An assessment of the ability of three-dimensional air quality models with current thermodynamic equilibrium models to predict aerosol NO<inf>3</inf> -

The partitioning of total nitrate (TNO3) and total ammonium (TNH4) between gas and aerosol phases is studied with two thermodynamic equilibrium models, ISORROPIA and the aerosol inorganics model (AIM), and three data sets: high time resolution measurement data from the 1999 Atlanta Supersite Experiment (summer case) and the 2002 Pittsburgh Air Quality Study (PAQS) Supersite Experiment (winter case), and 12-hour measurement data from the Clinton site, North Carolina, in 1999. At the Atlanta site, both models reproduced a large percentage of the observed aerosol NHj and HNO 3 (NH4: >94% and HNO3: >86%) within a factor of 1.5, whereas neither model reproduced a majority of observed aerosol NO3- and NH3 (NO3-:

Hygroscopicity and composition of California CCN during summer 2010

Athanasios Nenes

We present an overview and analysis of cloud condensation nuclei (CCN) sampled in California by a NOAA WP-3D aircraft during the 2010 CalNex project. Four distinct geographical regions are characterized, including the Los Angeles basin, the San Joaquin and Sacramento Valleys, and the eastern Pacific Ocean west of southern California. Median size distributions in the Central Valley were unimodal (Dg ∼ 25nm) with a larger fraction of organic species and smaller fraction of nitrate species in the Sacramento Valley aerosol than in the San Joaquin Valley aerosol. Size distributions in the Los Angeles basin and marine outflow were bimodal (geometric mean diameter, Dg ∼ 30, 90-100nm) with similar organic fractions and some replacement of nitrate with sulfate in the marine outflow. Both fine particle and CCN concentrations were found to decrease rapidly above the planetary boundary layer (∼2 km altitude), with CCN concentrations in the boundary layer ranging from ∼102-104cm-3 STP, while fine particle concentrations (0.004-1 m diameters) ranged from ∼103-10 5 cm-3 STP. The CCN-active number fraction varied between 0-100% in the Los Angeles Basin and Marine Outflow, but was substantially lower (0-40%) in the San Joaquin and Sacramento Valleys. Values of the hygroscopicity parameter, , inferred from the CCN measurements varied from 0.1-0.25, with the highest values in the marine outflow and the lowest values in the Sacramento Valley. The values agreed well with the predictions based on size-resolved aerosol composition, but were overpredicted by almost twofold when size-averaged composition was used. CCN closure was assessed for simplified compositional and mixing state assumptions, and it was found that assuming the aerosol to be internally mixed overpredicted CCN concentrations by 30-75% for all air mass types except within the Sacramento Valley, where good closure (overprediction < 10%) was achieved by assuming insoluble organics. Assuming an externally-mixed aerosol fraction or incorporating size-resolved composition data improved closure in the other three regions, consistent with the bimodal nature of the aerosol size distribution. © 2012 by the American Geophysical Union.

Blackwell Publishing Ltd2012

Thermodynamic characterization of Mexico City aerosol during MILAGRO 2006

Athanasios Nenes

Fast measurements of aerosol and gas-phase constituents coupled with the ISORROPIA-II thermodynamic equilibrium model are used to study the partitioning of semivolatile inorganic species and phase state of Mexico City aerosol sampled at the T1 site during the MILAGRO 2006 campaign. Overall, predicted semivolatile partitioning agrees well with measurements. PM2.5 is insensitive to changes in ammonia but is to acidic semivolatile species. For particle sizes up to 1μm diameter, semi-volatile partitioning requires 15-30 min to equilibrate; longer time is typically required during the night and early morning hours. Aerosol and gas-phase speciation always exhibits substantial temporal variability, so that aerosol composition measurements (bulk or size-resolved) obtained over large integration periods are not reflective of its true state. When the aerosol sulfate-to-nitrate molar ratio is less than unity, predictions improve substantially if the aerosol is assumed to follow the deliquescent phase diagram. Treating crustal species as "equivalent sodium" (rather than explicitly) in the thermodynamic equilibrium calculations introduces important biases in predicted aerosol water uptake, nitrate and ammonium; neglecting crustals further increases errors dramati- cally. This suggests that explicitly considering crustals in the thermodynamic calculations is required to accurately predict the partitioning and phase state of aerosols. © 2009 Author(s).

Copernicus GmbH2009

CCN predictions: Is theory sufficient for assessments of the indirect effect?

Athanasios Nenes

This study quantitatively assesses the sensitivity of cloud droplet number (CDNC) to errors in cloud condensation nuclei (CCN) predictions that arise from application of Köhler theory. The CDNC uncertainty is assessed by forcing a droplet activation parameterization with a comprehensive dataset of CCN activity and aerosol size and chemical composition obtained during the ICARTT field campaign in August 2004. Our analysis suggests that, for a diverse range of updraft velocity, droplet growth kinetics and airmass origin, the error in predicted CDNC is (at most) half of the CCN prediction error. This means that the typical 20-50% error in ambient CCN closure studies would result in a 10-25% error in CDNC. For the first time, a quantitative link between aerosol and CDNC prediction errors is available, and can be the basis of a robust uncertainty analysis of the first aerosol indirect effect. Copyright 2006 by the American Geophysical Union.

2006

Hygroscopicity and composition of California CCN during summer 2010

Athanasios Nenes

Blackwell Publishing Ltd2012