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.