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Spring and summer contrast in new particle formation over nine forest areas in North America
Citation:
Yu, F., G. Luo, S. Pryor, P. Pillai, S. Lee, J. Ortega, J. Schwab, A. Hallar, W. Leaitch, V. Aneja, J. Smith, Johnt Walker, O. Hogrefe, AND K. Demerjian. Spring and summer contrast in new particle formation over nine forest areas in North America. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 15(24):13993-14003, (2015).
Impact/Purpose:
Newly formed atmospheric particles play a role in atmospheric radiative forcing. Global modeling simulations indicate that indirect radiative forcing associate with atmospheric aerosol is sensitive to schemes used to predict particle nucleation. This manuscript compares two nucleation models. The two nucleation schemes predict quite different concentrations of cloud condensation nuclei and aerosol first direct radiative forcing in North America, highlighting the need to reduce NPF uncertainties in regional and global earth system models.
Description:
Recent laboratory chamber studies indicate a significant role for highly oxidized low volatility organics in new particle formation (NPF), but the actual role of these highly oxidized low volatility organics in atmospheric NPF remains uncertain. Here, particle size distributions (PSDs) measured in nine forest areas in North America are used to characterize the occurrence and intensity of NPF and to evaluate model simulations conducted using an ion-mediated mechanism (excluding organics) and using an empirical formulation in which formation rate is a function of the concentration of sulfuric acid and biogenic oxidized organics (which is represented herein as low volatility products from alpha-pinene oxidation). On average, NPF occurred on ~ 70% of days during March for the four forest sites with springtime PSD measurements, while NPF occurred on only ~ 10% of days in July for all nine forest sites. Thus, this study focuses on contrasting spring and summer months. The two different nucleation schemes appear to capture the high frequency of NPF in spring, but the nucleation scheme considering the role of organics significantly overpredicts while the ion-mediated nucleation scheme underpredicts NPF and particle number concentrations in summer. The two schemes also predict quite different spatial patterns in nucleation rates in the spring over North America. Statistical analyses indicate that the scheme without organics agrees better overall with observations. The two nucleation schemes predict quite different concentrations of cloud condensation nuclei and aerosol first direct radiative forcing in North America, highlighting the need to reduce NPF uncertainties in regional and global earth system models.
