The diurnal variation of net ecosystem exchange (NEE) showed an unusual pattern at the Blodgett Forest Ameriflux site, with late afternoon NEE lower than early morning (indicating more uptake), while air temperature and atmospheric vapor pressure deficit were much higher. To investigate processes influencing this pattern, NEE was compared to several environmental variables during summer 2002. Unusual variations of NEE can be partly attributed to dew formation on the leaf surface. An empirical model is used to show that surface wetness reduced the net ecosystem uptake of CO2 during the morning by 11%. In addition, transport of air-pollution from the Central Valley to this site results in higher aerosol particle concentration, light extinction and light scattering during the afternoon than in the morning. Total irradiance was 11% lower during the afternoon than in the morning, while diffuse irradiance was 24% higher. The empirical model is used to show that the decrease in total radiation reduced photosynthesis during the afternoon, but the increase in diffuse radiation enhanced photosynthesis even more. Aerosol loading caused net uptake of CO2 by the forest to increase by 8% in the afternoon as a result of changes in direct and diffuse radiation.