Chapter 1

Aerosols, Clouds and Chemistry-Climate Interactions

The term aerosol in the present context refers to suspended particles in the atmosphere such as sulfate, mineral dust, carbonaceous particles, other organic aerosols and sea salt. The microphysical and chemical effect of aerosols on global climate is emerging as a new frontier in global change studies. While aerosol science is almost as old as the rest of atmospheric sciences, the urgency and the great impetus for studying it in the context of global change arise from several recent developments which demonstrate its significance to global climate and chemistry.

Figure 1. Potential anthropogenic radiative forcing of climate. Direct effect refers to to the increase in clear-sky albedo. The indirect effect is the increase in cloud albedo. From IPCC, 1994.
* First, and foremost, is the demonstration by model studies that the increase in anthropogenic sulfate particles is potentially a large contributor to the decadal change in the global radiative forcing of climate (Figure 1). Increase in sulfate aerosols can lead to a radiative cooling directly by enhancing clear sky albedo and indirectly through modification of the short-wave cloud forcing.

The magnitude of the estimated direct and indirect global radiative cooling effects of sulfates during the last century (~ -1 Wm-2), and more importantly, the uncertainty in the estimated global effects (0 to -3 Wm-2) are so large (Figure 1) that aerosols now rank with cloud feedback and ocean-atmosphere interactions as the major barriers toward a reliable prediction of decadal climate changes.

* A parallel development is the demonstration that the global radiative effects of other continental aerosols (e.g., mineral dust and carbonaceous particles) may be comparable to the sulfate effects [Andreae, 1995 and Li et al., 1995]. The radiative effects of these aerosols are expected to be particularly large over the tropical and sub-tropical oceans, because of their proximity to arid regions [Prospero, 1979 for mineral dust] and equatorial forests [Andreae, 1995 for black carbon].

* The experimental discovery of the dominant role of heterogeneous chemistry in the Antarctic ozone hole has forced us to think about such heterogeneous chemistry effects in other regions as well. In particular, the role of heterogeneous chemistry within the cold tropical upper troposphere cirrus clouds, where temperatures equal minimum temperatures over the Antarctic, in determining the chemical composition, and thereby radiative properties of clouds is just one of many emerging issues. The observations of extremely low ozone mixing ratios in the equatorial Pacific upper troposphere during the Central Equatorial Pacific Experiment (CEPEX) [Kley et al., 1995; Crutzen, 1995] may be a case in point.

* Another fundamental issue concerns the inter hemispheric mixing of air. The gaseous pollutants and continental aerosols from the northern hemisphere encounter the relatively pristine air from the southern hemisphere in the vicinity of the ITCZ. The rapid vertical transports within the deep convective-cirrus cloud systems of the ITCZ and their subsequent horizontal distribution by fast zonal upper air flows provide a fundamental link between regional emissions of reactive gases and their global impacts on the radiatively active species such as ozone and aerosols. Our current understanding of the role of the ITCZ in the transport of trace gases and aerosols is largely based on model studies.

* The recent findings of previously unrecognized excess solar absorption within tropical clouds [Cess et al., 1995; Ramanathan et al., 1995; Pilewskie and Valero, 1995], and its potential significance to atmospheric and oceanic general circulation and climate [Kiehl et al., 1994] is the best example of how poorly understood aerosol and cloud processes can dramatically influence global climate. These studies show that clouds over the western tropical Pacific warm pool enhance atmospheric solar absorption by as much as 35 Wm-2. Furthermore, an earlier study [Ohmura and Gilgen, 1993] which employed a global network of surface pyranometers concluded that the net solar radiation reaching the surface is smaller by as much as 25 Wm-2 than earlier estimates. Globally, the excess absorption effect may be as large as 25 Wm-2 (Figure 2). These findings have to be confirmed by more elaborate spatial and temporal sampling techniques. In the interim, these studies imply that there may be an important heat source in the tropical atmosphere which has been ignored in models. The ITCZ cloud systems, just like those over the warm pool, reflect as much as 60 to 100 Wm-2 solar radiation back to space, on an annual-mean basis, which raises the following question:

Figure 2. Evolution of the understanding of the atmospheric absorption in the global energy budget (in Wm-2). [Ohmura and Gilgen, 1993; Cess et al., 1995; Pilewskie and Valero, 1995; Ramanathan et al., 1995]

Do tropical convective-cirrus systems, including those in the ITCZ, enhance the atmospheric solar absorption?

If so, the potential climatic effects are substantial. It can increase the tropical meridional diabatic heating gradient within the troposphere by about 50% and decrease it in the oceans by a comparable magnitude. It can also suppress the average evaporation and precipitation by 20 to 40% [Kiehl et al., 1995].

In addition, we need to examine how continental aerosols (particularly dust, black carbon and organics) and soluble species modify the cloud solar absorption in the tropics. One basic flaw in our models, as well as in our general approach to the cloud-radiation interaction problem, is that we assume radiative properties of pure water and ice without any consideration for the radiative effects of absorbing aerosols or chemical species within cloud drops and ice crystals. In short, the physics and chemistry of the cloud-climate problem are treated in isolation.

The fundamental premise of this white paper is that the time is ripe now, to deal with the aerosol-cloud-physical-chemical interaction and its impact on the macro-scale climate processes. In particular, INDOEX will focus on the following issues:

* the significance of sulfates and other continental aerosols for global radiative forcing

* the magnitude of solar absorption in ITCZ cloud systems

* the role of the ITCZ in the transport of trace species and pollutants

As described next, the equatorial Indian Ocean, during a brief period between January to April of each year, performs a unique natural experiment that enables us to address all of the above issues simultaneously.
Table 1. Estimate of Present-Day Global Emission of Major Aerosol Types (in Tg/year)

Present flux

Low High Best
Soil dust (mineral aerosol) 1,000 3,000 1,500
Sea salt 1,000 10,000 1,300
Volcanic dust 4 10,000 33
Biological debris 26 80 50
Sulfates from biogenic gases 60 110 90
Sulfates from volcanic SO2 4 45 12
Organic matter from biogenic NMHC* 10 200 55
Nitrates from NOx 10 40 22

Industrial dust etc. 40 130 100
Black carbon (soot and charcoal) 10 30 20
Sulphates from SO2 120 180 140
Biomass burning (w/o black carbon) 50 140 80
Nitrates from NOx 20 50 36
Organics from anthropogenic NMHC* 5 25 10

Total 2,390 24,000 3,450

* NMHC: non-methane hydrocarbons.
(From M. Andreae, 1995)