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.

Cities are concentrated areas of CO emissions and have become the foci of policies for mitigation actions. However, atmospheric measurement networks suitable for evaluating urban emissions over time are scarce. Here we present a unique long-term (decadal) record of CO mole fractions from five sites across Utah's metropolitan Salt Lake Valley. We examine "excess" CO above background conditions resulting from local emissions and meteorological conditions. We ascribe CO trends to changes in emissions, since we did not find long-term trends in atmospheric mixing proxies. Three contrasting CO trends emerged across urban types: negative trends at a residential-industrial site, positive trends at a site surrounded by rapid suburban growth, and relatively constant CO over time at multiple sites in the established, residential, and commercial urban core. Analysis of population within the atmospheric footprints of the different sites reveals approximately equal increases in population influencing the observed CO, implying a nonlinear relationship with CO emissions: Population growth in rural areas that experienced suburban development was associated with increasing emissions while population growth in the developed urban core was associated with stable emissions. Four state-of-the-art global-scale emission inventories also have a nonlinear relationship with population density across the city; however, in contrast to our observations, they all have nearly constant emissions over time. Our results indicate that decadal scale changes in urban CO emissions are detectable through monitoring networks and constitute a valuable approach to evaluate emission inventories and studies of urban carbon cycles.

Wintertime ammonium nitrate aerosol pollution is a severe air quality issue affecting both developed and rapidly urbanizing regions from Europe to East Asia. In the United States, it is acute in western basins subject to inversions that confine pollutants near the surface. Measurements and modeling of a wintertime pollution episode in Salt Lake Valley, Utah, demonstrate that ammonium nitrate is closely related to photochemical ozone through a common parameter, total odd oxygen, Ox,total. We show that the traditional nitrogen oxide and volatile organic compound (NOx‐VOC) framework for evaluating ozone mitigation strategies also applies to ammonium nitrate. Despite being nitrate‐limited, ammonium nitrate aerosol pollution in Salt Lake Valley is responsive to VOCs control and, counterintuitively, not initially responsive to NOx control. We demonstrate simultaneous nitrate limitation and NOx saturation and suggest this phenomenon may be general. This finding may identify an unrecognized control strategy to address a global public health issue in regions with severe winter aerosol pollution.

Anthropogenic modification of the water cycle involves a diversity of processes, many of which have been studied intensively using models and observations. Effective tools for measuring the contribution and fate of combustion-derived water vapor in the atmosphere are lacking, however, and this flux has received relatively little attention. We provide theoretical estimates and a first set of measurements demonstrating that water of combustion is characterized by a distinctive combination of H and O isotope ratios. We show that during periods of relatively low humidity and/or atmospheric stagnation, this isotopic signature can be used to quantify the concentration of water of combustion in the atmospheric boundary layer over Salt Lake City. Combustion-derived vapor concentrations vary between periods of atmospheric stratification and mixing, both on multiday and diurnal timescales, and respond over periods of hours to variations in surface emissions. Our estimates suggest that up to 13% of the boundary layer vapor during the period of study was derived from combustion sources, and both the temporal pattern and magnitude of this contribution were closely reproduced by an independent atmospheric model forced with a fossil fuel emissions data product. Our findings suggest potential for water vapor isotope ratio measurements to be used in conjunction with other tracers to refine the apportionment of urban emissions, and imply that water vapor emissions associated with combustion may be a significant component of the water budget of the urban boundary layer, with potential implications for urban climate, ecohydrology, and photochemistry. A nthropogenic perturbation of the atmospheric water cycle is expressed over a wide range of spatial and temporal scales. Recent global changes resulting from warming-associated increases in saturation vapor pressure have been observed in satellite and reanalysis data (1, 2). Regional impacts related to large-scale land use change are detectible in precipitation data and models (3-5). Humidity anomalies (both positive and negative) have been observed in many urban centers and associated with changes in land cover, direct anthropogenic sources, and interaction of evapotranspiration and condensation processes with the urban heat island effect (6-10).Fossil fuel combustion releases water vapor to the atmosphere. Assuming an average molar ratio of H 2 O to CO 2 emission of 1.5 (see Water of Combustion) and current anthropogenic carbon emission rates of 9.5 Pg C/y (11), global combustion vapor emissions total ∼21 Pg/y. At the global scale, these numbers are four orders of magnitude smaller than the gross global exchange of water vapor between the Earth surface and the atmosphere, which totals more than 480,000 Pg/y (12). However, anthropogenic emissions are highly concentrated in space and time and, locally, may be a significant source of vapor and impact atmospheric water cycling, ambient humidity, and photochemistry. Water of combustion has been hypothesized to be an important contributor...

Numerous mountain valleys experience wintertime particulate pollution events, when persistent cold air pools (PCAPs) develop and inhibit atmospheric mixing, leading to the accumulation of pollutants. Here we examine the relationships between trace gases and criteria pollutants during winter in Utah's Salt Lake Valley, in an effort to better understand the roles of transport versus chemical processes during differing meteorological conditions as well as insights into how targeted reductions in greenhouse gases will impact local air quality in varying meteorological conditions. CO2 is a chemically inert gas that is coemitted during fossil fuel combustion with pollutants. Many of these coemitted pollutants are precursors that react chemically to form secondary particulate matter. Thus, CO2 can serve as a stable tracer and potentially help distinguish transport versus chemical influences on pollutants. During the winter of 2015–2016, we isolated enhancements in CO2 over baseline levels due to urban emissions (“CO2ex”). CO2ex was paired with similar excesses in other pollutant concentrations. These relationships were examined during different wintertime conditions and stages of pollution episodes: (a) Non‐PCAP, (b) beginning, and (c) latter stages of an episode. We found that CO2ex is a good indicator of the presence of gaseous criteria pollutants and a reasonable indicator of PM2.5. Additionally, the relationships between CO2ex and criteria pollutants differ during different phases of PCAP events which provide insight into meteorological and transport processes. Lastly, we found a slight overestimation of CO:CO2 emission ratios and a considerable overestimation of NOx:CO2 by existing inventories for the Salt Lake Valley.