We present global direct radiative effect (DRE) calculations of carbonaceous aerosols emitted from biomass/biofuel burning addressing the interplay between two poorly constrained contributions to DRE: mixing state of black carbon (lensing) and light absorption by organic aerosol (OA) due to the presence of brown carbon (BrC). We use the parameterization of Saleh et al. (2014) which captures the variability in biomass/biofuel OA absorption. The global mean effect of OA absorption is +0.22 W/m2 and +0.12 W/m2 for externally and internally mixed cases, while the effect of lensing is +0.39 W/m2 and +0.29 W/m2 for nonabsorbing and absorbing OA cases, signifying the nonlinear interplay between OA absorption and lensing. These two effects can be overestimated if not treated simultaneously in radiative transfer calculations. The combined effect of OA absorption and lensing increases the global mean DRE of biomass/biofuel aerosols from −0.46 W/m2 to +0.05 W/m2 and appears to reduce the gap between existing model‐based and observationally constrained DRE estimates. We observed a strong sensitivity to these parameters in key regions, where DRE shifts from strongly negative (< −1 W/m2) to strongly positive (> +1 W/m2) when accounting for lensing and OA absorption.

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Most urban and regional models used to predict levels of organic particulate matter (OPM) are based on fundamental equations for gas/particle partitioning, but make the highly simplifying, anonymized-view (AV) assumptions that OPM levels are not affected by either: a) the molecular characteristics of the condensing organic compounds (other than simple volatility); or b) cocondensation of water as driven by non-zero relative humidity (RH) values. The simplifying assumptions have allowed parameterized chamber results for formation of secondary organic aerosol (SOA) (e.g., "two-product" (2p) coefficients) to be incorporated in chemical transport models. However, a return towards a less simplistic (and more computationally demanding) molecular view (MV) is needed that acknowledges that atmospheric OPM is a mixture of organic compounds with differing polarities, water, and in some cases dissolved salts. The higher computational cost of MV modeling results from a need for iterative calculations of the composition-dependent gas/particle partition coefficient values. MV modeling of OPM that considered water uptake (but not dissolved salts) was carried out for the southeast United States for the period August 29 through September 7, 2006. Three model variants were used at three universities: CMAQ-RH-2p (at PSU), UCD/CIT-RH-2p (at UCD), and CMAQ-RH-MCM (at TAMU). With the first two, MV structural characteristics (carbon number and numbers of functional groups) were assigned to each of the 2p products used in CMAQv.4.7.1 such that resulting predicted K p,i values matched those in CMAQv.4.7.1. When water uptake was allowed, most runs assumed that uptake occurred only into the SOA portion, and imposed immiscibility of SOA with primary organic aerosol (POA). (POA is often viewed as rather non-polar, while SOA is commonly viewed as moderately-to-rather polar. Some runs with UCD/CIT-RH-2p were used to investigate the effects of POA/SOA miscibility.) CMAQ-RH-MCM used MCM to generate oxidation products, and assumed miscibility of SOA and POA. In a ~500 km wide band from ACCEPTED MANUSCRIPT Louisiana through to at least North Carolina, CMAQ-RH-2p and UCD/CIT-RH-2p predicted that water uptake can increase SOA levels by as much as 50 to 100% (from a range of ~1 to 2 µg m-3 to a range of ~1 to 4 µg m-3). CMAQ-RH-MCM predicted much lower effects of water uptake on SOA levels (<10% increase). The results from CMAQ-RH-2p and UCD/CIT-RH-2p are considered more reflective of reality. In the Alabama/Georgia hotspot, both CMAQ-RH-2p and UCD/CIT-RH-2p predicted aerosol water levels that are up to nearly half the predicted SOA levels, namely ~0.5 to 2 µg m-3. Such water levels in SOA will affect particle optical properties, viscosity, gas/particle partitioning times, and rates of hydrolysis and water elimination reactions.

Abstract. Aerosols directly affect the radiative balance of the Earth through the absorption and scattering of solar radiation. Although the contributions of absorption (heating) and scattering (cooling) of sunlight have proved difficult to quantify, the consensus is that anthropogenic aerosols cool the climate, partially offsetting the warming by rising greenhouse gas concentrations. Recent estimates of global direct anthropogenic aerosol radiative forcing (i.e., global radiative forcing due to aerosol-radiation interactions) are −0.35 ± 0.5 W m −2 , and these estimates depend heavily on aerosol simulation. Here, we integrate a comprehensive suite of satellite and ground-based observations to constrain total aerosol optical depth (AOD), its fine-mode fraction, the vertical distribution of aerosols and clouds, and the collocation of clouds and overlying aerosols. We find that the direct fine-mode aerosol radiative effect is −0.46 W m −2 (−0.54 to −0.39 W m −2 ). Fine-mode aerosols include sea salt and dust aerosols, and we find that these natural aerosols result in a very large cooling (−0.44 to −0.26 W m −2 ) when constrained by observations. When the contribution of these natural aerosols is subtracted from the fine-mode radiative effect, the net becomes −0.11 (−0.28 to +0.05) W m −2 . This net arises from total (natural + anthropogenic) carbonaceous, sulfate and nitrate aerosols, which suggests that global direct anthropogenic aerosol radiative forcing is less negative than −0.35 W m −2 .

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Aerosols directly affect the radiative balance of the Earth through absorption and scattering of solar radiation. Although the contributions of absorption (heating) and scattering (cooling) of sunlight have proved difficult to quantify, the consensus is that anthropogenic aerosols cool the climate, partially offsetting the warming by rising greenhouse gas concentrations. Recent estimates of global direct aerosol radiative forcing are &amp;#8722;0.35 &amp;#177; 0.5 Wm&lt;sup&gt;&amp;#8722;2&lt;/sup&gt;, and these estimates depend either entirely or heavily on aerosol simulation. Here, we integrate a comprehensive suite of satellite and ground-based observations to constrain total AOD, its fine-mode fraction, the vertical distribution of aerosols and clouds, and the co-location of clouds and overlying aerosols. We find that fine-mode forcing is &amp;#8722;0.46 Wm&lt;sup&gt;&amp;#8722;2&lt;/sup&gt; (&amp;#8722;0.54 ~ &amp;#8722;0.39 Wm&lt;sup&gt;&amp;#8722;2&lt;/sup&gt;). Fine-mode aerosols include sea salt and dust aerosols, and we find that these natural aerosols pose a very large cooling (&amp;#8722;&lt;/sup&gt;0.44 ~ &amp;#8722;0.26 Wm&lt;sup&gt;&amp;#8722;2&lt;/sup&gt;) when constrained by observations. When the contribution of these natural aerosols is subtracted from the fine-mode forcing, the net becomes &amp;#8722;0.10 (&amp;#8722;0.28 ~ +0.05) Wm&lt;sup&gt;&amp;#8722;2&lt;/sup&gt;. The net forcing arises from carbonaceous, sulfate and nitrate aerosols. Despite uncertainties in the anthropogenic fraction of these aerosols, this &amp;#8722;0.28 ~ +0.05 Wm&lt;sup&gt;&amp;#8722;2&lt;/sup&gt; range compels the direct aerosol forcing to be near zero.&lt;/p&gt;