1. Adverse Effects of Drought on Air Quality in the US
Yuxuan Wang, Yuanyu Xie, Wenhao Dong, Yi Ming, Jun Wang, Lu Shen, Zijian Zhao
3 May 2017
IGC8
School of Software Tsinghua University

2. Drought: complex climate extreme
Land surface
Low precipitation
Drought
Chemistry & deposition
High
temperature
Fires
Atmospheric Composition
School of Software Tsinghua University

3. Drought is a quite ‘frequent’ extreme in the US
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 %

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

5. Observed correlation between SPEI and Pollution
Slope of linear correlation at sites with more than 10% drought
Ozone
PM2.5
negative
positive
negative
positive
More pollution
Less pollution
More pollution
Less pollution
Negative correlations throughout the US: higher O3 and PM2.5 at the surface with increasing drought severity
Wang et al., ACPD, 2017

6. GEOS-Chem simulated drought effects
Slope of linear correlation at sites with more than 10% drought
Ozone
Ozone
Obs
Model
PM2.5
PM2.5
Obs
Model
Model captures the general relationship between ozone and SPEI
Model is not able to capture the negative correlation between PM2.5 and SPEI

7. Performance of climate-chemistry models
Observations
NCAR-CAM3.5
GISS-E2-R
Ozone
PM2.5
Climate-chemistry model outputs from ACCMIP archive
Wang et al., ACPD, 2017

8. Drought effects on ozone
Higher temperatures and lower cloud covers lead to higher rates of ozone production (most models get this)
Drought reduces dry deposition of ozone (Kavassalis and Murphy, 2017; Huang et al., 2016) (models are lacking this response)
Response of biogenic emissions to drought is highly uncertain
Obs isoprene (regrid to model)
Collocated Model (N=2388)
EPA’s PAMS network
Isoprene increases with drought (except for Great Plains)
Model isoprene is not sensitivity to drought (except for SE US)
Sparse observations
drought
drought
SPEI
SPEI

9. Drought effects on aerosol species
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.

10. OA - drought relationship
Slope of linear regression between SPEI drought index and OA
Obs
Model
Observed OA-SPEI slope is positive over all regions, with larger sensitivity over northwestern and southeastern US.
Model captures the sign and spatial pattern of the slope, but largely underestimates the slope

11. Sulfate – drought relationship
Slope of linear regression between SPEI drought index and sulfate
Model.
Obs
Observations show sulfate increase during drought, due to lower wet deposition and higher gas-phase oxidation
Model shows a large decrease in the South
Model has excessive reduction in aqueous phase SO4 in the south
Model aqueous production of SO4 vs SPEI

12. Conclusion
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
Models (CTM and ACCM) reproduce the direction of changes for surface ozone, but not for PM2.5
Model deficiencies include but not limited to:
Too weak a response of surface emissions (e.g. BVOCs) to drought
Lack of dry deposition response
Aerosols: relative response of gas-phase vs. aqueous phase to drought
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
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: /acp , 2017

13. Back-up slides

14. Model Evaluation: 1990-2015 (Mar to Oct) mean distribution
Ozone (ppbv)
Obs.
Mod.
Mod. – Obs.
PM2.5 (ug/m3)
Obs.
Mod.
Mod. – Obs.
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

15. Correlation r between model and observations (1990-2014)
Model Evaluation
Correlation r between model and observations ( )
Ozone
PM2.5
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~ )

16. Pollution enhancements caused by drought

17. Drought effects on ozone
Comparison between model simulated and observed isoprene
Obs
Mod
Corr r
Observation
(regrid to model)
Model collocates with obs (N=2388)