Organic and Elemental Carbon (OC & EC) vs Altitude in Asian Outflow During ACE-Asia

Because airborne sampling legs are usually short relative to typical sampling times for carbonaceous aerosols, the number of published OC and EC altitude profiles is limited. It is only recently that Novakov et al. suggested that there may be as much carbonaceous aerosol as sulfate in the free troposphere, a radical change from the "almost all sulfate" view of many authors. We measured OC and EC during ACE-Asia from the NSF/NCAR C-130 aircraft, using a PC-BOSS sampler developed by Delbert Eatough of BYU. This sampler not only preconcentrated the samples (improving our S/N by about 3-5x for a given sampling time), but it also eliminated or minimized two major OC sampling artifacts: 1) a VOC denuder made of carbon-impregnated-glass (CIG) filter strips minimized the positive artifact due to adsorption of organic vapors on our quartz filters and 2) a CIG filter behind the quartz collected any volatilized OC aerosol that would have caused a negative artifact. Our data show that the positive artifact would often have been many times the actual OC, while the negative artifact would have been between 10 and 50% of the actual OC. Rarely was our S/N less than 2, in part because of relatively high OC and EC concentrations in Asian outflow and because of the enhancement by the PC-BOSS sampler. OC varied from 0.3 to 34 ug C/m3, with a median of 4 ug C/m3. EC was between 0.1 and 37 ug C/m3, with a median of 1.6 ug C/m3. These concentrations of black carbon will have a significant radiative impact. The median EC/OC ratio was 0.4, with an average of 0.8. The median ratio of OC (as carbon, with no multiplier for other elements) to NSS was 0.6, with an average of 1.6. Clearly organic aerosol concentrations are comparable to those of sulfate in Asian outflow.