Adverse Effects of Drought on Air Quality in the US

3 May 2017 IGC8

Yuxuan Wang, Yuanyu Xie, Wenhao Dong, Yi Ming, Jun Wang, Lu Shen, Zijian Zhao

Drought: complex climate extreme

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High temperature

Low precipitation Drought

Atmospheric Composition

Land surface

Fires

Chemistry & deposition

Drought is a quite ‘frequent’ extreme in the US

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•  SPEI drought index (Standardized Precipitation Evapotranspiration Index) •  Drought longer than one

month (1-month SPEI < -1.3)

•  Focus on regions with at least 10% drought (20 months)

Mar-Oct 1990-2014

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Data and method Surface observations: •  Ozone (MDA8): EPA, CASNET; •  PM2.5: EPA, IMPROVE •  1990-2014; March to October (growth season) •  Daily mean averaged to monthly GEOS-Chem simulation: •  MERRA 2o x 2.5o

•  1990-2014, Monthly mean •  Year-to-year changes of US anthropogenic emissions (Xing et al.,

2013) •  GFED4 monthly fire emissions

Drought - Pollution Relationship: •  Air pollution data were detrended and deseasonalized by

removing 7-year moving averages from each month

Observed correlation between SPEI and Pollution

Slope of linear correlation at sites with more than 10% drought

Negative correlations throughout the US: higher O3 and PM2.5 at the surface with increasing drought severity

negative negative positive positive

Ozone PM2.5

Wang et al., ACPD, 2017

More pollution Less pollution More pollution Less pollution

GEOS-Chem simulated drought effects

•  Model captures the general relationship between ozone and SPEI •  Model is not able to capture the negative correlation between PM2.5 and

SPEI

Obs Model

Obs

Ozone

PM2.5

Model

Slope of linear correlation at sites with more than 10% drought

Ozone

PM2.5

Performance of climate-chemistry models

Ozone

PM2.5

Climate-chemistry model outputs from ACCMIP archive

Observations NCAR-CAM3.5 GISS-E2-R

Wang et al., ACPD, 2017

Drought effects on ozone

l  Higher temperatures and lower cloud covers lead to higher rates of ozone production (most models get this)

l  Drought reduces dry deposition of ozone (Kavassalis and Murphy, 2017; Huang et al., 2016) (models are lacking this response)

l  Response of biogenic emissions to drought is highly uncertain

•  Isoprene increases with drought (except for Great Plains)

•  Model isoprene is not sensitivity to drought (except for SE US)

Obs isoprene (regrid to model)

Collocated Model (N=2388)

EPA’s PAMS network

SPEI SPEI

Sparse observations

drought drought

Drought effects on aerosol species

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Observed drought anomalies (drought minus normal)

§  Organic aerosol (OA) shows the largest response to drought in all regions

§  Sulfate is the second largest

§  Dust change is evident in the West and Great Plains.

OA - drought relationship

Slope of linear regression between SPEI drought index and OA

Obs Model

l  Observed OA-SPEI slope is positive over all regions, with larger sensitivity over northwestern and southeastern US.

l  Model captures the sign and spatial pattern of the slope, but largely underestimates the slope

Sulfate – drought relationship Slope of linear regression between SPEI drought index and sulfate

Obs Model.

l  Observations show sulfate increase during drought, due to lower wet deposition and higher gas-phase oxidation

l  Model shows a large decrease in the South

l  Model has excessive reduction in aqueous phase SO4 in the south

Model aqueous production of SO4 vs SPEI

Conclusion

l  Observations show significant adverse effects of drought on ozone and PM2.5 air quality:

3.5 ppbv (8%) for ozone and 1.6 µg m-3 (17%) for PM2.5 during drought compared to normal conditions l  Models (CTM and ACCM) reproduce the direction of changes for

surface ozone, but not for PM2.5 l  Model deficiencies include but not limited to:

Ø  Too weak a response of surface emissions (e.g. BVOCs) to drought Ø  Lack of dry deposition response Ø  Aerosols: relative response of gas-phase vs. aqueous phase to drought

l  Future projection: 1-6% increase for ground-level O3 and 1-16% for PM2.5 in the US by 2100 (compared to the 2000s) due to increasing drought alone

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Reference: 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. Discuss., doi:10.5194/acp-2017-234, 2017

Back-up slides

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Model simulated ozone is within 10% of observed value over most regions •  ~ 10 ppb underestimate at California •  ~ 10 ppb overestimate at southeast US Model simulated PM2.5 is ~20% lower than observations •  ~ 5 ug/m3 underestimation at southeast US

Model Evaluation: 1990-2015 (Mar to Oct) mean distribution

Obs. Mod. Mod. – Obs.

Ozone (ppbv)

PM2.5 (ug/m3)

Obs. Mod. Mod. – Obs.

Model Evaluation

•  Grids with less than 5 years data are excluded •  Model simulated ozone correlates well with observation

(r > 0.7 over most regions) •  Model simulated PM2.5 show moderate correlation with

observation (r~0.3-0.7)

Correlation r between model and observations (1990-2014)

Ozone PM2.5

Pollution enhancements caused by drought

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Drought effects on ozone

Obs Mod Corr r

Comparison between model simulated and observed isoprene

Observation (regrid to model)

Model collocates with obs (N=2388)