Abstract
Abstract. In order to elucidate the effect of galactic cosmic rays on cloud formation, we investigate the optical response of marine aerosols to Forbush decreases – abrupt decreases in galactic cosmic rays – by means of modeling. We vary the nucleation rate of new aerosols, in a sectional coagulation and condensation model, according to changes in ionization by the Forbush decrease. From the resulting size distribution we then calculate the aerosol optical thickness and Angstrom exponent, for the wavelength pairs 350, 450 nm and 550, 900 nm. In the cases where the output parameters from the model seem to compare best with atmospheric observations we observe, for the shorter wavelength pair, a change in Angstrom exponent, following the Forbush Decrease, of −6 to +3%. In some cases we also observe a delay in the change of Angstrom exponent, compared to the maximum of the Forbush decrease, which is caused by different sensitivities of the probing wavelengths to changes in aerosol number concentration and size. For the long wavelengths these changes are generally smaller. The types and magnitude of change is investigated for a suite of nucleation rates, condensable gas production rates, and aerosol loss rates. Furthermore we compare the model output with observations of 5 of the largest Forbush decreases after year 2000. For the 350, 450 nm pair we use AERONET data and find a comparable change in signal while the Angstrom Exponent is lower in the model than in the data, due to AERONET being mainly sampled over land. For 550, 900 nm we compare with both AERONET and MODIS and find little to no response in both model and observations. In summary our study shows that the optical properties of aerosols show a distinct response to Forbush Decreases, assuming that the nucleation of fresh aerosols is driven by ions. Shorter wavelengths seem more favorable for observing these effects and great care should be taken when analyzing observations, in order to avoid the signal being drowned out by noise.
Highlights
A Forbush Decrease (FD) is a sudden drop in the amount of galactic cosmic rays observed on Earth, due to large Coronal Mass Ejections from the sun (Forbush, 1937; Cane, 2000)
Natural sea salt can have a wide range of sizes (Pierce and crease (d50), and a 50% increase (i50), producing the 9 plots the aerosols to Forbush decreases (AOT) at 350 nm increases the most as the radius of the particle population grows, this causes an increase in the Angstrom exponent (AE)
If an ion induced values for both the model and MODIS. This is because the mechanism is working as in our model, it is expected that land based AERONET stations have more small mode frac- observations based on the shorter wavelength pair would be tion particles due to aerosols from e.g. pollution, dust, and the most favourable for seeing the FD effect. biomass burnings1
Summary
A Forbush Decrease (FD) is a sudden drop in the amount of galactic cosmic rays observed on Earth, due to large Coronal Mass Ejections from the sun (Forbush, 1937; Cane, 2000). A correlation between galactic cosmic rays (controlled by solar activity) and cloud cover has been shown (Marsh and Svensmark, 2003; Harrison and Stephenson, 2006) If this correlation is due to a physical mechanism a response in cloud cover could be expected during or after a Forbush decrease. Svensmark: Model of optical response of marine aerosols to Forbush decreases (AOT) and the Angstrom exponent (AE). Sulphuric acid molecule up to 70 molecules This represents stable particle formaTtiwonobyrecent model papers arrive at different conclusions on nm) followed by a slowly increasing bin size to around 450 nucleation. – Optical properties: the total optical depth and Angstrom exponent is calculated as a function of time for the combined sea salt distribution and sulphuric acid particles
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