Findings of Near Road Nitrogen Oxides (NO_x) 30 Month Monitoring Program at Dewey Square Tunnel in Boston, Massachusetts

摘要

The four-station nitrogen oxides (NO_x) monitoring network at the Dewey Square Tunnel (DST) in Boston, Massachusetts was deployed in April 2011 by Massachusetts DOT, and operated until September 2013. The study measured hourly traffic data, carbon monoxide (CO), NO_x, and nitrogen dioxide (NO2) 150 feet inside the tunnel exit portal, and at three sidewalk locations up to 260 feet downstream above the tunnel exit boat section. The purpose of this monitoring program was to derive a method for estimating new NO_x tunnel emission limits for the renewal of the 2006 Operating Certification of the Central Artery/Tunnel (CA/T) Ventilation System. Compliance with the one hour NO_2 NAAQS of 100 parts per billion (ppb) required a more refined analysis than the one performed at the tunnel opening in 2006; and a better understanding of how much nitrous oxide (NO) produced by motor vehicle exhaust is actually converted to NO_2 in the vicinity of the tunnel exhaust points. The first year of data, (April 2011 to March 2012) was used to support the modeling effort for the compliance demonstration to determine the revised NOx and CO tunnel emission limits. The collected data were used to update the CO-NO_x regression equation developed in 2006 (to reflect the current vehicle emission profile using the tunnel), and the NO to NO_2 conversion factor, which, at less than 10% proved to be much lower than it has been reported in the literature. A summary of these findings was presented at the 2012 AWMA annual meeting (Ginzburg et.al. #418). This paper presents the findings of the 30 month monitoring program; analyzing the tunnel and sidewalk data on a monthly basis by using average, maximum and 98th percentile concentrations. The findings indicate that DST emission levels (including average and peak levels) have steadily decreased during the three years of monitoring; and the NO_2 levels at the two sidewalk sites (#1 and #2) that are mostly influenced by the DST emissions had lower average, 98th percentile levels, and less number of days with NO_2 concentrations above 100 ppb in 2013 when compared to 2011.