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...

Emissions from gasoline and diesel vehicles are predominant anthropogenic sources of reactive gas-phase organic carbon and key precursors to secondary organic aerosol (SOA) in urban areas. Their relative importance for aerosol formation is a controversial issue with implications for air quality control policy and public health. We characterize the chemical composition, mass distribution, and organic aerosol formation potential of emissions from gasoline and diesel vehicles, and find diesel exhaust is seven times more efficient at forming aerosol than gasoline exhaust. However, both sources are important for air quality; depending on a region's fuel use, diesel is responsible for 65% to 90% of vehicular-derived SOA, with substantial contributions from aromatic and aliphatic hydrocarbons. Including these insights on source characterization and SOA formation will improve regional pollution control policies, fuel regulations, and methodologies for future measurement, laboratory, and modeling studies.motor vehicle emission factors | photochemical oxidation | urban air quality | volatile organic compound emissions | petroleum fuel composition O rganic aerosol (OA) in the atmosphere is detrimental to human health and represents a highly uncertain forcing of climate change (1). The use of petroleum-derived fuels is an important source of reactive gas-phase organic carbon that provides key precursors to the formation of secondary OA (SOA) and tropospheric ozone (1). Controlling these emissions from gasoline and diesel vehicles is central to air quality mitigation policies in urban areas (2). Previous work has concluded that further research is necessary to elucidate all organic sources of SOA precursors (3, 4). Significant controversy exists over the contributions of precursors from gasoline and diesel vehicles, and the relative importance of each for SOA formation remains in question, in part because of insufficient chemical characterization of fuels and emissions, and the difficulty of ambient measurements of gasphase compounds emitted from diesel sources (1, 4-8).In the United States, diesel fuel accounts for 21% of on-road fuel use (by volume), with off-road sources increasing total use to 28% diesel. In California, the diesel share of on-road use ranges from approximately 10% in coastal cities to more than 30% in agricultural regions (SI Appendix, Table S1) (2, 9, 10). Noncombusted hydrocarbons from the fuels are emitted in the exhaust of gasoline and diesel engines, and also via evaporation from gasoline vehicles and service stations. These compounds in unburned gasoline and diesel fuel dominate vehicular emissions of reactive gas-phase carbon that have the potential to form SOA (11,12). Previous work has shown nontailpipe emissions account for ∼30% of gasoline-related emissions in urban regions, but limited work exists constraining the emissions and SOA formation potential of gas-phase organic carbon from gasoline and diesel sources (13). By using extensive fuel analyses and field data from two sites that include ...

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Organonitrate (ON) groups are thought to be important substituents in secondary organic aerosols (SOAs). Model simulations and laboratory studies indicate a large fraction of ON groups in aerosol particles, but much lower quantities are observed in the atmosphere. Hydrolysis of ON groups in aerosol particles has been proposed recently to account for this discrepancy. To test this hypothesis, we simulated formation of ON molecules in a reaction chamber under a wide range of relative humidity (RH) (0 to 90%). The mass fraction of ON groups (5 to 20% for high-NO x experiments) consistently decreased with increasing RH, which was best explained by hydrolysis of ON groups at a rate of 4 day −1 (lifetime of 6 h) for reactions under RH greater than 20%. In addition, we found that secondary nitrogen-containing molecules absorb light, with greater absorption under dry and high-NO x conditions. This work provides the first evidence for particle-phase hydrolysis of ON groups, a process that could substantially reduce ON group concentration in atmospheric SOAs.

Laboratory studies have established a number of chemical pathways by which nitrogen oxides (NO(x)) affect atmospheric organic aerosol (OA) production. However, these effects have not been directly observed in ambient OA. We report measurements of particulate organic nitrates in Bakersfield, California, the nighttime formation of which increases with NO(x) and is suppressed by high concentrations of organic molecules that rapidly react with nitrate radical (NO(3))--evidence that multigenerational chemistry is responsible for organic nitrate aerosol production. This class of molecules represents about a third of the nighttime increase in OA, suggesting that most nighttime secondary OA is due to the NO(3) product of anthropogenic NO(x) emissions. Consequently, reductions in NO(x) emissions should reduce the concentration of organic aerosol in Bakersfield and the surrounding region.

Organic aerosols (OAs) in the atmosphere affect Earth's energy budget by not only scattering but also absorbing solar radiation due to the presence of the so-called "brown carbon" (BrC) component. However, the absorptivities of OAs are not represented or are poorly represented in current climate and chemical transport models. In this study, we provide a method to constrain the BrC absorptivity at the emission inventory level using recent laboratory and field observations. We review available measurements of the light-absorbing primary OA (POA), and quantify the wavelength-dependent imaginary refractive indices (kOA, the fundamental optical parameter determining the particle's absorptivity) and their uncertainties for the bulk POA emitted from biomass/biofuel, lignite, propane, and oil combustion sources. In particular, we parametrize the kOA of biomass/biofuel combustion sources as a function of the black carbon (BC)-to-OA ratio, indicating that the absorptive properties of POA depend strongly on burning conditions. The derived fuel-type-based kOA profiles are incorporated into a global carbonaceous aerosol emission inventory, and the integrated kOA values of sectoral and total POA emissions are presented. Results of a simple radiative transfer model show that the POA absorptivity warms the atmosphere significantly and leads to ∼27% reduction in the amount of the net global average POA cooling compared to results from the nonabsorbing assumption.

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An enhanced aerosol layer near the tropopause over Asia during the June-September period of the Asian summer monsoon (ASM) was recently identified using satellite observations. Its sources and climate impact are presently not well-characterized. To improve understanding of this phenomenon, we made in situ aerosol measurements during summer 2015 from Kunming, China, then followed with a modeling study to assess the global significance. The in situ measurements revealed a robust enhancement in aerosol concentration that extended up to 2 km above the tropopause. A climate model simulation demonstrates that the abundant anthropogenic aerosol precursor emissions from Asia coupled with rapid vertical transport associated with monsoon convection leads to significant particle formation in the upper troposphere within the ASM anticyclone. These particles subsequently spread throughout the entire Northern Hemispheric (NH) lower stratosphere and contribute significantly (∼15%) to the NH stratospheric column aerosol surface area on an annual basis. This contribution is comparable to that from the sum of small volcanic eruptions in the period between 2000 and 2015. Although the ASM contribution is smaller than that from tropical upwelling (∼35%), we find that this region is about three times as efficient per unit area and time in populating the NH stratosphere with aerosol. With a substantial amount of organic and sulfur emissions in Asia, the ASM anticyclone serves as an efficient smokestack venting aerosols to the upper troposphere and lower stratosphere. As economic growth continues in Asia, the relative importance of Asian emissions to stratospheric aerosol is likely to increase.