Optical properties of a heated aerosol in an urban atmosphere: a case study

JOHN BACKMAN (1), Aki Virkkula (1), Tuukka Petäjä (1), Minna Aurela (2), Anna Frey (2), Risto Hillamo (2)

(1) University of Helsinki, FINLAND (2) Finnish Meteorological Institute

     Abstract Number: 259
     Last modified: May 3, 2010

Abstract
Black carbon (BC) is the most effective and dominant absorber of visible solar radiation in the atmosphere. The internally mixed state of BC with aerosol constituents can enhance forcing by a factor of two.

Soot is externally mixed when it is introduced in to the atmosphere, but during transport, the soot particles may coagulate with other particles or get coated with sulphates, nitrates or organic species by condensation which results in different mixing states of BC.

The most commonly used filter-based measurement techniques for determining absorption of light in an aerosol is disturbed by light scattering aerosols. Light scattering volatile compounds such as sulphates, nitrates and most of the organic species are evaporated at 300 ºC were BC is still non-volatile.

The light scattering material changes the optical path of light in the filter matrix increasing the light absorption of the filter. Measurement equipment interpret this wrongly as light absorption by the aerosol. Most light scattering constituents in a sub-micron aerosol are volatile by their nature.

The optical impacts on volatilization was studied during a short field campaign with two suites of equipment measuring in parallel for six days in April of 2009 in Helsinki. When heated, the light scattering constituents were evaporated thus reducing the single-scattering albedo (omega$_0) of the aerosol by as much as 0.4.

With reduced influence of light scattering constituents in the aerosol phase the mass absorption cross section (MAC) of soot was calculated for the heated aerosol and the cool aerosol. The oven was also set to scan different temperatures which revealed the volatility of the urban aerosol at different temperatures as well as omega$_0 dependence on the non-volatile volume fraction (NVFR).

We found that absorption coefficients measured at different temperatures showed a temperature dependency possibly indicating initially different mixing states of the non-volatile constituents.