These earlier datasets were not designed for detecting aerosols, therefore the retrieval ability and data quality of the aerosol products suffer by several limitations of the instruments. For example, AVHRR does not provide the aerosol retrieval over land and TOMS has to exclude large amount of data that are affected by the clouds. Nonetheless, the AVHRR Climate Data Record dataset we used in this study has been created for the purpose of assessing aerosol trends, albeit only over oceans. TOMS data, on the other hand, are used mainly for features, not for quantitative assessment. The positive oceanic aerosol trends from TOMS is within the uncertainty range of the TOMS data, particularly over the low AOD areas, so it should not be referred as “trends”. Over high AOD regions such as high dust and pollution areas, trends from TOMS are consistent with the expected AOD changes.

Questioner: S.T. Rao

Question: From your analysis, what can you say about the aerosol impact on climate from volcanoes versus anthropogenic forcings given different residence times of aerosols from these two sources? How do you attribute aerosols from anthropogenic or volcanic sources?

Answer: Volcanic effects are mostly in the free troposphere and lower stratosphere because of the emission injection heights, in contrast with the near-surface anthropogenic emissions. However, volcanic aerosol have longer lifetime than anthropogenic aerosol, mainly because they are located far above the surface thus not subject to efficient dry and wet removals. Globally, volcanic forcing is much smaller than the anthropogenic forcing, but in the UTLS it can be the dominant aerosol component. We use the model to separate volcanic and anthropogenic aerosols by “tagging” the emission sources.

Questioner: Jose M. Maldasano

Question: In the results presented with respect to the Middle East area, it has indicated that the AOD had increase in the past ten to fifteen years due to mineral dust, when in that area in recent years has been the Iraq and Syria wars, and also a strong urban and industrial development in all gulf countries, has an explanation for this? Was threshold value has been used to discriminate the AOD due to mineral dust?

Answer: The Middle East AOD increase in the past decade has been observed by satellite data. The Angstrom exponent from the ground-based AERONET observations over the Middle East shows that such an increase is mainly due to the coarse-mode dust aerosols, indicated by the decreasing of the Angstrom exponent in the past decade. Our model simulation supports such findings with our tagged source simulations that show the AOD increase over the Middle East is mainly due to the increase of dust rather than anthropogenic aerosols.