Radiative forcing of phytogenic aerosols

The principal objective of this study is to investigate the radiative influences produced by phytogenic aerosols over a forested area in eastern Canada where measured aerosol concentrations resulted from α-pinene and β-pinene oxidation processes. The studied forest ecosystem produced moderate amounts of biogenic hydrocarbons, with isoprene and pinene mixing ratios reaching 1.6 parts per billion. Once oxidized, these pinene levels gave rise to maximum phytogenic aerosol concentrations of circa 5000 particles per cm³. The amount of diffuse and attenuated solar irradiance resulting from the interaction of aerosols with incoming irradiance was quantified using a one-dimensional spectral radiative transfer model and measured aerosol sized distributions and concentrations. Results show that aerosols in the atmospheric boundary layer contributed to only moderate levels of diffuse irradiance but generated substantial attenuation of the incoming solar irradiance stream. For the irradiance levels measured in eastern Canada during cloudless days in July and with aerosol concentrations in the range of 2000 to 5000 particles per cm³, average daytime solar irradiance attenuation amounted to 0.04 W m^-2 with a diffuse component of 0.01 W m^-2. The maximum solar irradiance extinction reached 0.2 W m^-2. Assuming a uniform spatial aerosol distribution, this negative radiative influence could offset substantial fractions of the regional thermal forcing resulting from increased levels of greenhouse gases such as carbon dioxide. It is concluded that greater radiative influences (cooling) could be present over regions dominated by hydrocarbon productive forest ecosystems.