Observations of Formaldehyde and Related Atmospheric Species Using Multi-Axis Spectroscopy Christopher P. Beekman and Dr. Heather C. Allen Department of Chemistry, and Environmental Sciences Graduate Program The Ohio State University June 19, 2007

Introduction Understanding the concentrations and distributions of photochemical species and aerosols in an urban air-shed Combination of spectroscopic and meteorological data with photochemical/radiative transfer models Important for atmospheric chemistry, health, and climate change modeling

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) Sensitivity to tropospheric absorbers Vertical profiling of trace gases NO 2, O 3, HCHO, HONO, BrO, ClO 2 Ocean Optics USB-2000 –0.7 nm resolution –Coupled to 1” telescope w/ multimode fiber –Low power requirements 8 hours with 12 V battery U. Platt (1994). Air Monitoring by Spectroscopic Techniques. M. W. Sigrist G. Hönninger, C. Von Friedeburg and U. Platt. Atmospheric Chemistry and Physics 4, 2004.

Collection of scattered sunlight in the UV-VIS, along different lines of sight Analysis of raw atmospheric spectra using modified Beer-Lambert law The most basic measured quantity is Slant Column Density (SCD) Slant columns at each elevation angle converted to Vertical Column Densities (VCD) Conversion factor is Air Mass Factor (AMF), calculated using model of solar radiative transfer UVSPEC/DISORT: libRadtran Package Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) VCD SCD B. Mayer and A. Kylling. Atmospheric Chemistry and Physics 5, pp , 2005.

Slant Column Density (SCD) Retrieval

Radiative Transfer Accurate knowledge of vertical structure of atmosphere required Aerosol optical depth and vertical distribution are key parameters Inclusion improves the model retrieval of Air Mass Factors

Aerosol Profile Retrieval  Greenblatt, G. D., J. B. Burkholder, A. R. Ravishankara. Journal of Geophysical Research, 95,  Wagner, T., B. Dix, C. v.Friedeburg,et al. Journal of Geophysical Research, 109, Aerosol profiling requires a species with known vertical profile O 4 is most appropriate in UV- Vis region Concentration proportional to [O 2 ] 2 12 typical profiles of aerosols simulated within model Comparison of measured and modeled Air Mass Factors of O 4 yields best match profiles

Aerosol Optical Depth Profiles

Volume Mixing Ratios Vertical Column Densities can be converted to volume mixing ratios Need to define the mixing height  h Height of lowest layer determined by several methods –Radiosonde data  location of 1 st inversion –Height of Aerosol Profile –Box Air Mass Factors  Iterative profile variation R. Sinreich, U. FrieS, T. Wagner, U. Platt. Faraday Discussions 130, 2005.

HCHO Primarily an oxidation product of other VOCs Indicator of VOC photochemistry 1989: 17% of atmospheric HCHO in Columbus attributed to vehicle emissions –R. Mukund, T. J. Kelly, C. W. Spicer. Atmospheric Environment 30 (20), : MAX-DOAS measurements of HCHO in Italy –A. Heckel, A. Richter, et al. Atmospheric Chemistry and Physics 5, HCHO + hv  H + HCO H + O 2  HO 2 HO 2 + NO  NO 2 + OH NO 2 + hv  NO + O O + O 2  O 3 A. Heckel, A. Richter, et al., 2005

HCHO, O 3 and NO 2 Both days: poor air quality : Strong spike in AM : Elevated NO 2, possible O 3 suppression during AM Need more information to characterize regimes

Conclusions and Outlook MAX-DOAS  Effective technique for probing atmospheric photochemistry HCHO measurements: –1989: 4.7 ppb –May and June 2007: 3.0 ppb Measurements of O 4 : enabled the retrieval of the first vertical profiles of aerosols in central Ohio –Extend to long term, incorporate into photochemical models

Acknowledgements Professor Heather C. Allen, Allen Lab Members Professor Katherine Calder, Dr. HongFei Li Arve Kylling and Bernhard Mayer