Inter-annual Comparison of New Particle Formation Chemistry and Cloud Condensation Nuclei Measurements at a Remote Rural Mountain Site

Ultrafine particles (≤100 nm) in the atmosphere represent a large source of cloud condensation nuclei (CCN), which alter cloud physical and radiative properties. Understanding the major sources of CCN is critical to understanding cloud formation as well as long-term regional climate, particularly how various sources contribute different CCN species each year. The mixing states of single particles during new particle formation (NPF) events were determined in real-time at a remote rural site in the Sierra Nevada Mountains during two consecutive winters (2009 and 2010). Atmospheric conditions conducive to NPF were similar during both winters with high relative humidity, low temperature, and increased solar radiation. The characteristics of NPF events were different between the two winters likely due to differences in the extent of aerosol scavenging during storms and availability of gas-phase precursors. Gas-phase precursor concentrations showed variable contributions each year: in 2009 there were higher O3 concentrations during the first NPF period and higher SO2 concentrations during the second, and in 2010 concentrations varied with each NPF event, with the highest SO2 and lowest O3 concentrations during the beginning of the study. We observed variation in single-particle chemistry as well with increases in amine-containing and organic carbon-containing species during NPF events in 2009 and 2010, respectively. In addition, new measurements of high mass organic and amine species in particles down to 45 nm were observed during NPF in 2010. Sources of SO2 and O3 varied by event and by year leading to differences in single particle chemistry. A wider range in calculated growth rates occurred in 2010 (1.2-14.9 nm/h) compared to 2009 (1.7-8.0 nm/h), resulting in rapid growth from the smallest sizes to 100 nm in ≤12 hours on average in 2010 and ≤10 hours in 2009 on average. Interestingly, growth rates were larger during periods of highest SO2 concentrations during both winters, suggesting SO2 largely regulates the evolution of these new particles. Growth to 100 nm increases the ability of the new particles to become CCN. Overall, increases in measured CCN showed a strong correlation with the NPF events during both years. Understanding the chemistry of newly-formed particles and their ability to become CCN is critical for predicting the impact of NPF on properties of orographic clouds formed during lifting along the Sierra Nevada mountain range. The possibility that higher CCN concentrations from NPF events impact clouds and potentially precipitation patterns needs to be considered broadly when evaluating the indirect climate impact of aerosols.