A comparison of aerosol chemical and optical properties from the 1st and 2nd Aerosol Characterization Experiments

Shipboard measurements of aerosol chemical composition and optical properti es were made during both ACE-1 and ACE-2, ACE-1 focused on remote marine ae rosol minimally perturbed by continental sources. ACE-2 studied the outflow of European aerosol into the NE Atlantic atmosphere. A variety of air mass es were sampled during ACE-2 including Atlantic, polar, Iberian Peninsula, Mediterranean, and Western European. Reported here are mass size distributi ons of non-sea salt (nss) sulfate, sea salt, and methanesulfonate and submi cron and supermicron concentrations of black and organic carbon. Optical pa rameters include submicron and supermicron aerosol scattering and backscatt ering coefficients at 550 nm, the absorption coefficient at 550 +/- 20 nm, the Angstrom exponent for the 550 and 700 nm wavelength pair, and single sc attering albedo at 550 nm. All data are reported at the measurement relativ e humidity of 55%. Measured concentrations of nss sulfate aerosol indicate that, relative to ACE-1, ACE-2 aerosol during both marine and continental f low was impacted by continental sources. Thus, while sea salt controlled th e aerosol chemical composition and optical properties of both the submicron and supermicron aerosol during ACE-1, it played a relatively smaller role in ACE-2. This is confirmed by the larger average Angstrom exponent for ACE -2 continental aerosol of 1.2 +/- 0.26 compared to the ACE-1 average of -0. 03 +/- 0.38. The depletion of chloride from sea salt aerosol in ACE-2 conti nental air masses averaged 55 +/- 25% over all particle sizes. This compare s to the ACE-2 marine average of 4.8 +/- 18% and indicates the enhanced int eraction of anthropogenic acids with sea salt as continental air masses are transported into the marine atmosphere. Single scattering albedos averaged 0.95 +/- 0.03 for ACE-2 continental air masses. Averages for ACE-2 and ACE -1 marine air masses were 0.98 +/- 0.01 and 0.99 +/- 0.01, respectively.