Retrieval of atmospheric elemental carbon records using lake sediments: Implications in radiative forcing

Elemental or black carbon (EC or BC) aerosols produced during incomplete combustion strongly absorb solar radiation and contribute to global warming, and cause cardiopulmonary disease. Long-term atmospheric EC measurements, [EC]atm, are needed to validate global climate models to estimate the impact of EC on earth's temperature. Such data is sparse.;In this work, a new technique was developed to retrieve the historical record of [EC]atm in the Northeastern US for the past two centuries. Measurements of [EC]atm were made in the monthly composites of daily filters collected over ∼30 yr at Whiteface Mountain (WFM), NY using the thermal optical method. Bottom sediment cores were collected from four lakes near WFM. They were sliced in horizontal sections, freeze dried, and their ages determined 210Pb dating technique. EC in sediments was chemically separated and its concentration determined using the same thermal-optical method. It was shown that [EC]sed = K [EC]atm where K is constant (m3/g). Measurements of [EC]atm and [EC]sed for the ∼1978 to 2005 period was used to determine the value for K. The value of K and [EC]sed for periods before 1978 were used to determine [EC]atm for the past ∼100 yrs. [EC]atm in the preindustrial period in US, ∼1850, varied between 38 and 73 ng/m3, with a mean value of 56 +/- 14 ng/m3. [EC]atm was found to increase sharply with rapid industrialization and reached its maximum value of 751 +/- 265 ng/m3 during 1920s, which was a factor of ∼12 higher compared to the mean preindustrial level. The [EC]atm declined gradually until ∼1980 and then decreased sharply. Directly measured values of [EC]atm are only ∼25% higher compared to the mean preindustrial level. Model US EC emissions estimates of Novakov et al. (2003), based on energy consumptions, reproduce our [EC]sed trends quite well for the ∼1900 to 1930 period. Subsequently, the model EC values drop-off more rapidly than our [EC]atm. To extend the technique where long tern [EC]atm are not available, a new generalized mathematical model expression to determine K was developed. The value of K calculated using the model agreed within +/-30% with the measurements.