The Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT), developed by NOAA’s Air Resources Laboratory, is one of the most widely used models for atmospheric trajectory and dispersion calculations. We present the model’s historical evolution over the last 30 years from simple hand-drawn back trajectories to very sophisticated computations of transport, mixing, chemical transformation, and deposition of pollutants and hazardous materials. We highlight recent applications of the HYSPLIT modeling system, including the simulation of atmospheric tracer release experiments, radionuclides, smoke originated from wild fires, volcanic ash, mercury, and wind-blown dust.

Urban areas are responsible for a substantial fraction of anthropogenic emissions of greenhouse gases (GHGs) including methane (CH₄), with the second largest anthropogenic direct radiative forcing relative to carbon dioxide (CO₂). Quantification of urban CH₄ emissions is important for establishing GHG mitigation policies. Comparison of observation‐based and inventory‐based urban CH₄ emissions suggests possible improvements in estimating CH₄ source emissions in urban environments. In this study, we quantify CH₄ emissions from the Baltimore‐Washington area based on the mass balance aircraft flight experiments conducted in Winters 2015 and 2016. The field measurement‐based mean winter CH₄ emission rates from this area were 8.66 ± 4.17 kg/s in 2015 and 9.14 ± 4.49 kg/s in 2016, which are 2.8 times the 2012 average U.S. GHG Inventory‐based emission rate. The observed emission rate is 1.7 times that given in a population‐apportioned state of Maryland inventory. Methane emission rates inferred from carbon monoxide (CO) and CO₂ emission inventories and observed CH₄/CO and CH₄/CO₂ enhancement ratios are similar to those from the mass balance approach. The observed ethane‐to‐methane ratios, with a mean value of 3.3% in Winter 2015 and 4.3% in Winter 2016, indicate that the urban natural gas system could be responsible for ~40–60% of total CH₄ emissions from this area. Landfills also appear to be a major contributor, providing 25 ± 15% of the total emissions for the region. Our study suggests there are grounds to reexamine the CH₄ emissions estimates for the Baltimore‐Washington area and to conduct flights in other seasons.