Multi-Month Observations of Spatial and Temporal Variation of CH4 and CO2 in New York City Using Open-Path Measurements over Km-Scale Paths

Understanding urban greenhouse emissions is complicated due to the complex mixture of sources present. These include strong point sources such as power generation facilities and wastewater treatment plants, as well as distributed sources such as from natural gas distribution and combustion. Most previous urban greenhouse gas observations come either from stationary point sampling sensors or from area-wide measurements. However, there are very few long-term observations of spatial variability of greenhouse gas at the kilometer scale. Here, we show horizontal path-integrated measurements of CO₂ and CH₄ concentrations across multiple paths in New York City over a multiple month time period with minute temporal resolution. These measurements overlapped with a several large campaigns around the NYC area (NYC-METS, AEROMMA, STAQS). We observe large spatial and temporal variability in the concentrations of these gases with CH₄ concentrations regularly reaching 2.6-2.7 ppm. Many of these CH₄ plumes occur over 1-2 hour time periods. In addition, we compare the open-path measurements to point measurements around the city as well as to airborne observations.

In order to better understand CH₄ source contributions, we can use additional tracer molecules. First, we observe a wide range of correlation factors between CH₄ and CO₂, which is indicative of a variety of different sources observed including combustion and non-combustion sources. The background CH₄/CO₂ ratio provides constraints on the relative emissions of these two gases and is similar to ratios observed in LA. In addition, we look at correlations with point measurements of other gases during the NYC-METS campaign. Finally, we discuss how these observations can be combined with back trajectory calculations to identify major greenhouse gas sources in the NYC area.