Long-term monitoring of air quality in urban areas is often required for regulatory purposes, but these measurements can sometimes produce unexpected and exciting results. When city-wide lockdowns in the early months of the Covid-19 pandemic halted research activities with more complex instrumentation, a Vocus Elf installed in the Changzhou Environmental Monitoring Center in the Yangtze River Delta region of eastern China continued its operation. This instrument recorded changes in volatile organic compound (VOC) composition and abundance as urban activity shut down, creating a unique dataset.
An international team of researchers from China and the United States, including TOFWERK scientists, have used these data to publish companion papers examining how the Covid lockdown affected air quality in the city. Jensen et al. (GRL, 2021) took a close look into the VOC measurements and used positive matrix factorization (PMF) to recognize the chemical fingerprints of different urban and industrial sources. They found that VOC emissions from vehicles decreased by 70% during the lockdowns. Several industrial sources of VOCs were identified, and it was found that emissions from each industry behaved differently: emissions from the pharmaceutical industry decreased by 60%, while those from the textile industry decreased by only 40%. This information is important to regulators and mathematical modelers seeking to understand how policy affects emissions.
Zhang et al. (ACPD, 2021, under review) carried the analysis further, looking into changes in the concentration of ozone, a secondary pollutant whose formation is often difficult to control because it depends on both VOCs and nitrogen oxides (NOx) in a nonlinear way. Counterintuitively, they found that ozone increased in many Chinese urban areas during lockdowns, despite the overall reduced emissions. Combining measurements from the Vocus Elf and from conventional O3 and NOx monitors, the researchers were able to calculate the crucial ratio of NOx to VOC before and during the lockdowns, quantifying the shift in reaction regime that caused increased ozone formation. They were also able to separate the effects of meteorology and emissions reduction on ozone formation.