Background levels of black carbon over remote marine locations

Abstract

Approximately 80% of the Southern Hemisphere (SH) surface is oceanic, while the Northern Hemisphere (NH) hosts 85% of the world's population along with a broadly equivalent anthropogenic emissions contribution. Consequently, the so-called ‘background’ marine air loading would be expected to be notably lower, or cleaner, in the SH than that over the NH. To test this supposition, we investigate the levels of air pollution over the Northeast Atlantic, at the Mace Head Global Atmospheric Watch station (located 53.3253°N, 9.9044°W), and on a ship-borne cruise in the Southern Ocean (50–65°S and 40–65°W). The highly sensitive single-particle soot photometer (SP2) was deployed to measure refractory black carbon (BC) at one-minute temporal resolution during the Austral Summer from January 2nd – February 12th 2015 for the SH campaign and April 2nd – 20th 2015 for the NH campaign. The aerosol organic matter (OM) was also measured as a relevant adjunct measurement. We found the percentage ‘frequency-of-occurrence’ of BC mass concentration ranging from 0 to 1000 ng m−3. For both the SH and NH distributions, we found that these distributions could be fitted by a series of lognormal distributions (8-modes). The SH distribution had four modes with BC concentrations <0.1 ng m−3, while the NH distribution had only one mode at concentrations <0.1 ng m−3. Both distributions had a maximum modal occurrence of ~270 ng m−3. The lognormal distributions are statistical representations of particular aerosol sources or ageing. In this context, we can regard the lower concentration modes in the frequency-of-occurrence distribution as representative of the marine background. Consequently, we conclude, using BC as an anthropogenic tracer, that while both oceans show low background concentrations of refractory BC, the SH campaign was cleaner much more frequently. The addition of regression and correlation analysis of OM vs BC also define what levels of BC can be regarded as thresholds for distinguishing between natural and anthropogenic OM over oceans, particularly in the NH where biogenic OM may be significantly greater than in the SH.