title: Investigation of a systematic offset in the measurement of organic carbon with a semicontinuous analyzer.
abstract: Organic carbon (OC) was measured semicontinuously in laboratory experiments of steady-state secondary organic aerosol formed by hydrocarbon + nitrogen oxide irradiations. Examination of the mass of carbon measured on the filter for various sample volumes reveals a systematic offset that is not observed when performing an instrumental blank. These findings suggest that simple subtraction of instrumental blanks determined as the standard analysis without sample collection (i.e., by cycling the pump and valves yet filtering zero liters of air followed by routine chemical analysis) from measured concentrations may be inadequate. This may be especially true for samples collected through the filtration of small air volumes wherein the influence of the systematic offset is greatest. All of the experiments show that filtering a larger volume of air minimizes the influence of contributions from the systematic offset. Application of these results to measurements of ambient concentrations of carbonaceous aerosol suggests a need for collection of sufficient carbon mass to minimize the relative influence of the offset signal.
authors.affiliation: U.S. Environmental Protection Agency, National Exposure Research Laboratory, Human Exposure Atmospheric Sciences Division, Research Triangle Park, NC 27711, USA. offenberg.john@epa.gov
authors.forename: John H
authors.initials: JH
authors.lastname: Offenberg
authors.forename: Michael
authors.initials: M
authors.lastname: Lewandowski
authors.forename: Edward O
authors.initials: EO
authors.lastname: Edney
authors.forename: Tadeusz E
authors.initials: TE
authors.lastname: Kleindienst
authors.forename: Mohammed
authors.initials: M
authors.lastname: Jaoui
mesh_terms: D000388:Air / Q000032:analysis; D000393:Air Pollutants / Q000032:analysis*; D002244:Carbon / Q000032:analysis* / Q000737:chemistry; D004784:Environmental Monitoring / Q000379:methods*; D005374:Filtration; D006838:Hydrocarbons / Q000032:analysis / Q000737:chemistry; D009589:Nitrogen Oxides / Q000032:analysis / Q000737:chemistry; D010777:Photochemistry
chemical_list: D000393:Air Pollutants; D006838:Hydrocarbons; D009589:Nitrogen Oxides; D002244:Carbon