Decrease in radiative forcing by organic aerosol nucleation, climate, and land use change.

Organic nucleation is an important source of atmospheric aerosol number concentration, especially in pristine continental regions and during the preindustrial period. Here, we improve on previous simulations that overestimate boundary layer nucleation in the tropics and add changes to climate and land use to evaluate climate forcing. Our model includes both pure organic nucleation and heteromolecular nucleation of sulfuric acid and organics and reproduces the profile of aerosol number concentration measured in the Amazon.

Observed suppression of ozone formation at extremely high temperatures due to chemical and biophysical feedbacks.

Ground level ozone concentrations ([O(3)]) typically show a direct linear relationship with surface air temperature. Three decades of California measurements provide evidence of a statistically significant change in the ozone-temperature slope (Δm(O3-T)) under extremely high temperatures (> 312 K). This Δm(O3-T) leads to a plateau or decrease in [O(3)], reflecting the diminished role of nitrogen oxide sequestration by peroxyacetyl nitrates and reduced biogenic isoprene emissions at high temperatures.

The Use of Photochemical Indicators to Evaluate Ozone-NO-Hydrocarbon Sensitivity: Case Studies from Atlanta, New York, and Los Angeles.

This study examines the use of ambient measurements of a number of "photochemical indicators" as a basis for determining ozone-NO-hydrocarbon sensitivity and for evaluating the performance of ozone models. The successful photochemical indicators are: 0/NO , 0/NO (where NO = NO-NO), 0/HN0, H0/HN0, and H0/NO. Results of Urban Airshed Model (UAM-IV) simulations for Atlanta, GA, New York, NY, and Los Angeles, CA, show that high values of these species ratios are correlated with NO-sensitive chemistry and low values are associated with reactive organic gases (ROG)-sen-sitive chemistry.