Mechanisms Driving New Particle Formation Throughout the Troposphere

New particle formation is an ubiquitous atmospheric phenomenon, observed in various environments throughout the troposphere. This phenomenon provides around

three-quarters of the total particles at ground level, and almost all of the particles in the free troposphere. These particles not only mask half of the global radiative forcing caused since the industrial revolution by greenhouse gases, but also engender air pollution and consequently millions of premature deaths every year. Despite its global importance, new particle formation is driven by various yet poorly understood mechanisms that are rather specific to the regional conditions. In this dissertation, I investigate these mechanisms with experiments performed under typical atmospheric conditions in the CERN CLOUD chamber (Cosmics Leaving OUtdoor Droplets). The first region addressed in this dissertation is the urban atmosphere. New particle formation is a major contributor to urban smog, but it is often puzzling how urban new particles survive the rapid scavenging by abundant pre-existing particles. Here I

show that below about +5 C, nitric acid and ammonia vapors can readily condense onto sub-10 nm particles. Since these vapors are often 1000 times ampler than sulfuric

acid, the resulting particle growth rates can be extremely high, reaching well above 100 nm/h. In view of the strong temperature dependence measured for the gas-phase

supersaturations, one would expect the gas-particle ammonium nitrate system to be out of equilibrium in inhomogeneous urban settings, especially in wintertime with

strong local sources. This process is thus fast enough to shepherd particles through the smallest size range where they are most vulnerable to scavenging loss, greatly

increasing their survival probability. The urban atmosphere contains much more than inorganic acid-base chemistry.

Formation and growth of secondary organic aerosols (SOA) by oxidized organic vapors contributes substantially to both particle number and mass. The particlephase volatility measurements confirm that oxidation products of aromatic hydrocarbons can contribute to the initial growth of newly-formed particles. Toluenederived (C7) oxidation products have a similar volatility distribution to that of a- pinene-derived (C10) oxidation products, while naphthalene-derived (C10) oxidation products are much less volatile than those from toluene or a-pinene; they are thus stronger contributors to growth. Aromatic oxidation products also appear to undergo multiple generations of oxidation more rapidly than products of monoterpenes. Such multi-generation oxidation leads to functional groups with much lower volatility per

added oxygen atom, such as hydroxyl and carboxylic groups instead of hydroperoxide groups, decreasing product volatility by several orders of magnitude. This pathway

increases the aromatic SOA yields from a few percent to a few tens of percent at typical ambient concentrations, dominating aromatic SOA formation. The second region addressed in this dissertation is the upper free troposphere, especially over the Asian monsoon. I show that under upper tropospheric conditions, nitric acid, sulfuric acid and ammonia form particles synergistically, at rates orders

of magnitude higher than those from any two of the three components. This particle formation mechanism could be limited by the availability of ammonia according to the conventional expectation that ammonia is efficiently scavenged by cloud and rain droplets during convection. However, strongly enhanced levels of ammonia and

ammonium nitrate are observed in the upper troposphere over the Asian monsoon region [1]. Once new particles have formed, co-condensation of ammonia and abundant

nitric acid alone is sufficient to drive rapid growth to CCN sizes. Furthermore, simultaneous ice nucleation measurements show these particles are efficient ice nucleating particles (INP). Complementary model simulations confirm that ammonia can be transported by convective systems, and subsequently drive rapid HNO₃–H₂SO₄–NH₃ nucleation in the upper troposphere during the Asian monsoon, producing particles that can travel across the mid-latitude Northern Hemisphere. The final region addressed in this dissertation is the marine atmosphere, especially the costal marine boundary layer. I present the application of a bromide chemical ionization mass spectrometer (Br-CIMS) method for simultaneous measurements of many iodine species, with detection limits down to 10⁵ molec. cm ^-3 quantify iodine species and sulfuric acid via offline calibrations using permeation tubes or a calibrator, and also via inter-method calibrations using calibrated instruments. The corresponding calibration coefficients appears to qualitatively agree with the calculated

binding enthalpies between the calibrated species and reagent ions. This indicates that the quantum chemical calculations can be employed along with the calibration

experiments to determine the sensitivities for unquantifiable species. Further, a separate study applying this method resolves the mechanisms of iodic new particle formation: ion-induced channel involves IO₃ – and the sequential addition of iodic acid (HIO₃); while neutral channel proceeds with the repeated sequential addition of iodous acid (HIO₂) followed by HIO₃, showing that HIO2 plays a key stabilizing

role. Further, Br-CIMS, coupled with thermal desorption analysis, shows that newly formed particles are composed almost entirely of HIO₃. Together, these instrumental

improvements and new particle formation experiments show that iodine oxoacids are at least as efficient as sulfuric acid in atmospheric new particle formation, indicating that nucleation involving iodine species may be widespread in air masses with marine influence.