ARL Weekly News – July 30, 2021

Recent Events

Reliford Defends Thesis.

Anaiya Reliford, a graduate student in the NOAA Cooperative Science Center in Atmospheric Sciences and Meteorology (NCAS-M) at Howard University, successfully defended her master’s thesis this week. Her project focused on design and testing of UAS-mounted particulate matter (PM) sensors. She plans to continue her UAS research as part of the doctoral program in Howard’s Department of Chemical Engineering. Anaiya was a NERTO intern under the mentorship of Drs. LaToya Myles and Bruce Baker. She will continue working closely with boundary layer scientists at ATDD for her dissertation research.

Recent Publications

Published: Volatile chemical product emissions enhance ozone and modulate urban chemistry,

Coggon, M.M., G.I. Gkatzelis, B.C. McDonald, J.B. Gilman, R.H. Schwantes, N. Abuhassan, K.C. Aikin, M.F. Arend, T.A. Berkoff, Steven.S.Brown, T.L. Campos, G. Gronoff, J.F. Hurley, G. Isaacman-VanWertz, A.R. Koss, M. Li, S.A. McKeen, F. Moshary, J. Peischl, V. Pospisilova, X. Ren, A. Wilson, Y. Wu, M. Trainer, and C. Warneke, Volatile chemical product emissions enhance ozone and modulate urban chemistry, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.2026653118, 2021.


Decades of air quality improvements have substantially reduced the motor vehicle emissions of volatile organic compounds (VOCs). Today, volatile chemical products (VCPs) are responsible for half of the petrochemical VOCs emitted in major urban areas. We show that VCP emissions are ubiquitous in US and European cities and scale with population density. We report significant VCP emissions for New York City (NYC), including a monoterpene flux of 14.7 to 24.4 kg ⋅ d−1 ⋅ km−2 from fragranced VCPs and other anthropogenic sources, which is comparable to that of a summertime forest. Photochemical modeling of an extreme heat event, with ozone well in excess of US standards, illustrates the significant impact of VCPs on air quality. In the most populated regions of NYC, ozone was sensitive to anthropogenic VOCs (AVOCs), even in the presence of biogenic sources. Within this VOC-sensitive regime, AVOCs contributed upwards of ∼20 ppb to maximum 8-h average ozone. VCPs accounted for more than 50% of this total AVOC contribution. Emissions from fragranced VCPs, including personal care and cleaning products, account for at least 50% of the ozone attributed to VCPs. We show that model simulations of ozone depend foremost on the magnitude of VCP emissions and that the addition of oxygenated VCP chemistry impacts simulations of key atmospheric oxidation products. NYC is a case study for developed megacities, and the impacts of VCPs on local ozone are likely similar for other major urban regions across North America or Europe.


Recent work in Los Angeles has shown that urban volatile organic compound (VOC) emissions from consumer and industrial products—termed volatile chemical products (VCPs)—are now an important source of ozone precursors. Using advancements in VOC instrumentation, we show that VCP emissions are ubiquitous in urban regions and can be identified via unique VOC fingerprints. Through detailed modeling, we show that VCPs are as important to ozone production as fossil fuel VOCs and that the chemistry of VCPs can have significant impacts on model simulations of key atmospheric processes. Consequently, air quality models must be updated to account for both the emissions and atmospheric chemistry of VCPs in order to capture their full impact on urban air quality.


Published: Exploring the Use of Standardized Soil Moisture as a Drought Indicator

Leeper, R. D., B. Petersen, M. A. Palecki, H.J. Diamond,  (2021) Exploring the Use of Standardized Soil Moisture as a Drought Indicator. J. Appl. Meteor. Climatol., 60, 1021-1033,


Agricultural drought has traditionally been monitored using indices that are based on above-ground measures of temperature and precipitation that have lengthy historical records. However, the period-of-record length for soil moisture networks is becoming sufficient enough to standardize and evaluate soil moisture anomalies and percentiles that are spatially and temporally independent of local soil type, topography, and climatology. To explore these standardized measures in the context of drought, the U.S. Climate Reference Network hourly standardized soil moisture anomalies and percentiles were evaluated against changes in the U.S. Drought Monitor (USDM) status, with a focus on onset, worsening, and improving drought conditions. The purpose of this study was to explore time scales (i.e., 1–6 weeks) and soil moisture at individual (i.e., 5, 10, 20, 50, and 100 cm) and aggregated layer (i.e., top and column) depths to determine those that were more closely align with evolving drought conditions. Results indicated that the upper-level depths (5, 10, and 20 cm, and top layer aggregate) and shorter averaging periods were more responsive to changes in USDM drought status. This was particularly evident during the initial and latter stages of drought when USDM status changes were thought to be more aligned with soil moisture conditions. This result indicates that standardized measures of soil moisture can be useful in drought monitoring and forecasting applications during these critical stages of drought formation and amelioration.

Significance Statement

Drought is normally monitored by making inferences from temperature and precipitation observations. In this study, we explored whether soil moisture data would improve our ability to monitor evolving drought conditions. Results showed that soil moisture observations were drier than usual prior to U.S. Drought Monitor onset for nearly 80% of events and worsening drought weeks. For improving weeks, soil moisture observations were only slightly drier than usual or near normal. This was more pronounced in the initial and final few weeks of drought. This suggests that applications of soil moisture measurements to monitor and anticipate evolving drought conditions are best focused on the critical stages of drought formation and termination.


Papers accepted: Dynamic global vegetation models underestimate net CO2 flux mean and inter-annual variability in dryland ecosystems

Natasha MacBean, Russell L Scott, Joel Biederman, Philippe Peylin, Thomas Kolb, Marcy Litvak, Praveena Krishnan, Tilden P Meyers, Vivek K Arora, Vladislav Bastrikov, Daniel Goll, Danica L. Lombardozzi, Julia E. M. S. Nabel, Julia Pongratz, Stephen Sitch, Anthony P Walker, Sonke Zaehle and David J Moore, 2021, Dynamic global vegetation models underestimate net CO2 flux mean and inter-annual variability in dryland ecosystems, Environmental Research Letters, DOI: 10.1088/1748-9326/ac1a38


Despite their sparse vegetation, dryland regions exert a huge influence over global biogeochemical cycles because they cover more than 40% of the world surface. It is thought that drylands dominate the inter-annual variability (IAV) and long-term trend in the global carbon (C) cycle. Projections of the global land C sink therefore rely on accurate representation of dryland C cycle processes; however, the dynamic global vegetation models (DGVMs) used in future projections have rarely been evaluated against dryland C flux data. Here, we carried out an evaluation of 14 DGVMs (TRENDY v7) against net ecosystem exchange (NEE) data from 12 dryland flux sites in the southwestern US encompassing a range of ecosystem types (forests, shrub- and grasslands). We find that all of the models underestimate both mean annual C uptake/release as well as the magnitude of NEE IAV, suggesting that improvements in representing dryland regions may improve global C cycle projections. Across all models, the sensitivity and timing of ecosystem C uptake to plant available moisture was at fault. Spring biases in gross primary production (GPP) dominate the underestimate of mean annual NEE, whereas models’ lack of GPP response to water availability in both spring and summer monsoon are responsible for inability to capture NEE IAV. Errors in GPP moisture sensitivity at high elevation, forested sites were more prominent during the spring, while errors at the low elevation shrub and grass-dominated sites were more important during the monsoon. We propose a range of hypotheses for why model GPP does not respond sufficiently to changing water availability that can serve as a guide for future dryland DGVM developments. Our analysis suggests that improvements in modeling C cycle processes across more than a quarter of the Earth’s land surface could be achieved by addressing the moisture sensitivity of dryland C uptake.