Reactive Nitrogen Emissions from Marcellus Shale Natural Gas Activity and Implications for Regional Deposition

According to the U.S. EPA National Emission Inventory, electric generating unit (EGU) emissions of NO_x (NO + NO₂) have declined by ~75% since 1995.  However, recent increases in unconventional natural gas (UNG) extraction activity (i.e. Marcellus Shale) could counteract the substantial air quality improvements realized under the Clean Air Act and Amendments.  For example, in 2013, industry-reported UNG NO_x emissions were equivalent to ~10% of total EGU NO_x emissions in Pennsylvania, while emissions factor estimates ranged from 2 to 29% of EGU NO_x emissions.  While existing wet and dry deposition monitoring networks effectively capture the regional impact of EGU NO_x emissions, it is currently unknown whether existing monitoring networks capture increasingly prevalent UNG emissions.  To determine whether current monitoring data reflects the increased UNG activity, we compared wet and dry reactive nitrogen (Nr) deposition [NO₃^-, HNO₃ and particulate NO₃^- from National Atmospheric Deposition Program – National Trends Network (NADP-NTN) and Clean Air Status and Trends Network (CASTNET) sites] and ambient NO₂ concentrations (EPA Urban Air Database) throughout Pennsylvania to spatial patterns in UNG well pad locations and associated NO_x emissions.  From 2011 to 2013, our analyses indicate that dry HNO₃ deposition, wet NO₃^- deposition, and NO₂ concentrations are negatively correlated with the number of well pads within a 25 km buffer around monitoring sites.  This suggests that factors other than well pad distributions and associated NO_x emissions are controlling Nr deposition at the monitoring sites. 

In order to better constrain Nr emission estimates and associated deposition fluxes occurring around UNG extraction sites, we collaborated with scientists at an experimental research well pad in Morgantown, West Virginia (USA) called the Marcellus Shale Energy and Environment Laboratory (MSEEL).  At this site, we established ~300 m transects of passive samplers upwind and downwind of MSEEL.  Passive samplers are effective recorders of time-integrated ambient concentrations and depositional fluxes of NO₂, HNO₃, and NH₃.  Samplers were deployed biweekly from May-December 2015 to quantify temporal and spatial variations in NO₂, HNO₃, and NH₃ concentrations and deposition fluxes.  While previous UNG studies report daily NO_x ambient air concentration during different phases of extraction, this study addresses a key knowledge gap with important ecosystem and water quality implications; what is the spatial extent of Nr deposition from UNG emissions?  We present changes in biweekly concentrations and deposition fluxes of nitrogen dioxide (NO₂), nitric acid (HNO₃) and ammonia (NH₃) during all phases of unconventional natural gas extraction including top hole drilling, horizontal drilling, hydraulic fracturing, and well production.