AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
In-situ Investigation of Near-highway Secondary Aerosol Formation in a Potential Aerosol Mass (PAM) Flow Reactor
PROVAT SAHA, Stephen Reece, Andrew Grieshop, North Carolina State University
Abstract Number: 700 Working Group: Urban Aerosols
Abstract Motor vehicles are a major source of primary aerosols and aerosol precursors that are linked with adverse health and climate effects. Here we present an in-situ investigation of near-highway secondary aerosol formation using a Potential Aerosol Mass (PAM) oxidation flow reactor at a site 10 m from Interstate 40 (I-40), outside Durham, North Carolina. In July, 2015, ambient air at this near-highway site was exposed to high oxidant (O3 and OH; hydroxyl radical) concentrations in a newly developed PAM reactor. The reactor is constructed of stainless steel, has a short, large-diameter inlet and was operated with a residence time of ~70 s. Online OH exposure in the reactor was measured via the decay of externally injected carbon-monoxide (CO); estimated OH exposure ranged from 0.3-14 days of equivalent atmospheric aging assuming an atmospheric OH concentration of 1.5e6 molec cm-3. PAM-processed and ambient air (bypass) were alternately sampled by several instruments (e.g., ACSM, SMPS, PAX). The volatility of PAM-oxidized and ambient aerosol were measured using a multi-tube thermodenuder (TD)-SMPS system at 60 and 120 ˚C temperature with a 30 s TD residence time. Additional measurements included black carbon, NOx and CO2 concentration, meteorological conditions and traffic volume and speed. Oxidation produced substantial additional mass of organic and nitrate species, whereas secondary sulfate production was not observed. Secondary aerosol concentrations varied strongly with OH exposure and between day and night time. Night time enhancement was significantly higher than day time measurements, despite the higher daytime traffic volume. Peak mass enhancement of aerosol mass by a factor of 2 to 4 was observed with 2-5 days of equivalent atmospheric aging, while aerosol mass was lost during very high exposures. The volatility and optical properties of ambient and PAM-oxidized aerosol will be discussed and results will be compared from other recent campaigns.