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Abstract.Results from a regional air quality forecast model, AIRPACT-3, are compared to OMI tropospheric NO 2 integrated column densities for an 18 month period over the Pacific Northwest. AIRPACT column densities are well correlated (r = 0.75) to cloud-free (<35%) retrievals of tropospheric NO 2 for monthly averages without wildfires, but are poorly correlated (r = 0.21) with significant model overpredictions for months with wildfires when OMI and AIR-PACT are compared over the entire domain. AIRPACT predicts higher NO 2 in some northwestern US urban areas, and lower NO 2 in the Vancouver, BC urban area, when compared to OMI. Model results are spatially averaged to the daily OMI swath. The Dutch KNMI (DOMINO) and NASA (Standard Product) retrievals of tropospheric NO 2 from OMI (Collection-3) are compared. The NASA product is shown to be significantly different than the KNMI tropospheric NO 2 product. The average difference in tropospheric columns, after applying the averaging kernels of the respective products to the model results, is shown to be larger in the summer (±50%) than winter (±20%).

Accurate measurements of the spectral solar flux reaching the surface in cloud free conditions are required to determine the aerosol radiative impact and to test aerosol models that are used to calculate radiative forcing of climate. Spectral flux measurements are hampered in many locations by persistent broken cloud fields. Here we develop a new technique to derive the diffuse solar spectral flux reaching the surface from principal plane measurements conducted in the last 6 years by the AErosol RObotic NETwork (AERONET). This 50-100 instrument global network measures the principal plane radiances in four spectral bands (0.44-1.02 µm) approximately every hour every day. These instruments also measure the spectral optical thickness, and derive the aerosol size distribution and other properties from sky measurements. The advantage of the AERONET measurements is that collimated sky radiance is measured for each 1°x1° field of view. Clouds and cloud shadows are rejected before the total sky brightness is reconstructed and the flux is derived. The results compare favorably with shadow-band measurements and with aerosol models. We study smoke aerosol in Brazil, Saharan dust in Cape Verde and urban/industrial pollution in Creteil near Paris, France and near Washington DC, USA. The spectral attenuation of total (diffuse+direct) solar flux reaching the surface is given by: f λ =exp(-a λ-b λ τ λ), where a λ is attenuation by an atmosphere with no aerosol and b λ is the aerosol attenuation coefficient. Remarkably, we find that for these sites except for the Washington DC site, the spectrally averaged value of b λ does not vary significantly from one aerosol type to another: {b λ }=0.35±0.03 (for solar zenith angle of 50°). The measured 24 hour average aerosol impact on the solar flux at the surface per unit optical thickness is ∆F/∆τ=-80 W/m 2 in these sites, almost independent of the aerosol type: smoke, dust or urban/industrial pollution. In Washington DC it is suspected, and demonstrated in a back of the envelope calculation, that the high amount of broken cloudiness and its correlation with the aerosol optical thickness is responsible for the apparent small aerosol forcing at the surface of ∆F/∆τ=-50 W/m 2 .

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Forecasts using mesoscale meteorological modeling and a photochemistry grid model, running nightly, provide air quality predictions for a high-resolution Puget Sound domain via the Web. The expanding need for information that pervades our society extends to a need to know about the environment in which we live. At the same time, the explosion of information and computer technol-AFFILIATIONS: VAUGHAN, LAMB, AND FREI