Impacts of Droughts on Atmospheric

Composition and Potential Chemistry

Feedbacks

1 Dept. of Earth and Atmospheric Sciences, University of Houston2 Geophysical Fluid Dynamics Laboratory

18 June, 2019 CESM Workshop, Boulder, CO

Yuxuan Wang1, Sing-Chun Wang1,

Yuanyu Xie2

Drought is a hydroclimate extreme

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https://www.bbc.com/news/world-africa-42731084

Cape Town drought, 2018 Colorado drought, 2017-2018

• Drought is a slow-developing process (time scale: weeks

and longer)

• Land-atmosphere feedbacks are known to be important

for drought persistence

https://www.denverpost.com/2019/05/06/colorad

o-drought-nearly-officially-over/

Goal: Feedback loops between atmospheric composition

and drought

Ridgway Reservoir, CO

The “Dust Bowl”: drought-dust connection

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A dust storm in Texas, 1935

Enhanced dust emissions were critical in enhancing the

severity and determining the geographical distribution

of the Dust Bowl drought (e.g. Cook et al., 2009; 2013)

Contemporary Droughts

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• 1-month SPEI drought index

(Standardized Precipitation

Evapotranspiration Index)

• Analyzed surface sites with at least 10%

drought occurrences

March - October, 1990-2015

%

Negative correlations: higher O3 and

PM2.5 at the surface with increasing

drynessWang Y. et al., ACP, 2017

Ozone

Drought

frequency

PM2.5 (dry mass)

Drought-induced pollution enhancement

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Mean enhancements

Ozone: 8% (3.4 ppbv)

PM2.5: 17% (1.7 μg/m3)

(Wang Y. et al., ACP, 2017)

2011 Southern US drought: 75% higher PM2.5

(Wang Y. et al., JAS, 2015; Zhao, Wang, et al., ES&T, 2019)

Aerosol response is

driven by

secondary

aerosols:

SOA and Sulfate

Drought influence on secondary pollutants

Fire emissions + (wildfires) - (prescribed fires)

Anthropogenic emissions + (~10%)

Biogenic VOC emissions ?

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As wet deposition is greatly reduced during drought, the response

of secondary pollutants is largely determined by precursor

emissions and atmospheric processing.

Isoprene at EPA’s PAMS network

drought(Wang Y. et al.,

2017)

wet

Drought influence on secondary pollutants

Fire emissions + (wildfires) - (prescribed fires)

Anthropogenic emissions + (~10%)

Biogenic VOC emissions + (mild drought)

- (severe drought)

Atmospheric processing:

Aqueous chemistry/processing -

Gas-phase chemistry +

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As wet deposition is greatly reduced during drought, the response

of secondary pollutants is largely determined by precursor

emissions and atmospheric processing.

Aerosol composition

Size

Hygroscopity

Coupled Climate-Chemistry Model:

Ozone-Drought Relationship

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• Models largely reproduce the ozone increase during drought

• All the models underestimate the magnitude of the ozone

increase (e.g. missing BVOC emission response to drought,

dry deposition, clouds)Wang et al., ACP, 2017

ObservationsACCMIP models (Lamarque et al., 2013)

Coupled Climate-Chemistry Model:

Aerosol-Drought Relationship

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• Most models fail to predict the sign of aerosol change under

drought

• CAM3.5 appears to capture the observed correlation, but that

was due to incorrect dust responseWang et al., ACP, 2017

ObservationsACCMIP models (Lamarque et al., 2013)

CESM 2.1.0 Performance

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• Model outputs from CMIP6 CESM2.1.0 AMIPhindcast with WACCM6 and interactivechemistry (TSMLT1), interactive land, data ocean,prescribed sea ice and non-evolving land ice.

• Time period is March – Oct, 1990-2015 (same asin Wang et al., 2017).

• Model PM2.5 is the summation of OC, BC, SO42-,

NO3-, and fine-mode dust

CESM2.1.0 analysis is performed by Sing-Chun Wang (participant of 2017 CESM Tutorial)

CESM 2.1 Drought-Ozone Relationship

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• Significant improvements in correlation and slope

• CESM2.1 still underestimate the magnitude of ozone response

to drought (e.g. California, Georgia, Central US)

Correlation

coefficient (r)

Correlation

slope

CESM2.1.0 Surface observations

CESM 2.1 Drought-PM2.5 Relationship

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• CESM2.1 has different spatial patterns than observations

• Model does not reproduce PM2.5 increase during drought in Central US;

instead it simulates a decrease

• Model overestimates PM2.5 increase during drought in eastern US

Correlation

coefficient

Correlation

slope

CESM2.1.0 Surface observations

?

CESM 2.1 Aerosol Component vs. SPEI

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Dust OA Sulfate

Observations

CESM2.1

• Dust patterns in the model are consistent with

observations

• Secondary aerosols – OA and sulfate – are not

consistent with observations

Response of OA to drought in Central US

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Central US

model

observation

observation

model

Wet month vs.

Dry monthdry

wet

dry

wet

• Model underestimates OA

seasonality

• Model overestimates OA under wet

conditions: inefficient wet

scavenging?

• Model underestimates OA under

dry conditions: emissions and

chemistry

Response of OA to drought in Southeast US

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Southeast US

model

observation

observation

model

Wet month vs.

Dry month dry

wet

dry

wet

• Model overestimates OA by a factor

of 2

• Model captures the dry-to-wet

decrease of OA

• But model overestimates OA under

both wet and dry conditions

Conclusion

Observational evidence: large response of ozone and

PM2.5 to drought

Aerosol response to contemporary drought is driven by

secondary aerosols

Current ESMs can reproduce the direction of change for

ozone, but not for aerosols

Possible reasons for model deficiencies in aerosol:

Emissions are not drought-aware (e.g. MEGAN3/CLM drought

stress algorithm for BVOC emissions)

Underestimate gas-phase formation of secondary aerosols?

Deposition process

Aerosol feedbacks to drought are likely to be important.

Need to improve our predictive capability of chemistry-

climate interactions under extremes such as drought16

More Information

Wang, Y., Y. Xie, W. Dong, Y. Ming, J. Wang, L. Shen, Adverse

effects of increasing drought on air quality via natural processes,

Atmos. Chem. Phys., 17, 12827–12843, doi:10.5194/acp-2017-234,

2017

Wang Y., Y. Xie, L. Chai, W. Dong, Q. Zhang, and L. Zhang, Impact

of the 2011 southern US drought on ground-level fine aerosol

concentration in summertime, J. Atmos. Sci., 72, 1075–1093, 2015

Zhao, Z., Y. Wang, M. Qin, Y. Hu, Y. Xie, A.G. Russell: Drought

Impacts on Secondary Organic Aerosol: a Case Study in the

Southeast United States, Environ. Sci. Technol., 53(1), 242-250, 2019

Xie, Y., Y. Wang, W. Dong, J.S. Wright, L. Shen, Z. Zhao: Evaluating

the response of sulfate to hydroclimate variations in the GEOS-Chem

model: role of meteorological inputs, J. Geophys. Res., 124, 1662-

1679, 2019

https://www.yxwanggroup.net/publications17