The Role of Aerosols in the Troposphere: Radiative Forcing, Model Response, and Uncertainty Analysis

Pan, W.
Ph.D. Thesis, MIT Dept. of Earth, Atmospheric and Planetary Sciences, Report Nr. 0

This thesis addresses the role of aerosols in the troposphere from three perspectives: (1) the radiative forcing by aerosols; (2) responses of meteorological and chemical fields to the aerosol radiative forcing; (3) uncertainty analysis of the radiative forcing by anthropogenic sulfate aerosols.

The sensitivity of the direct radiative forcing by anthropogenic sulfate aerosols to their optical properties, concentrations and the ambient humidity has been investigated in an explicit radiative transfer model with available aerosol and meteorological data. Results indicate that aerosol concentrations and optical properties contribute about equally to the factor-of-three difference in the estimates of this forcing in the literature. The use of constant humidity scaling factors for aerosol optical properties is a good approximation, provided that these factors in the visible wavelength are kept the same as the observed ones. Neglecting the humidity effect on aerosol single-scattering albedo and asymmetry factor will only lead to a 10% overestimate of the result.

The global distribution of radiative flux changes at the top of the atmosphere and the surface due to climatological aerosols in d'Almeida et al. [1991] is calculated with the radiative transfer scheme by Fu and Liou [1992]. At the top of the atmosphere aerosols decrease the net downward short-wave radiation in most parts of the global. Increases occur mainly in the Saharian desert region, its downwind equatorial east Atlantic, and part of Australia. At the surface the decrease of the net downward short-wave radiation is more than a factor of 2 larger than that at the top of the atmosphere. Decreases of the net outgoing long-wave radiation are about an order of magnitude smaller than changes of the short-wave radiation. Changes in short- and long-wave radiation are characterized by large spatial variations and gradients. The annual global mean radiative forcing owing to aerosols increases by about a factor of 2 when the humidity effect on aerosol optical properties is included. Replacing the boundary layer heights as prescribed in the aerosol data set by the actual values decreases the result by a similar factor.

The mechanism and magnitude of meteorological and chemical responses to aerosol radiative forcing are studied in three different models: a one-dimensional radiative-convective equilibrium model, a mesoscale meteorological model, and a photochemical air quality model. The simulations in the one-dimensional radiative-convective model show that the prescribed time-invariant short-wave heating in the lower troposphere causes an increase of the long-wave heating in the upper troposphere and a decrease of the convective heating throughout the troposphere. As a result, the model atmosphere at the new equilibrium is cooler and drier. Analysis of the surface energy balance in the model shows that the equilibrium temperature change depends not only on the external forcing but also on the internal feedbacks. A negative feedback between the surface temperature and the sensible and latent heat fluxes and a positive feedback between the surface temperature and atmospheric water vapor are evident. The negative feedback coefficient increases by a factor of 4 to 5 when the equilibrium is reached. In the meantime, the positive feedback coefficient also increases to its equilibrium value, which is comparable to the magnitude of the negative feedback. As a result, the initial sensitivity of surface temperature change to aerosol radiative forcing is about a factor of 4 to 5 smaller than the equilibrium sensitivity. The transient surface temperature change approaches the equilibrium one with a characteristic time scale, which depends on both feedback factors and surface properties.

The mesoscale model is set up in the Southern California Air Quality Study (SCAQS) region and model predictions without aerosols are validated against the SCAQS measurements. Four aerosol types with low- and high-level concentrations from the climatological aerosol data set in d'Almeida et al. [1991] are introduced into the mesoscale model. The single-column simulations show that in response to large decreases in the incoming solar radiation due to aerosols surface temperature changes are less sensitive than those in the one-dimensional radiative-convective model. This attributes to the difference between initial and equilibrium sensitivity of surface temperature change to aerosol radiative forcing. The three-dimensional mesosclae simulations indicate that the domain averaged relative changes are -30 to 10% for boundary layer height, -30 to 40% from wind speed, and -20 to 0% for net downward SW radiation. Temperature changes vary from -1.5 to 0.5 �C, and wind direction changes vary within 50 degrees.

Simulations in the photochemical air quality model with either uniformly perturbed or aerosol-induced meteorological fields show that the chemical fields are more sensitive to changes in temperature, wind, and boundary layer height. The domain-averaged relative changes of ground level concentrations of chemical species range up to 10% due to the prescribed low-level aerosol loading. Shifting aerosol loading from the low-level to high-level appears to almost double the concentration response of species during the day. Changes at specific sites can be much larger than the domain-averaged changes. The spatial pattern in the response of chemical fields bears little or no resemblance to that in any single meteorological perturbation.

The uncertainty of the direct and indirect radiative forcing by anthropogenic sulfate aerosols has been addressed with a new uncertainty analysis technique. The probability density function of the direct radiative forcing under the influence of 9 uncertain parameters is calculated for four different models. The mean value of the result varies from 9.3 to 1.3 W/m2 with a 95% confidence range of 0.1 to 4.2 W/m2. Variance analysis identifies the sulfate yield and lifetime as the two primary uncertain parameters. The probability density function of the indirect radiative forcing has been evaluated in 5 different models with respect to 20 uncertain parameters. The mean value of the indirect forcing varies from 1.2 to 1.7 W/m2 with a 95% confidence range of 0.1 to 5.2 W/m2. Variance analysis ranks aerosol size distribution as the leading contributor to the model uncertainty.