Comment on acp-2020-1234

<strong class="journal-contentHeaderColor">Abstract.</strong> In this study, we present a novel approach for assessing nearshore seepage atmospheric emissions through modeling of air quality station data, specifically a Gaussian plume inversion model. A total of 3 decades of air quality station meteorology and total hydrocarbon concentration, THC, data were analyzed to study emissions from the Coal Oil Point marine seep field offshore California. THC in the seep field directions was significantly elevated and Gaussian with respect to wind direction, <span class="inline-formula"><i>θ</i></span>. An inversion model of the seep field, <span class="inline-formula"><i>θ</i></span>-resolved anomaly, THC<span class="inline-formula">^′</span>(<span class="inline-formula"><i>θ</i>)</span>-derived atmospheric emissions is given. The model inversion is for the far field, which was satisfied by gridding the sonar seepage and treating each grid cell as a separate Gaussian plume. This assumption was validated by offshore in situ data that showed major seep area plumes were Gaussian. Plume total carbon, TC (TC <span class="inline-formula">=</span> THC <span class="inline-formula">+</span> carbon dioxide, CO<span class="inline-formula">₂</span>, <span class="inline-formula">+</span> carbon monoxide), 18 % was CO<span class="inline-formula">₂</span> and 82 % was THC; 85 % of THC was CH<span class="inline-formula">₄</span>. These compositions were similar to the seabed composition, demonstrating efficient vertical plume transport of dissolved seep gases. Air samples also measured atmospheric alkane plume composition. The inversion model used observed winds and derived the 3-decade-average (1990–2021) field-wide atmospheric emissions of 83 400 <span class="inline-formula">±</span> 12 000 m<span class="inline-formula">³</span> THC d<span class="inline-formula">^−1</span> (27 Gg THC yr<span class="inline-formula">^−1</span> based on 19.6 g mol<span class="inline-formula">^−1</span> for THC). Based on a 50 : 50 air-to-seawater partitioning, this implies seabed emissions of 167 000 m<span class="inline-formula">³</span> THC d<span class="inline-formula">^−1</span>. Based on atmospheric plume composition, C<span class="inline-formula">₁</span>–C<span class="inline-formula">₆</span> alkane emissions were 19, 1.3, 2.5, 2.2, 1.1, and 0.15 Gg yr<span class="inline-formula">^−1</span>, respectively. The spatially averaged CH<span class="inline-formula">₄</span> emissions over the <span class="inline-formula">∼</span> 6.3 km<span class="inline-formula">²</span> of 25 <span class="inline-formula">×</span> 25 m<span class="inline-formula">²</span> bins with sonar values above noise were 5.7 <span class="inline-formula">µ</span>M m<span class="inline-formula">^−2</span> s<span class="inline-formula">^−1</span>. The approach can be extended to derive emissions from other dispersed sources such as landfills, industrial sites, or terrestrial seepage if source locations are constrained spatially.