Evolution of multispectral aerosol optical properties in a biogenically-influenced urban environment during the CARES campaign

摘要: Abstract. Ground-based aerosol measurements made in June 2010 within Sacramento urban area (site T0) and at a 40-km downwind location T1) the forested Sierra Nevada foothills are used to investigate evolution of multispectral optical properties as aerosols aged interacted with biogenic emissions. Along black carbon non-refractory mass composition observations, spectral absorptio (βabs), scattering (βsca), extinction (βext) coefficients for wavelengths ranging from 355 1064 nm were measured both sites using photoacoustic (PA) instruments integrating nephelometers cavity ring-down (CRD) instruments. The daytime average Angstrom exponent absorption (AEA) was ~1.6 wavelength pair 405 870 T0, while it ~1.8 T1, indicating modest wavelength-dependent enhancement throughout study. Mie theory calculations βsca showed good correlation (R2=0.85–0.94). contribution supermicron (mainly composed sea salt particles advected Pacific Ocean) total coefficient ranged less than 20% greater 80% nm. From 22 28 June, secondary organic increased significantly due emissions coupled intense photochemical activity air recirculation area. During this period, short gradually increase size submicron aerosols. At same time, BC mass-normalized cross-section (MAC) values ultraviolet T1 by ~60% compared relatively T0 site. In contrast, MAC identical sites. These results suggest formation moderately brown formed biogenically-influenced air.