Tracking Transport and Transformation of Aerosols using C and O-triple Isotopic Composition of Carbonates: CSI La Jolla

Aerosols affect climate in numerous ways, including change in the earth's energy balance by absorbing and scattering solar radiations, alteration of the hydrological cycle by serving as cloud condensation nuclei, change in biogeochemical cycles by providing nutrients. Another significant process is the effect on the chemical composition of the atmosphere by providing surfaces for heterogeneous chemical reactions. Fine particles of aerodynamic diameter less than 2.5μm (PM2.5) also impinge upon human health by admission to the respiratory system causing a range of cardiopulmonary diseases. Both climate and public health aspects depend on their physical and chemical properties, therefore, understanding physico-chemical and photochemical transformations on aerosol surfaces is important for predicting their effects on climate change, atmospheric chemistry and human health. Here we present initial findings on the processes occurring on aerosol surfaces using isotopes to delineate day and night time chemistry, thus resolving photochemistry effects, and to identify their sources by way of the carbon isotopes. Aerosols were collected on filter papers for 12h during the day and at night time from June-Dec. 2011in La Jolla, CA., using high volume, multi stage cascade impactors. CO₂ released after treating these filter papers with 100% phosphoric acid at 27^oC was collected, purified chromatographically and analyzed for both C and O isotopes. Our data indicate that both C and O isotopes can be used to distinguish between heterogeneous and photochemical transformations. Aerosol carbonates collected during the day time were depleted in δ¹³C_day = -23 to -28‰ and δ¹⁸O_day = +3 to +10‰ and were isotopically distinct from the carbonates collected at night time δ¹³C_night = 0 to -12‰, δ¹⁸O_nightnight = +23 to +32‰. Higher chloride concentration in the samples collected at night time indicated the transport of marine air masses whereas higher nitrate and sulfate concentration indicated urban air pollution in the samples collected during the day time. To further constraint the origin of air masses being collected at La Jolla, Ca, NOAA Hysplit back trajectories were analyzed for 24 and 48h at three different altitudes (500m, 1000m, <1500m). Our analysis found dominance of urban air masses from Los Angeles and San Diego area during the day time whereas aerosols collected at night time had a significant component of marine air masses and some contribution from San Diego areas at our sampling site, as expected. To further understand the role of ozone in heterogeneous and photochemical transformation, oxygen triple isotopic composition of the carbonates will be measured. Scanning electron microscopy will also be used to characterize the physical and chemical features of the aerosols. The comprehensive approach will facilitate to define changes in surface chemistry of the aerosols occuring during their transport and aging.