Abstract. Dichotomous aerosols (nominal super-and subµm-diameter size fractions) in sectored on-shore flow were sampled daily from July 2006 through June 2009, at the Tudor Hill Atmospheric Observatory (THAO) on the western coast of Bermuda (32.27 • N, 64.87 • W) and analyzed for major chemical and physical properties. FLEXPART retroplumes were calculated for each sampling period and aerosol properties were stratified accordingly based on transport from different regions. Transport from the northeastern United States (NEUS) was associated with significantly higher (factors of 2 to 3 based on median values) concentrations of bulk particulate non-sea-salt (nss) SO 2− 4 , NO − 3 , and NH + 4 and associated scattering and absorption at 530 nm, relative to transport from Africa (AFR) and the oceanic background. These differences were driven primarily by higher values associated with the sub-µm size fraction under NEUS flow. We estimate that 75(± 3) % of the NEUS nss SO 2− 4 was anthropogenic in origin, while only 25(± 9) % of the AFR nss SO 2− 4 was anthropogenic. Integrating over all transport patterns, the contribution of anthropogenic sulfate has dropped 14.6 % from the early 1990s. Bulk scattering was highly correlated with bulk nss SO 2− 4 in all flow regimes but the corresponding regression slopes varied significantly reflecting differential contributions to total scattering by associated aerosol components. Absorption by super-µm aerosol in transport from the NEUS versus AFR was similar although the super-µm aerosol size fraction accounted for a relatively greater contribution to total absorption in AFR flow. Significantly greater absorption Ångström exponents (AAEs) for AFR flow reflects the wavelength dependence of absorption by mineral aerosols; lower AAEs for NEUS flow is consistent with the dominance of absorption by combustionderived aerosols. Higher AOD associated with transport from both the NEUS and AFR relative to oceanic background flow results in a top of atmosphere direct radiative forcing on the order of −1.6 to −2.5 W m −2 , respectively, showing these aerosols drive cooling. The dominance of transport from the NEUS on an annual basis coupled with the corresponding decreases in anthropogenic nss SO 2− 4 aerosols since the early 1990s implies that emission reductions in the US account for a decline in atmospheric cooling over the western North Atlantic Ocean during this period.

Abstract.Light scattering by size-resolved aerosols in nearsurface air at Tudor Hill, Bermuda, was measured between January and June 2009. Vertical distributions of aerosol backscattering and column-averaged aerosol optical properties were characterized in parallel with a micro-pulse lidar (MPL) and an automated sun-sky radiometer. Comparisons were made between extensive aerosol parameters in the column, such as the lidar-retrieved extinction at 400 m and the aerosol optical depth (AOD), and scattering was measured with a surface nephelometer. Comparisons were also made for intensive parameters such as the Ångström exponent and calculations using AERONET(Aerosol Robotic Network)-derived aerosol physical parameters (size distribution, index of refraction) and Mie theory, and the ratio of submicron scattering to total scattering for size-segregated nephelometer measurements. In these comparisons the r 2 was generally around 0.50. Data were also evaluated based on back trajectories. The correlation between surface scattering and lidar extinction was highest for flows when the surface scattering was dominated by smaller particles and the flow had a longer footprint over land then over the ocean. The correlation of AOD with surface scatter was similar for all flow regimes. There was also no clear dependence of the atmospheric lapse rate, as determined from a nearby radiosonde station, on flow regime. The Ångström exponent for most flow regimes was 0.9-1.0, but for the case of air originating from North America, but with significant time over the ocean, the Ångström exponent was 0.57 ± 0.18. The submicron fraction of aerosol near the surface (R sub-surf ) was significantly greater for the flows from land (0.66 ± 0.11) than for the flows which spent more time over the ocean (0.40 ± 0.05). When comparing R sub-surf and the columnintegrated submicron scattering fraction, R sub-col , the correlation was similar, r 2 = 0.50, but R sub-surf was generally less than R sub-col , indicating more large particles contributing to light scattering at the surface, contrary to conditions over continents and for polluted continental transport over the ocean. In general, though, the marginal correlations indicate that the column optical properties are weakly correlated with the surface optical measurements. Thus, if it is desired to associate aerosol chemical/physical properties with their optical properties, it is best to use optical and chemical/physical measurements with both collected at the surface or both collected in the column.

This study presents the spectral monthly and seasonal variation of aerosol optical depth (τAOD), single scattering albedo (SSA), and aerosol absorption optical depth (AAOD) between 2010 and 2018 obtained from the Aerosol Robotic Network (AERONET) over Pokhara, Nepal. The analysis of these column-integrated aerosol optical data suggests significant monthly and seasonal variability of aerosol physical and optical properties. The pre-monsoon season (March to May) has the highest observed τAOD(0.75 ± 0.15), followed by winter (December to February, 0.47 ± 0.12), post-monsoon (October and November, 0.39 ± 0.08), and monsoon seasons (June to September, 0.27 ± 0.13), indicating seasonal aerosol loading over Pokhara. The variability of Ångström parameters, α, and β, were computed from the linear fit line in the logarithmic scale of spectral τAOD, and used to analyze the aerosol physical characteristics such as particle size and aerosol loading. The curvature of spectral τAOD, α’, computed from the second-order polynomial fit, reveals the domination by fine mode aerosol particles in the post-monsoon and winter seasons, with coarse mode dominating in monsoon, and both modes contributing in the pre-monsoon. Analysis of air mass back trajectories and observation of fire spots along with aerosol optical data and aerosol size spectra suggest the presence of mixed types of transboundary aerosols, such as biomass, urban-industrial, and dust aerosols in the atmospheric column over Pokhara.

Two reflectance techniques, based on Kubelka-Munk (K-M) theory and on the Beer-Lambert (B-L) law, were used to measure the absorption coefficient of aerosol particles collected on a filter. The two methods agreed, with the B-L technique being higher than the K-M method by a factor of 1.10, but with a correlation, r 2 , between the two methods of 0.99. The aerosol absorption Å ngstr€ om exponents (AAE) between the two methods also agreed within 0.4 and were in the range of measurements reported in the literature with other techniques. The precision of the two methods depends on the volume of air sampled, but a typical sampling scheme (100 L min 21 , 10 cm 2 sampling area, full day of sampling) results in a precision in the measurement of the aerosol light absorption coefficient of 0.05 Mm 21 .

Abstract. Light scattering and spectral absorption by size-resolved aerosols in near-surface air at Tudor Hill, Bermuda were measured continuously between January and June 2009. Vertical distributions of aerosol backscattering and column-averaged aerosol optical properties were characterized in parallel with a Micro-pulse lidar (MPL) and an automated sun-sky radiometer. Aerosol optical properties measured near the surface were often significantly correlated with those averaged over the column. These include scattering by near-surface bulk aerosol at 530 nm vs. column aerosol optical depth (AOD), near-surface sub-μm scattering fraction vs. column averaged sub-μm scattering fraction, the column averaged Angstrom exponent derived using a column integrated size distribution and complex refractive index. The relative contribution of submicron aerosol light scattering to total aerosol light scattering shows a slight enhancement of the column contribution of submicron particles over the surface measurements. Physical factors such as surface level wind speed have a more important affect on bulk aerosol light scattering at the surface.