It’s over 30°C in the aircraft cabin, and every few minutes a different airport tower tries to divert us from their airspace… That’s what flying in summer at 300 m over Los Angeles is like. It is also exciting – it is unbelievable what we are able to see from this low altitude over the city. Not only sightseeing-worth views of the port and beaches (including, from time to time, a school of dolphins in the ocean), or of the downtown skyscrapers – but as someone who is new to Los Angeles, I am also flabbergasted by the oil wells inside the city, and the dystopian size of the highways cutting through the metropolis.
Sampling the air and the vertical wind speed at low, constant altitude with a slow aircraft has a special purpose: We performed airborne VOC and NOx flux measurements. Different from conventional concentration measurements, emission fluxes provide direct information about the emission sources within a certain area below the point of measurement, without the need to take into account long range transport. Measured emission fluxes can almost directly be compared to emission inventories that are kept by regulatory agencies such as the US Environmental Protection Agency or the California Air Resources Board, without running chemical transport models.
VOCs and NOx are precursors to the air pollutants PM2.5 and ozone, which have decreased in California as in most industrialized countries over several decades. This was thanks to emission regulations. But since approximately 2010, their concentrations have stagnated in California and our mission is to find potential mismatches between the inventories and actual emissions, and potentially underestimated emission sources.
While 9 of our 16 flights cover the Los Angeles Basin, we also have another region on our radar: The San Joaquin Valley is an enormous agricultural production area, with millions of dairy cows, and crops of almost every variety you can imagine. Flying over the San Joaquin Valley gets quite a bit more boring than over Los Angeles – the landscape looks almost desert-like with the brown, dusty fields and incredibly large dairy farms. Even from our airplane we can often smell the manure lagoons.
The cabin temperatures here get to around 40°C, and even though we have installed several extra fans to its rack, we have to hold ice packs against our mass spectrometer to keep it from shutting down. This is because the air inside the San Joaquin Valley is quite stagnant and does not offer the sea breeze that we get in Los Angeles.
Anyone who has ever participated in an aircraft campaign will know the constant stress level that is caused by the pressure for everything to work perfectly within the very limited – because expensive - timeframe of the campaign. So, we were in a celebratory mood when the campaign was over without any major instrument failure. But the largest chunk of work was only beginning after we got home to Berkeley: Data processing. From raw mass spectra measured at 10 Hz time resolution to final VOC flux data for a few hundred masses/compounds it took almost one and a half years of full-time work.
The results were interesting: We found out that emission inventories perform well for typical traffic emissions, but that they underestimate most oxygenated VOC emissions. These are more typical for solvents and other volatile chemical products (VCPs), or for cooking emissions. The inventories also were not great at representing terpenoid emissions, which stem from plant emissions and fragrances. In the San Joaquin Valley, soil NOx emissions and dairy farm VOC emissions were also by far underestimated.
What does this mean? This means that air quality predictions and regulations may be improved by taking into account that a significant fraction of the emissions that are precursors to ozone and particulate air pollution is from underestimated sources.
The RECAP-CA campaign was a collaboration between the University of California, Berkeley, the Navy Postgraduate School, CalTech, NOAA, the California Air Resources Board and the South Coast Air Quality Management District. You can find out more about the results from this campaign in:
Pfannerstill, E. Y., Arata, C., Zhu, Q., Schulze, B. C., Woods, R., Seinfeld, J. H., Bucholtz, A., Cohen, R. C., and Goldstein, A. H.: Volatile organic compound fluxes in the agricultural San Joaquin Valley – spatial distribution, source attribution, and inventory comparison, Atmos. Chem. Phys., https://doi.org/10.5194/egusphere-2023-723, 2023.
Zhu, Q., Place, B., Pfannerstill, E. Y., Tong, S., Zhang, H., Wang, J., Nussbaumer, C. M., Wooldridge, P., Schulze, B. C., Arata, C., Bucholtz, A., Seinfeld, J. H., Goldstein, A. H., and Cohen, R. C.: Direct observations of NO x emissions over the San Joaquin Valley using airborne flux measurements during RECAP-CA 2021 field campaign, Atmos. Chem. Phys., 23, 9669–9683, https://doi.org/10.5194/acp-23-9669-2023, 2023.
Pfannerstill, E. Y., Arata, C., Zhu, Q., Schulze, B. C., Woods, R., Harkins, C., Schwantes, R. H., McDonald, B. C., Seinfeld, J. H., Bucholtz, A., Cohen, R. C., and Goldstein, A. H.: Comparison between Spatially Resolved Airborne Flux Measurements and Emission Inventories of Volatile Organic Compounds in Los Angeles, Environ. Sci. Technol., XXX, https://doi.org/10.1021/acs.est.3c03162, 2023.
Nussbaumer, C. M., Place, B. K., Zhu, Q., Pfannerstill, E. Y., Wooldridge, P., Schulze, B. C., Arata, C., Ward, R., Bucholtz, A., Seinfeld, J. H., Goldstein, A. H., and Cohen, R. C.: Measurement report: Airborne measurements of NO x fluxes over Los Angeles during the RECAP-CA 2021 campaign, Atmos. Chem. Phys., 23, 13015–13028, https://doi.org/10.5194/acp-23-13015-2023, 2023.