Measuring UV aerosol absorption

Why is aerosol UV absorption important ? Change in boundary layer ozone mixing ratios as a result of direct aerosol forcing + 10%-20% Aerosol “dimming” effects on UV trends may enhance reduce, or reverse effects of stratospheric O 3 change Satellite (TOMS/OMI) overestimation of surface UV irradiance 3) Aerosol effects on photochemical smog production: aerosol scattering increases photolysis rates; while aerosol absorption decreases it: %

What we need to measure:  abs or Single Scattering Albedo (SSA):  abs =(1-SSA)  ext

Since 2002, the NASA TOMS, USDA UVB, AERONET and Aura/OMI programs have shared equipment, personnel and analysis tools to quantify aerosol UV absorption using a combination of ground based radiation measurements and RT modeling AEROSOL UV absorption retrievals: Brewer spectrometer : ozone, SO 2, NO 2 UV Multifilter Rotating Shadowband Radiometer AERONET CIMEL sun-sky radiometers + +

1) UV-MFRSR extinction AOT compares well with AERONET and Brewer measurements AERONET Cloud-screened  UVMFRSR

SSA retrieval AERONET retrievals of size distribution (PSD) and effective refractive index (real) at 440nm 1-min Diffuse/Total relative measurement (3% error) absolute radiometric calibration of sky measurements (5% error) Imaginary refr index, k, n 440 A-priori information Fitting of absolute sky radiances in almucantar SSA at 325nm, 332nm and 368nm SSA at 440nm, 670nm,870nm, 1020nm Mie calculations PSD = const n=n 440 k  ext T  ozone

2) MFRSR SSA measurements in UV complement AERONET almucantar SSA inversions (440nm – 870nm): MFRSR: -high frequency measurements -Low solar zenith angles -High precision (no absolute calibration necessary)  AERONET and MFRSR AOT 368  Single Scattering Albedo (SSA) MFRSR: SSA 368 AERONET SSA 440

Figure 1, upper: Correction for NO 2 Cede, A. J. Herman, A. Ritcher, N. Krotkov, J. Burrows, Measurements of Nitrogen Dioxide Total Column Amounts at Goddard Space Flight Center Using a Brewer Spectrometer in Direct Sun Mode, JGR 2006

1. Single scattering albedo (  =  scattering /  ext ) NO 2 Amount November :  a ~0.1 and  NO2 ~0.03 NO 2 Amount

1. Single scattering albedo (  =  scattering /  ext ) November :  a ~0.1 and  NO2 ~0.03 NO 2 Amount November :  /  ~  NO2 /  a ~ 0.2

 versus  a (no NO2 correction) Decrease of  with  a is due to uncorrected NO 2 at LOW  a  SSA Retrieval neglecting NO 2

 versus  a (with NO2 correction) SSA retrieval correcting for NO 2

 versus  a (325nm ) SSA retrieval correcting for NO 2

What is SSA spectral dependence in the UV and visible wavelengths ? UV -MFRSR has been modified to add 440nm filter used by AERONET Good agreement between AERONET SSA and UV/VIS MFRSR

Modified UV/VIS MFRSR compared well with AERONET SSA at 440nm Small  ext

SCOUT-03 field campaign in Thessaloniki, Greece, July 2006 Roof of the Physics Dept., of the Aristotle University of Thessaloniki.

Modified UV/VIS MFRSR compared well with AERONET SSA at 440nm

Modified UV/VIS MFRSR compared well with AERONET SSA at 440nm:

Spring AGU, May 2007 MILAGRO session Volkamer and Barnard, MILAGRO JGR special issue in prep. ?

Qi et al., AGU Dec 2006

Models underpredict SOA Volkamer et al., 2006

Pollution Impacts on Biological Surface UV Radiation 1pm local time S. Madronich - Spring AGU 2007, Acapulco, Mexico

NO 2 + h  nm)  NO + O( 3 P)

O 3 + h  nm)  O( 1 D) + O 2

Single scattering albedo of pollution aerosols may be considerably smaller at UV than visible wavelengths  Secondary organic aerosols? Increased optical depth and ssa at visible wavelengths Absorption at UV wavelengths Composition is largely unknown Large effects on  Biologically active UV at surface, calculation of UV Index  Photolysis rates in PBL A challenge for air quality models  Urban O 3 production is approx. proportional to J NO2  Secondary organic aerosols also produced by photochemistry  Negative feedback locally?  Increased export of pollutants from megacity to region

Aerosol effect on OMI SO2 retrievals East AIRE filed experiment in NE China, April submitted to the JGR Aura Special Issue Aircraft in-situ SO2 measurements (red) and aerosol scattering profile (blue)

Future work: 1.In order to fully characterize absorption properties of the atmosphere one has to measure both NO 2 and aerosol extinction (  ) and single scattering albedo (  ). 2.With these new measurements we aim to discriminate between black carbon (BC) and organic carbon (OC) in pollution and smoke aerosols. 3.Validation of satellite UV, SO2 and aerosol absorption products from OMI

END Mean aerosol optical depth: 0.82 Mean aerosol single scattering albedo: 0.90

END

2. Single scattering albedo (  =  scattering /  ext ) NO 2 Amount May :  /  ~  NO2 /  a ~ 0.03

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