281155 The Role of Nitrate in the Formation of Atmospheric Nanoparticles: Insights From Ambient Measurements and Chemical Transport Models
Atmospheric nanoparticles affect climate – directly, by scattering or absorbing solar radiation and indirectly, by acting as cloud condensation nuclei (CCN) and thereby influencing the formation and properties of clouds . These effects are highly complex; in fact, direct and indirect particle impacts are the dominant uncertainty in predicting anthropogenic radiative forcing and future climate [1, 2]. Nanoparticles also affect human health by, among others, damaging the respiratory and cardiovascular systems [3, 4]. Improving our understanding of atmospheric particles will allow us to better predict radiative forcing and future climate, and to develop better-informed policy actions aimed at mitigating atmospheric nanoparticle concentrations and their adverse health effects.
Nanoparticles can either be emitted directly into the atmosphere, or they can form from atmospheric vapors. Recent field observations and modeling studies have found that new particle formation (NPF) significantly impacts the concentrations of atmospheric nanoparticles [5, 6] and CCN [7, 8]. The formation of new particles consists of a complex set of processes including the production of nanometer-size clusters from gaseous vapors, the growth of these clusters, and the removal of growing clusters by coagulation with pre-existing particles [9-12] . While atmospheric new particle formation has been observed to take place almost everywhere [13, 14], significant gaps in our knowledge regarding this phenomenon still exist. The condensation of sulfuric acid (H2SO4) vapors appears to be involved in almost all NPF [15, 16]. However, there is abundant evidence that the early growth of particles involves much more than sulfuric acid [17-20].
We previously developed the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS), which can measure the chemical composition of 10-30 nm particles and thereby provides insights into the chemical species involved in early nanoparticle growth . We have recently upgraded the TDCIMS with a high-resolution time-of-flight mass spectrometer by Tofwerk AG, which allows us to collect data at higher time and mass resolution. We report the first measurements with this upgraded instrument taken in Hyytiälä, Finland, in April 2011. The TDCIMS measurements show that nitrate is the dominant species in 10-30 nm particles, suggesting its importance in nanoparticle growth. The abundance of nitrate in the nanoparticles is consistent with other measurements taken at the site.
We developed a box model to better understand the species and processes involved in NPF, building upon the previously developed Dynamic Model for Aerosol Nucleation (DMAN) . The model simulates nucleation, coagulation and condensation/evaporation for a population of particles consisting of multiple components. It uses the TwO-Moment Aerosol Sectional (TOMAS) algorithm , which simultaneously tracks the mass and number concentration in each size section. Using results from laboratory chamber experiments, we developed parameterizations for the role of organic nitrate compounds in NPF and added these parameterizations to the model.
We compare model results of the relative contributions of different components to nanoparticle growth to measurements in two very different environments: Hyytiälä, Finland, a relatively clean environment with high biogenic emissions and Atlanta, GA, a city with large anthropogenic influence but also high biogenic emissions.
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