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Acid-catalyzed reactions between gas- and particle-phase constituents are critical to atmospheric secondary organic aerosol (SOA) formation. The aerosol-phase state is thought to influence the reactive uptake of gas-phase precursors to aerosol particles by altering diffusion rates within particles. However, few experimental studies have explored the precise role of the aerosol-phase state on reactive uptake processes. This laboratory study systematically examines the reactive uptake coefficient (γ) of trans-β-isoprene epoxydiol (trans-β-IEPOX), the predominant IEPOX isomer, on acidic sulfate particles coated with SOA derived from α-pinene ozonolysis. γIEPOX is obtained for core-shell particles, the morphology of which was confirmed by microscopy, as a function of SOA coating thickness and relative humidity. γIEPOX is reduced, in some cases by half of the original value, when SOA coatings are present prior to uptake, especially when coating thicknesses are > 15 nm. The diurnal trend of IEPOX lost to acid-catalyzed reactive uptake yielding SOA compared with other known atmospheric sinks (gas-phase oxidation or deposition) is derived by modeling the experimental coating effect with field data from the southeastern United States. IEPOX-derived SOA is estimated to be reduced by 16-27% due to preexisting organic coatings during the afternoon (12:00 to 7:00 p.m., local time), corresponding to the period with the highest level of production.

International audience; As part of the EU-FP7 ACTRIS program, a large international intercomparison study of 15 aerosol mass spectrometers (13 Q-ACSM, 1 ToF-ACSM and 1 HR-ToF-AMS) has been performed from 15 Nov. to 2 Dec. 2013 at the LSCE in-situ atmospheric platform which is part of the French SIRTA observatory (http://sirta.ipsl.fr) located at 20km southwest of Paris. During this period, each instrument measured the major non-refractory submicron aerosols (NR-PM1) components (organic matter, nitrate, sulfate, and ammonium) in ambient air. The accuracy of Q-ACSM instruments was determined by comparison with various co-located instruments (TEOM-FDMS, SMPS, OPC, OC-EC Sunset Field analyzer, PILS-IC, aethalometers, nephelometers, and filter sampling). The measurement precision was also evaluated by intercomparing the correlation of absolute mass concentrations for all the NR-PM1 species. The ACSM analytical uncertainties were then estimated by applying a statistical approach in order to evaluate the analytical standard deviations between ACSMs and to highlight any bias or influencing factor on the ACSM measurements. For this purpose, the Z-score indicator representing performance criteria was used making it possible to measure its relative deviation to the assigned value. Fig. 1 summarizes the Z-Score results applied to the 13 Q-ACSM datasets for the major chemical species of the NR-PM1. All the ACSMs present satisfactory Z-Score values (a Z-Score value of 3 is considered as the limit value) whatever the considered parameter highlighting the instrument precision.

As part of the EU-FP7 ACTRIS program (Aerosols, Clouds, and Trace gases Research InfraStructure Network; http://www.actris.net), 14 of 19 ACSMs (Aerosol Chemical Scpeciation Monitor; Aerodyne Research Inc., MA, USA) have been deployed for long-term observations within background monitoring stations, among which the French SIRTA atmospheric supersite (Site Instrumental de Recherche par Télédétection Atmosphérique; http://sirta.ipsl.fr) located 20km southwest of Paris. In order to have homogenous quality-controlled ACSM datasets at an European scale, an intercomparison study was conducted from 15 Nov. to 2 Dec. 2013 at the SIRTA station. The real-time mass concentrations and chemical composition (Organic, nitrate, sulfate, ammonium) of nonrefractory submicronic aerosols (NR-PM1) measured by the ACSMs were compared providing new insights on the precision of this instrument. Accuracy of these measurements was then evaluated with various co-located instruments (TEOM-FDMS, SMPS, OPC, OC-EC Sunset Field analyzer, PILS-IC, HR-ToF-AMS, aethalometers, nephelometers, filter sampling). Preliminary results of this significant intercomparison stduy are presented which compare ACSM measurement precision, accuracy and validity of source apportionment data.

Black carbon (BC) and light-absorbing organic carbon (brown carbon, BrC) play key roles in warming the atmosphere, but the magnitude of their effects remains highly uncertain. Theoretical modelling and laboratory experiments demonstrate that coatings on BC can enhance BC’s light absorption, therefore many climate models simply assume enhanced BC absorption by a factor of ~1.5. However, recent field observations show negligible absorption enhancement, implying models may overestimate BC’s warming. Here we report direct evidence of substantial field-measured BC absorption enhancement, with the magnitude strongly depending on BC coating amount. Increases in BC coating result from a combination of changing sources and photochemical aging processes. When the influence of BrC is accounted for, observationally constrained model calculations of the BC absorption enhancement can be reconciled with the observations. We conclude that the influence of coatings on BC absorption should be treated as a source and regionally specific parameter in climate models.