Aerosols in and Above the Bornean Rainforest

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Abstract

Atmospheric aerosols affect climate directly by scattering and absorbing solar radiation, and indirectly by affecting the albedo and lifetime of clouds through their role as cloud condensation nuclei. Aerosol sources, and the processes that govern their evolution in the atmosphere are not well understood, making the aerosol effects a significant source of uncertainty in future climate predictions. The tropics experience a large solar flux meaning that any radiative forcing in this region is particularly important. Despite this, there is a paucity of data from the tropics, with the majority of previous studies performed in the northern mid-latitudes. The few in-situ studies of aerosol composition that have been performed are all in the continental settings of Amazonia or Africa. Until now the “maritime continent” region of South East Asia has remained unstudied. Presented here are Aerosol Mass Spectrometer composition measurements from the Oxidant and Particulate Processes Above a South East Asian Rainforest project, performed from ground and airborne measurement platforms in and around the rainforest of Borneo, South-East Asia. Unlike the previous tropical studies, this allows for the characterisation of a region of mixed terrestrial and marine biogenic emissions. The region is also undergoing rapid land use change, with forest being converted for agriculture, particularly the cultivation of oil palms. This study also allows for the characterisation of a region that is beginning to undergo land use change, providing insight into emissions from different land use types, and providing a benchmark to measure the effects of land use change against in the future.Total sub-micron aerosol loadings were found to be lower than studies in the northern mid-latitudes, similar to previous tropical studies. However, aerosol composition was different to that observed in Amazonia, with much greater sulphate loadings in Borneo. A regional background of sulphate and highly oxidised organic aerosol was identified, with organic aerosol that is less oxidised originating inland. Aerosol confined to a shallow marine boundary layer upwind of Borneo is lofted higher into the troposphere as it advects across the island, with regional aerosol being removed and biogenic terrestrial aerosol added. The lofting of this aerosol is expected to extend its atmospheric lifetime and change its role in the Earth’s radiative budget. A novel organic aerosol signal was identified which correlated with gas phase isoprene oxidation products, strongly suggesting that it was significant of isoprene SOA. Aerosol associated with this signal made up a substantial fraction of the organic aerosol loading. This opens up the opportunity for future studies to make isoprene SOA measurements using the Aerosol Mass Spectrometer in other studies. A substantial amount of the organic aerosol in Borneo was attributed to isoprene oxidation.

Layman's Abstract

Aerosols (microscopic particles suspended in the air) play an important role in the climate of the Earth. They can absorb or reflect sunlight, changing the amount of radiation that goes into heating the Earth. They also play an important role in cloud formation, acting as “nuclei” for cloud droplets to form upon. They tend to make clouds whiter and longer lasting, acting to reflect more sunlight back into space. Their role in cloud formation and heating of the atmosphere can also affect rainfall, potentially even affecting systems as large as the Indian Monsoon. Despite the important influence aerosol can have on climate, their affects are still not precisely understood, in fact the Intergovernmental Panel for Climate Change state that they are one of the biggest unknowns in predicting future climate change.To predict the effect man-made aerosols have on climate, we first need to understand the role of natural aerosols. This is particularly important as these natural systems are being rapidly changed thanks to deforestation for timber, crops and to build new settlements. We can only predict what effects this land use change will have on the environment if we have a good understanding of the natural systems. Many studies of atmospheric aerosol have been conducted around the more developed parts of the Northern Hemisphere (e.g. Europe, North America, Japan) however there have been very few studies in the tropics, in fact only a handful in South America and Africa. It is particularly important to understand aerosol climate effects in the tropics as the large amount of sun the region experiences means it can make a bigger difference than in other, less sunny parts of the World.One region of the tropics that has received particularly little attention in recent studies is the “maritime continent” — South East Asia. This region is made up of a network of tropical islands, so it is likely to be very different from the other continental regions that have previously been studied in the tropics. The measurements presented in this thesis are from Borneo, a very large island in South East Asia and home to one of the largest rainforests in theWorld. The forest is being rapidly cut down to make way for plantations to manufacture palm oil — a ubiquitous ingredient as “vegetable oil” in very many high street products.This work found that aerosols in Borneo are indeed very different from those measured in the Amazon Rainforest—it seems that emissions from the sea are a large source of aerosol. The project found that palm oil produces a very different mixture of gases from the rainforest and the work presented here shows that these gases react in an unexpected way to condense and make aerosol. It could even be the case that the sea emissions are the reason that so much of this unexpected aerosol is formed, although more work needs to be done to figure out the precise chemistry. When air is transported across Borneo, it tends to get lifted higher into the atmosphere, meaning it is above a lot of the rain (which can remove the aerosol though wash-out) and is likely to have a longer atmospheric lifetime. Results from other studies show that man-made pollution can increase the amount of natural gases that go into making aerosol. This means that if Borneo becomes more polluted in the future there could be a lot of aerosol produced, not just from the pollution itself, but from natural gases from the rainforest and crops which are not currently converted to aerosol.

Keyword(s)

Borneo; aerosol; atmospheric chemistry; climate change; deforrestation; rainforest; tropics

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