AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Volatility-Based Measurements of Aerosol Mixing State at an Urban Background Site in the Western United States
CHIRANJIVI BHATTARAI, Andrey Khlystov, Desert Research Institute
Abstract Number: 644 Working Group: Urban Aerosols
Abstract The volatility of organic aerosols (OA) is an important thermodynamic parameter for understanding their gas/particle partitioning, lifetime, and transport. Measurements of aerosol volatility using thermodenuders (TD) usually assume the aerosol to be internally mixed. Very little is known about the validity of this assumption. We will present measurements of ambient aerosol volatility aimed to determine aerosol mixing state at an urban background site in the western United States. The measurements took place on a roof of the Desert Research Institute in Reno, NV during July-September 2017, December-February 2018, and June-July 2018. A volatility tandem differential mobility analysis (VTDMA) with a TD residence time of 22 seconds was used to measure the aerosol volatility. Temperatures between 40°C and 200°C were scanned every 3 hours. Aerosol size distributions measured after the TD were processed to identify the main mode. The contribution (F) of the main mode to the total selected aerosol was calculated as the number concentration of the main mode divided by the total number concentration of single-charged particles selected by the VTDMA. F values below 1 indicate the presence of externally mixed particles. Our observations show that aerosols were externally mixed most of the time. The extent of the aerosol mixing state varied between winter and summer. The probability distribution of the main mode fraction (F) during summer time was negatively skewed with a mean F =0.70 (i.e., approximately 1/3 of the aerosol was found outside of the main mode). In winter, the distribution was positively skewed with the mean F=0.4. The higher degree of external mixing during winter is likely due to a stronger influence of local aerosol sources.