Our review was published at a very crucial time when it was realized that organic compounds make up a major fraction of atmospheric particles. This recognition led to a substantial effort to examine aerosol-phase chemical processes in order to better understand how aerosols are modified in the atmosphere. This review basically describes the first steps in assessing the transformation of organic matter in aerosols, the new methods used, and the state of the art in terms of scientific understanding.

The paper is a summary of work carried out in many laboratories. It describes new approaches in studying the reactivity of the organic component of airborne particles and learning about aerosol-phase reaction pathways and mechanisms.

Aerosols effects on climate represent one of the highest uncertainties in our current understanding of the atmosphere and the contribution of its different components to climate change and health. Until a few years ago, atmospheric chemists thought that the aerosol in the atmospheres contained mostly inorganic components. Recently, it was understood that, in fact, organic compounds compose a large fraction of the atmospheric aerosol and that these compounds determine many of the physical and chemical properties of these particles.

It was also realized that, through interactions with radicals and oxidants, the organic matter is transformed during its aerosol atmospheric lifetime. Our paper describes how experiments were designed to understand these processes and how laboratory experiments strive to derive useful information that can be applied to the environmental system by increasing the complexity and interpretation of these efforts.

I am a physical chemist whose studies in the laboratory focus on processes which are relevant to atmospheric change. It is indeed a very challenging field where one has to study systems, species, and processes which are relevant to the atmosphere, under specific conditions.

I am also challenged by the differences in time and spatial scales, going from the nano-scale to the global scale. I was intrigued by the organic fraction in aerosols due to the high complexity of the issue and the realization that it requires our immediate attention and educated input in order to address issues relevant to society.

We are now studying the properties of largely complex humic-like organic compounds that have been identified in atmospheric particles. We are especially interested in their transformations, their ability to act as cloud condensation nuclei, and their optical properties. All these properties are relevant to their environmental impacts.

The experimental systems that we use now are much more sophisticated than those that were described in our first review, as we hope to shed new light on the role of organic matter in Earth’s atmosphere.

We trust that the research in this field will help in reducing climatic and health-related uncertainties associated with particulate matter in the atmosphere.