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. Organic nucleation decreases the sum of the total aerosol direct and indirect radiative forcing by 12.5%. The addition of climate and land use change decreases the direct radiative forcing (-0.38 W m-2) by 6.3% and the indirect radiative forcing (-1.68 W m-2) by 3.5% due to the size distribution and number concentration change of secondary organic aerosol and sulfate. Overall, the total radiative forcing associated with anthropogenic aerosols is decreased by 16%.

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. Despite inclusion of these processes in global and regional chemistry-climate models, a statistically significant change in Δm(O3-T) has not been noted in prior studies. Future climate projections suggest a more frequent and spatially widespread occurrence of this Δm(O3-T) response, confounding predictions of extreme ozone events based on the historically observed linear relationship.

The Use of Photochemical Indicators to Evaluate Ozone-NOx-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-NOx-hydrocarbon sensitivity and for evaluating the performance of ozone models. The successful photochemical indicators are: 03/NO , 03/NOz (where NOz = NOy-NOx), 03/HN03, H202/HN03, and H202/NOz. 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 NOx-sensitive chemistry and low values are associated with reactive organic gases (ROG)-sen-sitive chemistry. Correlations between measured 03 and NO in Atlanta and between 03 and NOz in Los Angeles are consistent with theory and reflect the difference between likely NOx-sensitive chemistry in Atlanta and hydrocarbon-sensitive chemistry in Los Angeles. Measured 03, NOx and NO are used to evaluate model performance during two air pollution events in Atlanta and Los Angeles. The performance evaluation includes model scenarios for each city with different anthropogenic and biogenic emission rates and different NOx-ROG sensitivity predictions. Simulations with different NOx-ROG chemistry are found to give similar predictions for peak ozone but different values for photochemical indicators. Comparison with measured values of photochemical indicators provides a more stringent test of model performance than evaluation versus observed ozone.