Characterization of carbonaceous aerosols during the MINOS campaign in Crete, July–August 2001: a multi-analytical approach

[摘要] During the major part of the Mediterranean Intensive Oxidant Study(MINOS) campaign (summer 2001, Crete Isl.), the Marine Boundary Layer (MBL) air was influenced by longrange transport of biomass burning from the northern and western part of the Black Sea.During this campaign, carbonaceous aerosols were collected on quartz filters at a FreeTropospheric (FT) site, and at a MBL site together with size-resolved distribution of aerosols.Three Evolution Gas Analysis (EGA) protocols have been tested in order to bettercharacterize the collected aged biomass burning smoke: A 2-step thermal method(Cachier et al., 1989) and a thermo-optical technique using two different temperature programs. The latertemperature programs are those used for IMPROVE (Interagency Monitoring of ProtectedVisual Environments) and NIOSH 5040 (National Institute of Occupational Safety andHealth). Artifacts were observed using the NIOSH temperature program and identified asinteractions between carbon and dust deposited on the filter matrix at high temperature(T>550ºC) under the pure helium step of the analysis.

During the MINOS campaign, Black Carbon (BC) and Organic Carbon (OC) mass concentrations were on average respectively1.19±0.56 and 3.62±1.08 mgC/m³ for theIMPROVE temperature program, and 1.09±0.36 and 3.75±1.24 mgC/m³for the thermal method. Though these values compare well on average and the agreement between the TotalCarbon (TC) measurements sample to sample was excellent (slope=1.00, r²=0.93, n=56),important discrepancies were observed in determining BC concentrations from these twomethods (average error of 33±22%). BC from the IMPROVE temperature program comparedwell with non-sea-salt potassium (nss-K) pointing out an optical sensitivity to biomassburning. On the other hand, BC from the thermal method showed a better agreement withnon-sea-salt sulfate (nss-SO₄), considered as a tracer for fossil fuel combustion during theMINOS campaign. The coupling between these two methods for determining BC brings herenew insights on the origin of carbonaceous aerosols in a complex mixture of different sources.It brings also to our attention that important deviations in BC levels are observed using threewidely used EGA's techniques and most probably none of the EGA tested here are welladapted to fully characterize this aerosol mixture.

Spherical, smooth and silico-aluminated fly-ash observed by an Analytical Scanning ElectronMicroscope (ASEM) confirm the influence of coal combustion on the carbonaceous aerosolload throughout the campaign. A rough calculation based on a BC/nss-SO₄ mass ratiosuggests that biomass burning could be responsible for half of the BC concentration recordedduring the MINOS campaign.

From the plot of BC as a function of TC, two linear correlations were observed correspondingto 2 times series (before and after 12 August). Such good correlations suggest, from a firstlook, that both BC and OC have similar origin and atmospheric transport. On the other hand,the plot of BC as a function of TC obtained from the 2-step thermal method applied toDEKATI Low Pressure Cascade Impactor samples does not show a similar correlation andpoints out a non conservative distribution of this ratio with 2 super micron modes enriched inOC, correlated with sea salt aerosols and probably originating from gas-to-particleconversion.

[学科分类] 大气科学