Project Description and Overview: Objectives: Determining the overall impact of atmospheric aerosols on radiative balance requires knowledge of the relative amounts of scattering and absorbing aerosols, their distributions, and their chemical and optical properties. This proposal was a continuation of measurements of aerosol scattering and absorption begun in Mexico City in 2003 in collaboration with MCMA 2003 and continuing in the Atmospheric Science Program field study, Megacity Aerosol Experiment-Mexico City, (MAX-Mex) during March of 2006 aimed at determining the variability of aerosol optical properties. A suite of instrumentation was deployed in MAX-Mex at site TO, located in the northern part of the Mexico City Metropolitan Area, (MCMA), for the characterization of the aerosol optical properties in the field. Measurements were made of the following aerosol properties: (1) aerosol absorption as a function of wavelength, measured at two minute intervals with a 7-wavelength Aethalometer (2) aerosol scattering as a function of wavelength, measured at one minute intervals with a 3-wavelength nephelometer; 3) aerosol scattering as a function of relative humidity (RH), measured at one minute intervals with 2 single-wavelength nephelometers operated under dry (10% RH) and wet (80% RH) conditions; and 4) collection of size-fractionated aerosol samples on quartz fiber filters at 12 hour intervals (day/night) for further laboratory characterization. Aerosol filter samples were also collected at site Tl (located north of MCMA) for comparison with those collected in the city center. Preliminary results from in situ measurements have indicated an enhanced UV absorption in the afternoon over that expected from black carbon (BC) aerosols alone. These results are directly applicable to both modeling of aerosol radiative forcing and satellite optical depth retrieval algorithms. Both of these applications assume that the aerosol absorption is due only to BC with a wavelength dependence of A, " whereas results obtained in MAX-Mex show that the aerosol wavelength exponent varies over Mexico City from-0.7 to-1.5. All of the data collected in the field from the measurement sets 1-3 have been made available to the ASP community via the MILAGRO data site housed at NCAR. The laboratory characterization of aerosol samples collected in the ASP MAX-Mex field study compared results from Mexico City to samples collected at other sites, including Chicago, Little Rock, and Mt. Bachelor, OR. The project focused on obtaining complete spectral characterization of aerosols-especially their absorption characteristics as they relate to basic chemical functional groups. Particular attention was given to organics and from biogenic derived organic compounds. This included determinations of the UV-Visible-NIR characteristics of the aerosol absorption as reported as Angstrom Absorption Exponents. Correlation of these results with IR band observations of carboxylic acid, and carboxylate groups were conducted, along with past correlations with carbon...

Abstract. A suite of offline and real-time gas- and particle-phase measurements was deployed at Look Rock, Tennessee (TN), during the 2013 Southern Oxidant and Aerosol Study (SOAS) to examine the effects of anthropogenic emissions on isoprene-derived secondary organic aerosol (SOA) formation. High- and low-time-resolution PM2.5 samples were collected for analysis of known tracer compounds in isoprene-derived SOA by gas chromatography/electron ionization-mass spectrometry (GC/EI-MS) and ultra performance liquid chromatography/diode array detection-electrospray ionization-high-resolution quadrupole time-of-flight mass spectrometry (UPLC/DAD-ESI-HR-QTOFMS). Source apportionment of the organic aerosol (OA) was determined by positive matrix factorization (PMF) analysis of mass spectrometric data acquired on an Aerodyne Aerosol Chemical Speciation Monitor (ACSM). Campaign average mass concentrations of the sum of quantified isoprene-derived SOA tracers contributed to ~ 9 % (up to 28 %) of the total OA mass, with isoprene-epoxydiol (IEPOX) chemistry accounting for ~ 97 % of the quantified tracers. PMF analysis resolved a factor with a profile similar to the IEPOX-OA factor resolved in an Atlanta study and was therefore designated IEPOX-OA. This factor was strongly correlated (r2 > 0.7) with 2-methyltetrols, C5-alkene triols, IEPOX-derived organosulfates, and dimers of organosulfates, confirming the role of IEPOX chemistry as the source. On average, IEPOX-derived SOA tracer mass was ~ 26 % (up to 49 %) of the IEPOX-OA factor mass, which accounted for 32 % of the total OA. A low-volatility oxygenated organic aerosol (LV-OOA) and an oxidized factor with a profile similar to 91Fac observed in areas where emissions are biogenic-dominated were also resolved by PMF analysis, whereas no primary organic aerosol (POA) sources could be resolved. These findings were consistent with low levels of primary pollutants, such as nitric oxide (NO ~ 0.03 ppb), carbon monoxide (CO ~ 116 ppb), and black carbon (BC ~ 0.2 μg m−3). Particle-phase sulfate is fairly correlated (r2 ~ 0.3) with both methacrylic acid epoxide (MAE)/hydroxymethyl-methyl-α-lactone (HMML)- (henceforth called methacrolein (MACR)-derived SOA tracers) and IEPOX-derived SOA tracers, and more strongly correlated (r2 ~ 0.6) with the IEPOX-OA factor, in sum suggesting an important role of sulfate in isoprene SOA formation. Moderate correlation between the MACR-derived SOA tracer 2-methylglyceric acid with sum of reactive and reservoir nitrogen oxides (NOy; r2 = 0.38) and nitrate (r2 = 0.45) indicates the potential influence of anthropogenic emissions through long-range transport. Despite the lack of a clear association of IEPOX-OA with locally estimated aerosol acidity and liquid water content (LWC), box model calculations of IEPOX uptake using the simpleGAMMA model, accounting for the role of acidity and aerosol water, predicted the abundance of the IEPOX-derived SOA tracers 2-methyltetrols and the corresponding sulfates with good accuracy (r2 ~ 0.5 and ~ 0.7, respectively). The modeling and data combined suggest an anthropogenic influence on isoprene-derived SOA formation through acid-catalyzed heterogeneous chemistry of IEPOX in the southeastern US. However, it appears that this process was not limited by aerosol acidity or LWC at Look Rock during SOAS. Future studies should further explore the extent to which acidity and LWC as well as aerosol viscosity and morphology becomes a limiting factor of IEPOX-derived SOA, and their modulation by anthropogenic emissions.

[1] The study uses satellite observations, global assimilated aerosol data sets, Atmospheric Brown Clouds (ABC) observatories, a Monte Carlo aerosol-cloud-radiation model and a regional chemical transport model (STEM-2K) to characterize the spatial extent of brown clouds, regional and megacity ABC hot spots, chemical composition and the direct radiative forcing. It presents the first annual cycle of aerosol observations and forcing from the ABC observatories in the Indo-Asia-Pacific regions. East Asia, IndoGangetic Plains, Indonesian region, southern Africa and the Amazon basin are the regional hot spots defined by the criteria that anthropogenic aerosol optical depths (AODs) should exceed 0.3 and absorbing AOD > 0.03. Over these hot spots, as well as in other polluted oceanic regions, the EC mass exceeds 0.5 mg m À3 , the OC mass exceeds 2 mg m À3 and sulfate mass exceeds 10 mg m À3 from the surface to 3 km. The brown clouds also have strong seasonal dependence. In the tropics the seasonal dependence is driven by pollution accumulating during the dry seasons, December to February in Northern Hemisphere tropics and June to August in Southern Hemisphere tropics. In the extratropics the pollution peaks during the summer. The brown cloud problem is not restricted to the tropical regions. Over the eastern half of US and western Europe the AODs exceeds 0.2 and absorption AODs exceed 0.02. Brown clouds also extend well into the western Pacific Ocean, the Indian Ocean reaching as far south as 60°S and the eastern Atlantic Ocean. The largest total SO 2 emission occurs over China and US, while SO 2 emission per unit surface area is maximum over Germany and England. The largest total EC and OC emissions occur over China, but the largest OC emission per unit surface area occur over India. As a result, the maximum negative annual mean TOA direct forcing is over India and Germany. The surface annual-diurnal mean dimming over the regional hot spots is of the order of À10 W m À2 and À20 W m À2 over megacity hotpots. Citation: Ramanathan, V., et al. (2007), Atmospheric brown clouds: Hemispherical and regional variations in long-range transport, absorption, and radiative forcing,

Ambient sampling was conducted in Riverside, California during the 2005 Study of Organic Aerosols in Riverside to characterize the composition and sources of organic aerosol using a variety of state-of-the-art instrumentation and source apportionment techniques. The secondary organic aerosol (SOA) mass is estimated by elemental carbon and carbon monoxide tracer methods, water soluble organic carbon content, chemical mass balance of organic molecular markers, and positive matrix factorization of high-resolution aerosol mass spectrometer data. Estimates obtained from each of these methods indicate that the organic fraction in ambient aerosol is overwhelmingly secondary in nature during a period of several weeks with moderate ozone concentrations and that SOA is the single largest component of PM 1 aerosol in Riverside. Average SOA/OA contributions of 70-90% were observed during midday periods, whereas minimum SOA contributions of ∼45% were observed during peak morning traffic periods. These results are contrary to previous estimates of SOA throughout the Los Angeles Basin which reported that, other than during severe photochemical smog episodes, SOA was lower than primary OA. Possible reasons for these differences are discussed.