Atmospheric particulate matter plays an important role in the Earth's radiative balance. Over the past two decades, it has been established that a portion of particulate matter, black carbon, absorbs significant amounts of light and exerts a warming e ect rivalling that of anthropogenic carbon dioxide 1,2 . Most climate models treat black carbon as the sole light-absorbing carbonaceous particulate. However, some organic aerosols, dubbed brown carbon and mainly associated with biomass burning emissions 3-6 , also absorbs light 7 . Unlike black carbon, whose light absorption properties are well understood 8 , brown carbon comprises a wide range of poorly characterized compounds that exhibit highly variable absorptivities, with reported values spanning two orders of magnitude 3-6,9,10 . Here we present smog chamber experiments to characterize the e ective absorptivity of organic aerosol from biomass burning under a range of conditions. We show that brown carbon in emissions from biomass burning is associated mostly with organic compounds of extremely low volatility 11 . In addition, we find that the e ective absorptivity of organic aerosol in biomass burning emissions can be parameterized as a function of the ratio of black carbon to organic aerosol, indicating that aerosol absorptivity depends largely on burn conditions, not fuel type. We conclude that brown carbon from biomass burning can be an important factor in aerosol radiative forcing.Black carbon (BC) in atmospheric particulate matter is an important global warming agent (potentially second only to CO 2 ) with estimates of its direct radiative forcing (DRF) ranging between 0.17 and 1.48 W m −2 (ref. 2). The large uncertainty in BC DRF stems partly from the mismatch between BC light absorption (hence its DRF) estimated by climate models and that retrieved using remote sensing, with models usually reporting smaller values 2 . Open biomass burning contributes one-third of the global BC budget. Biomass burning is also a major source of organic aerosol (OA), contributing two-thirds of the global primary OA budget 2,12 , which most climate models treat as purely scattering. The cooling due to this scattering offsets the warming by BC from biomass burning, resulting in negative net DRF for biomass burning emissions 13 . However, there is a growing body of evidence that biomass burning OA contains substantial amounts of light-absorbing brown carbon 3-6 (BrC), which can shift the net biomass burning DRF to positive values 14 . Neglecting absorption by biomass burning OA might lead to misattribution of observed atmospheric particulate matter absorption to BC, contributing to the discrepancy between models and observations. There are substantial uncertainties in quantifying the effect of BrC. A major obstacle is the very high variability in reported light absorption properties of biomass burning OA, often attributed to fuel type and burn conditions 4,6 , which complicates their treatment in climate models.In this study, we show that the least volatile fraction (extreme...

Abstract. Motivated by the need to develop instrumental techniques for characterizing organic aerosol aging, we report on the performance of the Toronto Photo-Oxidation Tube (TPOT) and Potential Aerosol Mass (PAM) flow tube reactors under a variety of experimental conditions. The PAM system was designed with lower surface-area-tovolume (SA/V) ratio to minimize wall effects; the TPOT reactor was designed to study heterogeneous aerosol chemistry where wall loss can be independently measured. The following studies were performed: (1) transmission efficiency measurements for CO 2 , SO 2 , and bis(2-ethylhexyl) sebacate (BES) particles, (2) H 2 SO 4 yield measurements from the oxidation of SO 2 , (3) residence time distribution (RTD) measurements for CO 2 , SO 2 , and BES particles, (4) aerosol mass spectra, O/C and H/C ratios, and cloud condensation nuclei (CCN) activity measurements of BES particles exposed to OH radicals, and (5) aerosol mass spectra, O/C and H/C ratios, CCN activity, and yield measurements of secondary organic aerosol (SOA) generated from gas-phase OH oxidation of m-xylene and α-pinene. OH exposures ranged from (2.0 ± 1.0) × 10 10 to (1.8 ± 0.3) × 10 12 molec cm −3 s. Where applicable, data from the flow tube reactors are compared with published results from the Caltech smog chamber. The TPOT yielded narrower RTDs. However, its transmission efficiency for SO 2 was lower than that for the PAM. Transmission efficiency for BES and H 2 SO 4 particles was size-dependent and was similar for the two flowCorrespondence to: T. B. Onasch (onasch@aerodyne.com) tube designs. Oxidized BES particles had similar O/C and H/C ratios and CCN activity at OH exposures greater than 10 11 molec cm −3 s, but different CCN activity at lower OH exposures. The O/C ratio, H/C ratio, and yield of m-xylene and α-pinene SOA was strongly affected by reactor design and operating conditions, with wall interactions seemingly having the strongest influence on SOA yield. At comparable OH exposures, flow tube SOA was more oxidized than smog chamber SOA, possibly because of faster gas-phase oxidation relative to particle nucleation. SOA yields were lower in the TPOT than in the PAM, but CCN activity of flow-tubegenerated SOA particles was similar. For comparable OH exposures, α-pinene SOA yields were similar in the PAM and Caltech chambers, but m-xylene SOA yields were much lower in the PAM compared to the Caltech chamber.

Laboratory experiments investigated the relationship between oxidation level and hygroscopic properties of secondary organic aerosol (SOA) particles generated via OH radical oxidation in an aerosol flow reactor. The hygroscopic growth factor at 90% RH (HGF90%), the CCN activity (κORG,CCN) and the level of oxidation (atomic O:C ratio) of the SOA particles were measured. Both HGF90% and κORG,CCN increased with O:C; the HGF90% varied linearly with O:C, while κORG,CCN mostly followed a nonlinear trend. An average HGF90% of 1.25 and κORG,CCN of 0.19 were measured for O:C of 0.65, in agreement with results reported for ambient data. The κORG values estimated from the HGF90% (κORG,HGF) were 20 to 50% lower than paired κORG,CCN values for all SOA particles except 1,3,5‐trimethylbenzene (TMB), the least hygroscopic of the SOA systems. Within the limitations of instrumental capabilities, we show that differences in hygroscopic behavior among the investigated SOA systems may correspond to differences in elemental composition.