Adverse Effects of Drought on Air Quality in the US Yuxuan Wang, Yuanyu Xie, Wenhao Dong, Yi Ming, Jun Wang, Lu Shen, Zijian Zhao

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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

2 Drought: complex climate extreme Drought Low precipitation High temperature Land surface Chemistry & deposition Fires Atmospheric Composition 2

3 Drought is a quite frequent extreme in the US SPEI drought index (Standardized Precipitation Evapotranspiration Index) Mar-Oct % Drought longer than one month (1-month SPEI < -1.3) Focus on regions with at least 10% drought (20 months)

4 Data and method Surface observations: Ozone (MDA8): EPA, CASNET; PM 2.5 : EPA, IMPROVE ; March to October (growth season) Daily mean averaged to monthly GEOS-Chem simulation: MERRA 2 o x 2.5 o , 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 PM 2.5 negative positive negative positive More pollution Less pollution More pollution Less pollution Negative correlations throughout the US: higher O 3 and PM 2.5 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 Obs Model Ozone PM 2.5 Obs Model PM 2.5 Model captures the general relationship between ozone and SPEI Model is not able to capture the negative correlation between PM 2.5 and SPEI

7 Performance of climate-chemistry models Observations NCAR-CAM3.5 GISS-E2-R Ozone PM 2.5 Climate-chemistry model outputs from ACCMIP archive Wang et al., ACPD, 2017

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,

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) Sparse observations drought drought Model isoprene is not sensitivity to drought (except for SE US) 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 Obs Model. 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 SO 4 in the south Model aqueous production of SO 4 vs SPEI

12 Conclusion Observations show significant adverse effects of drought on ozone and PM 2.5 air quality: 3.5 ppbv (8%) for ozone and 1.6 µg m -3 (17%) for PM 2.5 during drought compared to normal conditions Models (CTM and ACCM) reproduce the direction of changes for surface ozone, but not for PM 2.5 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 Future projection: 1-6% increase for ground-level O 3 and 1-16% for PM 2.5 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

Model Evaluation: 1990-2015 (Mar to Oct) mean distribution Ozone (ppbv) Obs. Mod. Mod. Obs PM2.

Mod. Obs Model simulated ozone is within 10% of observed value over most regions ~ 10 ppb

14 Model Evaluation: (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

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

15 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 PM 2.5 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)

Particulate Matter Prediction and Source Attribution for U.S. Air Quality Management in a Changing World

Particulate Matter Prediction and Source Attribution for U.S. Air Quality Management in a Changing World Xin-Zhong Liang (PI), xliang@umd.edu University of Maryland, College Park, MD Co-PIs: Donald J.