E. R. Lovejoy

Impacts of sources and aging on submicrometer aerosol properties in the marine boundary layer across the Gulf of Maine

[1]xa0Measurements were made on board the NOAA RV Ronald H. Brown during the second New England Air Quality Study (NEAQS 2004) to determine the source of the aerosol in the region and how sources and aging processes affect submicrometer aerosol chemical composition and optical properties. Using the Lagrangian particle dispersion model FLEXPART in combination with gas phase tracer compounds, local (urban), regional (NE U.S. urban corridor of Washington, D.C.; New York; and Boston), and distant (midwest industries and North American forest fires) sources were identified. Submicrometer aerosol measured near the source region (Boston Harbor) had a molar equivalence ratio near one with respect to NH4+, NO3−, and SO4=, had a large mass fraction of particulate organic matter (POM) relative to SO4=, and had relatively unoxidized POM. As distance from the source region increased, the submicrometer aerosol measured in the marine boundary layer became more acidic and had a lower POM mass fraction, and the POM became more oxidized. The relative humidity dependence of light extinction reflected the change in aerosol composition being lower for the near-source aerosol and higher for the more processed aerosol. A factor analysis performed on a combined data set of aerosol and gas phase parameters showed that the POM measured during the experiment was predominantly of secondary anthropogenic origin.

Negative atmospheric ions and their potential role in ion‐induced nucleation

[1]xa0Mass identified ion cluster distributions were measured under ambient atmospheric conditions and compared with model predictions based on laboratory ion cluster thermodynamics data. The results are shown from several days where atmospheric sulfur concentrations were high and thus ion-induced cluster growth was anticipated. Atmospheric gas phase sulfuric acid, temperature, relative humidity, SO2, mobility distributions of ions and small charged particles, and aerosol size distributions were also measured in support of the model calculations. The relative agreement of measurement and model for the first and second sulfuric acid clusters (HSO4−(H2SO4)m) for m = 1 and 2 is quite good but suggests that sulfuric acid clustering may not occur at the collision rate. Clusters for higher m values were not observed, which is also consistent with model predictions for the conditions under which measurements were performed. The lack of both observed and predicted large ion clusters is also consistent with the independent measurements of ion mobility distributions and particle size distributions, which showed similar numbers of positively and negatively charged ultrafine particles, suggesting that neither positive nor negative ion-induced nucleation processes were likely to have contributed significantly to observed new particle formation rates during this study. The relatively low observed concentrations of the bisulfate ion also suggest that the processes leading to the first sulfuric acid/bisulfate cluster (HSO4−H2SO4) may be more complicated than simple sulfuric acid clustering or exchange reactions. While nucleation was observed on some days, measurements suggest that ion-induced nucleation did not contribute significantly to new particle production or growth during these events. This does not rule out the possibility that ion-induced nucleation could contribute significantly to atmospheric new particle formation under very different atmosphere conditions such as in areas with much lower temperatures and higher ion concentrations.

Tropospheric ionization and aerosol production: A model study

[1]xa0Recent observations of ultrafine aerosol production in the atmosphere support the hypothesis of ions acting as nucleation agents. We use a numerical model of ion-induced aerosol formation based on experimental cluster ion thermodynamics to study the production of ultrafine aerosol in the troposphere and its response to variations in background ionization. An analytical model is used to explain the findings. Our results show a considerable ultrafine particle and surface area production due to ion-induced nucleation. Both particle and surface area production readily respond to variations in ionization, such as resulting from the modulation of the galactic cosmic ray intensity by the 11 year solar cycle. However, this response may be positive or negative, depending on ambient conditions. We explain the mechanism responsible for this behavior. The large particle production seen in our simulations suggests that ion-induced nucleation is an important atmospheric process, which may promote the solar cycle signal to the troposphere.

A parameterization of ion-induced nucleation of sulphuric acid and water for atmospheric conditions

This paper describes a five-dimensional parameterization of ion-induced nucleation (IIN) that covers the complete range of conditions relevant to the lower atmosphere. The parameters are (1) temperature T (190-300 K), (2) relative humidity RH (0.05-0.95), (3) number concentration of H 2 SO 4 (10 5 -10 8 cm -3 ), (4) first-order loss of H 2 SO 4 to particles (0.00009-0.0245 s -1 ), and (5) ion source rate (2-50 ion pairs cm -3 s -1 ). The parameterization is based on a steady state version of the kinetic aerosol model Sulphuric Acid and Water Nucleation (SAWNUC) that uses experimentally measured thermodynamics for the ion clusters. Parameterized formulas are obtained for the following variables: (1) particle nucleation rate (cm -3 s -1 ), (2) H 2 SO 4 nucleation rate (cm -3 s -1 ), (3) number of H 2 SO 4 molecules in average nucleating cluster, (4) number of H 2 O molecules in average nucleating cluster, and (5) radius (nanometers) of average nucleating cluster. The parameterization generally reproduces the modeled nucleation rate to within an order of magnitude over the whole range of conditions, except when the nucleation rate is very low (<10 -6 cm -3 s -1 ), which corresponds to a rate of less than 0.1 particle d -1 cm -3 . This parameterization speeds up IIN calculations by a factor of ∼10 6 , as compared to the original SAWNUC model.

Hot-air Balloon Measurements of Vertical Variation of Boundary Layer New Particle Formation

In this study, we used a hot-air balloon as a platform for boundary layer particle and cluster measurements. We did altogether 11 flights during the spring of 2005 and 2006. During the spring of 2006, we observed five new particle formation days. During all days, new particle formation took place in the mixed boundary layer. During one of the days, we observed particle formation in the free troposphere, separate from that of the mixed layer. The observations showed that the concentration of freshly-formed 1.5-2 nm negative ions was several times higher than the concentration of positive ions. We also clearly observed that nucleation during one of the days, 13 March 2006, was a combination of neutral and ion-induced nucleation. During some of the days, particle growth stopped at around 3 nm, probably due to lack of condensable organic vapours. Simulations of boundary layer dynamics showed that particles are formed either throughout the mixed layer or in the lower part of it, not at the top of the layer.