With the pending withdrawal of the United States from the Paris Climate Accord, cities are now leading US actions toward reducing greenhouse gas emissions. Implementing effective mitigation strategies requires the ability to measure and track emissions over time and at various scales. We report CO emissions in the Boston, MA, urban region from September 2013 to December 2014 based on atmospheric observations in an inverse model framework. Continuous atmospheric measurements of CO from five sites in and around Boston were combined with a high-resolution bottom-up CO emission inventory and a Lagrangian particle dispersion model to determine regional emissions. Our model-measurement framework incorporates emissions estimates from submodels for both anthropogenic and biological CO fluxes, and development of a CO concentration curtain at the boundary of the study region based on a combination of tower measurements and modeled vertical concentration gradients. We demonstrate that an emission inventory with high spatial and temporal resolution and the inclusion of urban biological fluxes are both essential to accurately modeling annual CO fluxes using surface measurement networks. We calculated annual average emissions in the Boston region of 0.92 kg C·m·y (95% confidence interval: 0.79 to 1.06), which is 14% higher than the Anthropogenic Carbon Emissions System inventory. Based on the capability of the model-measurement approach demonstrated here, our framework should be able to detect changes in CO emissions of greater than 18%, providing stakeholders with critical information to assess mitigation efforts in Boston and surrounding areas.

Abstract. Factors influencing diurnal to interannual variability in Hg• over New England were investigated using multiyear measurements conducted by AIRMAP at the Thompson Farm (TF) coastal site, an inland elevated site at Pac Monadnock (PM), and two month measurements on Appledore Island (AI) in the Gulf of Maine. Mixing ratios of Hg • at TF showed distinct seasonality with maxima in March and minima in October. Hg • at AI tracked the trend at TF but with higher minima, while at PM the diurnal and annual cycles were dampened. In winter, Hg • was correlated most strongly with CO and NO y , indicative of anthropogenic emissions as their primary source. Our analysis indicates that Hg • had a regional background level of ∼160 fmol/mol in winter, a dry deposition velocity of ∼0.20 cm s −1 with a ∼16 day lifetime in the coastal boundary layer in summer. The influence of oceanic emissions on ambient Hg • levels was identified using the Hg • -CHBr 3 correlation at both TF and AI. Moreover, the lower Hg These trends could be explained by a homogeneous distribution of Hg • over North American in winter due to its long lifetime and/or rapid removal of reactive mercury from anthropogenic sources. We caution that during warmer winters, the Hg • -CO slope possibly reflects Hg • loss relative to changes in CO more than their emission ratio.

Three widely used Lagrangian particle dispersion models (LPDMs)-the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT), Stochastic Time-Inverted Lagrangian Transport (STILT), and Flexible Particle (FLEXPART) models-are evaluated with measurements from the controlled tracerrelease experiments Cross-Appalachian Tracer Experiment (CAPTEX) and Across North America Tracer Experiment (ANATEX). The LPDMs are run forward in time driven by identical meteorological inputs from the North American Regional Reanalysis (NARR) and several configurations of the Weather Research and Forecasting (WRF) model, and the simulations of tracer concentrations are evaluated against the measurements with a ranking procedure derived from the combination of four statistical parameters. The statistical evaluation reveals that all three LPDMs have comparable skill in simulating the tracer plumes when driven by the same meteorological inputs, indicating that the differences in their formulations play a secondary role. Simulations with HYSPLIT and STILT demonstrate the benefit of using customized hourly WRF fields with 10-km horizontal grid spacing over the use of 3-hourly NARR fields with 32-km horizontal grid spacing. All three LPDMs perform better when the WRF wind fields in the planetary boundary layer are nudged to NARR, with FLEXPART benefitting the most. Case studies indicate that the nudging corrects an overestimate in plume transport speed possibly caused by a positive bias in WRF wind speeds near the surface. All three LPDMs also benefit from the use of time-averaged velocity and convective mass flux fields generated by WRF, but the impact on HYSPLIT and STILT is much greater than on FLEXPART. STILT backward runs perform as well as their forward counterparts, demonstrating this model's reversibility and its suitability for application to inverse flux estimates.

The relationship between synoptic‐scale circulation patterns and surface ozone (O3) across the northeastern United States was investigated for summers 2000–2004. Observational data consisted of 1200 UT sea level pressure fields obtained from the National Centers for Environmental Prediction Global Final Analysis and O3 measurements from 474 Environmental Protection Agency and five AIRMAP monitoring sites. The five most common circulation patterns, or map types (I–V), were identified with a correlation‐based synoptic categorization technique, which persisted on 65% of the days during the study period. Map type I, characterized by stagnant warm conditions throughout the northeast, occurred most frequently (21%) with associated episodes of high O3. Interannual variability in O3 varied regionally from a seasonally averaged daily maximum value of 64 ppbv in 2002 to a minimum of 52 ppbv in 2004. By considering both the sea level pressure system intensity and frequency of each map type, 46% of the interannual variability in summertime O3 was reproduced with intensity being the dominant factor. The remaining interannual variability was possibly due to nonlinear relationships between climate and biogenic emissions and/or recent reductions of power plant emissions of nitrogen oxides (NOx) over the eastern United States. The storm track of cyclones in the eastern United States was a key determinate of the intensity of circulation patterns.

Abstract.The relationship between synoptic circulation patterns over the western North Atlantic Ocean in spring (March, April, and May) and tropospheric O 3 and CO was investigated using retrievals from the Tropospheric Emission Spectrometer (TES) for 2005 and 2006. Seasonal composites of TES retrievals reprocessed to remove the artificial geographic structure added from the a priori revealed a channel of slightly elevated O 3 (>55 ppbv) and CO (>115 ppbv) at the 681 hPa retrieval level between 30 • N and 45 • N extending from North America out over the Atlantic Ocean. Ozone and CO in this region were correlated at r=0.22 with a slope value of 0.13 mol mol −1 indicative of the overall impact of photochemical chemical processes in North American continental export. Composites of TES retrievals for the six predominant circulation patterns identified as map types from sea level pressure fields of the NCEP FNL analyses showed large variability in the distribution of tropospheric O 3 . Map types MAM2 and MAM3 featuring cyclones near the US east coast produced the greatest export to the lower free troposphere with O 3 >65 ppbv and a relatively well-defined O 3 -CO correlation (slope values near 0.20 mol mol −1 ). The ensembles of HYSPLIT backward trajectories indicated that the high O 3 levels were possibly a result of pollutants lofted to the free troposphere by the warm conveyor belt (WCB) of a cyclone. An important finding was that pollutant export occurred in the main WCB branch to the east of the cyclone and in a secondary branch circling to the back of the cyclone center. Conversely, a map type featuring a large anticyclone dominating the flow over the US east coast (MAM6) restricted export with O 3 levels generally <55 ppbv and CO levels generally <110 ppbv. There was also evidence of stratospheric intrusions particularly to the north of 45 • N in the 316 hPa composites predominately Correspondence to: J. Hegarty (jhegarty@ccrc.sr.unh.edu) for MAM1 which featured a large cyclone near Newfoundland. However, the concurrence of these intrusions with pollutant export, specifically in the southwestern North Atlantic Ocean, made it difficult to delineate their respective contributions to the 681 hPa O 3 composites.