Coupling between Chemical and Meteorological Processes under Persistent Cold-Air Pool Conditions: Evolution of Wintertime PM_2.5 Pollution Events and N₂O₅ Observations in Utah’s Salt Lake Valley

Abstract

The Salt Lake Valley experiences severe fine particulate matter pollution episodes in winter during persistent cold-air pools (PCAPs). We employ measurements throughout an entire winter from different elevations to examine the chemical and dynamical processes driving these episodes. Whereas primary pollutants such as NO and CO were enhanced twofold during PCAPs, O concentrations were approximately threefold lower. Atmospheric composition varies strongly with altitude within a PCAP at night with lower NO and higher oxidants (O) and oxidized reactive nitrogen (NO) aloft. We present observations of NO during PCAPs that provide evidence for its role in cold-pool nitrate formation. Our observations suggest that nighttime and early morning chemistry in the upper levels of a PCAP plays an important role in aerosol nitrate formation. Subsequent daytime mixing enhances surface PM by dispersing the aerosol throughout the PCAP. As pollutants accumulate and deplete oxidants, nitrate chemistry becomes less active during the later stages of the pollution episodes. This leads to distinct stages of PM pollution episodes, starting with a period of PM buildup and followed by a period with plateauing concentrations. We discuss the implications of these findings for mitigation strategies.

Detection and variability of combustion-derived vapor in an urban basin

“…Diurnal cyclicity was apparent in the relationship between d-excess and CO 2 as well, with periods of increasing (decreasing) CO 2 producing rapid decreases (increases) in d-excess with little change in q. These diurnal patterns are consistent with daytime growth of a shallow convective boundary layer at the surface with a stable layer aloft; the same interpretation was made in prior studies 10 of this event (Baasandorj et al, 2017). Diurnal cycle amplitudes of q, temperature, and CO 2 decreased for the second half of the PCAP (Fig.…”

“…9e), as noted in prior events (Fig. 7,8) (Baasandorj et al, 2017;Whiteman et al, 2014). Humidity and temperatures decreased on January 5 with the passage of a second cold front, which had little impact on d-excess, but promoted strong surface cooling 15 ahead of the third PCAP event.…”

“…7e), which likely indicated cloud development at 20 the base of an elevated inversion, a feature typical of extended PCAP events (Baasandorj et al, 2017;Whiteman et al, 2014).…”

“…prior studies of winter stability in the SLV (Baasandorj et al, 2017;Whiteman et al, 2014): ( 1) where c p is the specific heat capacity at constant pressure for dry air (1005 J kg…”

Observational assessment of the role of nocturnal residual-layer chemistry in determining daytime surface particulate nitrate concentrations

“…In winter, nighttime HNO 3 production can more efficiently compete with daytime photochemically driven production due to the low photolysis rates and hydroxyl radical concentrations (Wagner et al, 2013;Pusede et al, 2016). Multiday pollution events (i.e., periods with elevated particulate matter, PM, concentrations) can occur when meteorological conditions inhibit dispersion, as is the case with the persistent cold air pool formation often found in valley regions Baasandorj et al, 2017). During the daytime, sunlight-driven convection leads to an evolution of the near-surface temperature profile and causes the atmosphere to be reasonably well mixed up to some height (typically less than 1 km; cf.…”