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Difference in particle formation at a mountaintop location during spring and summer: Implications for the role of sulfuric acid and organics in nucleation

New particle formation (NPF) has been observed frequently at Storm Peak Laboratory (SPL), a high‐elevation mountaintop observatory in Colorado. A detailed analysis of field measurements taken in March and July 2012 at SPL reveals significant and interesting differences in NPF during the spring and summer months. Persistent long‐lasting NPF occurred on a daily basis in March but was absent in July. Bursts of ultrafine particles did occur frequently in July, but such bursts were short‐lasting and did not show any obvious pattern. A global chemical transport model (GEOS‐Chem) coupled with a size‐resolved advanced particle microphysics model is used to interpret in‐depth this observed nucleation phenomenon. The model captures well the observed persistent daily nucleation events in March and the absence of regional‐scale NPF in July. Model simulations indicate that aerosol precursors were dominated by H2SO4 gas in March and by low‐volatile secondary organic gases (LV‐SOGs) in July, which are consistent with previous particle composition measurements at SPL. The observed persistent daily NPF in March and the absence of regional‐scale nucleation in July at SPL indicate that H2SO4 gas plays a much more critical role in the initial nucleation process, although LV‐SOGs dominate particle growth in July. Our analysis suggests that the short‐lasting bursts of ultrafine particles observed at SPL in July are likely associated with nucleation in subgrid power plant plumes, where concentrations of H2SO4 rather than LV‐SOGs are expected to be higher.