AEROSOL

Colorado: StormVEX Aerosol Size Distribution

15 November 2010 - 25 April 2011

Lead Scientist: Gannet Hallar

Observatory: AMF, SBS

As shown recently in work by Merikanto et al. (Atmos. Chem. Phys., 9, 12999¨C13037, 2009), new particle formation (NPF) has a great impact on the number of global CCN (cloud condensation nuclei). Using the GLOMAP model, the importance of NPF in the free troposphere was characterized with approximately 35% of CCN (at 0.2% SS) in global low©\level clouds derived from this mechanism. Specifically, at higher elevations, the contribution of primary particles to CCN decreases with respect to the contribution from upper tropospheric nucleation (Merikanto et al., 2009). Additionally, recent evidence indicates that newly formed particles exhibit enhanced hydroscopic properties and therefore may be more effective as CCN and increase the indirect radiative effect beyond current estimates (O¡¯Halloran et al., Atmospheric Environment, 43, 22©\23, 2009). However, Dusek et al. (Geophy. Research Letters, 37, 2010) found that CCN activity was reduced when particle growth occurred through condensation of secondary organic material following NPF events. Thus, a critical requirement for understanding the role of aerosols in atmospheric chemistry and climate change is elucidation of their formation pathways, specifically, nucleation and growth. NPF has been observed regularly at SPL (Hallar et al., submitted 2010). The data set includes 400 days from January, 2001 to April, 2009. A NPF event occurred on approximately 62% of sampling days (total of 274 days). To evaluate the drivers for NPF, various atmospheric variables were explored to compare differences between days with and without NPF events. These included UV radiation, O3 concentration, temperature, wind direction, wind speed, and relative humidity. Overall, there was a statistically significantly higher level of UV irradiance at all measured wavelengths on days with identified NPF compared to days without these events. Prior studies have shown that increased UV irradiance is associated with increased atmospheric OH radical concentrations, which drive the formation of sulfuric acid, thought to be involved in aerosol nucleation (e.g. Clarke et al. Science, 282, 89¨C92, 1998; Weber et al., J. Geophys. Res., 102, 4375¨C4385, 1997). For example, Kulmala et al. (Atmos. Chem. Phys., 6, 787¨C793, 2006) showed that observed NPF rates appear to be a function of sulfuric acid concentration. More recent evidence suggests that species other than sulfuric acid and water (such as NH3, HNO3, and organic compounds) may affect nucleation rates in the real atmosphere (review by Yu et al., Atmos. Chem. Phys., 8, 2537¨C2554, 2008). Recent results from Zhang (Proc Natl Acad Sci, 106,42, 17650, 2009) indicate that interactions between biogenic organic acids and sulfuric acid enhance nucleation and initial growth of new particles such that anthropogenic emissions over pristine forested areas, like those near SPL, will strongly impact aerosol formation. The objectives of this proposal are as follows: 1. Determine the frequency of NPF with a SMPS deployed at the AOS. 2. Study the chemical mechanisms leading to these events at SPL. 3. Determine the relationship between CCN activity and the chemical composition of aerosols formed via NPF at SPL and the AOS site.