The 2020 coronavirus pandemic and the following quarantine measures have led to significant changes in daily life worldwide. Preliminary research indicates that air quality has improved in many urban areas as a result of these measures. This study takes a neighborhood-scale approach to quantifying this change in pollution. Using data from a network of citizen-hosted, low-cost particulate matter (PM) sensors, called Air Quality & yoU (AQ&U), we obtained high-spatial resolution measurements compared to the relatively sparse state monitoring stations. We compared monthly average estimated PM 2.5 concentrations from February 11 to May 11, 2019 at 71 unique locations in Salt Lake County, UT, USA with the same (71) sensors’ measurements during the same timeframe in 2020. A paired t -test showed significant reductions (71.1% and 21.3%) in estimated monthly PM 2.5 concentrations from 2019 to 2020 for the periods from March 11-April 10 and April 11-May 10, respectively. The March time period corresponded to the most stringent COVID-19 related restrictions in this region. Significant decreases in PM 2.5 were also reported by state monitoring sites during March (p < 0.001 compared to the previous 5-year average). While we observed decreases in PM 2.5 concentrations across the valley in 2020, it is important to note that the PM 2.5 concentrations did not improve equally in all locations. We observed the greatest reductions at lower elevation, more urbanized areas, likely because of the already low levels of PM 2.5 at the higher elevation, more residential areas, which were generally below 2 μg/m 3 in both 2019 and 2020. Although many of measurements during March and April were near or below the estimated detection limit of the low-cost PM sensors and the federal equivalent measurements, every low-cost sensor (51) showed a reduction in PM 2.5 concentration in March of 2020 compared to 2019. These results suggest that the air quality improvement seen after March 11, 2020 is due to quarantine measures reducing traffic and decreasing pollutant emissions in the region.

Aluminum (Al) is a preferred metal for designing deep-ultraviolet (DUV) surface plasmon resonance (SPR)-based sensors. The native oxide layer (alumina), which grows when the Al film is exposed to air, adds an extra layer to the multilayer stack and consequently affects the DUV-SPR sensing performance. To mitigate the performance loss in DUV-SPR-based sensing, new, to the best of our knowledge, approaches are considered here. We first consider chromium, indium (In), nickel, and platinum as alternative plasmonic materials to Al. In-film-based DUV-SPR sensors exhibit the best performance parameters compared to these alternative materials. We next consider the approach of replacing the native oxide layer by an ultrathin gold (Au) layer on top of bare Al or In. With an optimal Au thickness, higher sensitivity as compared to oxidized metals is observed. The next approach adds one or more graphene layers on top of the bare metal film. In this case, the performance depends on the number of graphene layers, but improvement in sensor characteristics in the DUV is also obtained. The use of Au or graphene overlayers increases the refractive index sensing dynamic range, which can be significant for In with these overlayers under certain operating conditions.

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