Quantifying the Impact of Biomass Burning Emissions on Major Inorganic Aerosols and Their Precursors in the U.S.

The primary sources for inorganic aerosols from biomass burning are rather negligible, but they are predominantly formed chemically following emission of their precursors (e.g., SO 2 , NH 3 , HO x , and NO x ). The biomass burning contributions to some of the precursors can be considerable. Accordingly, we quantify the impact of the emissions on major inorganic aerosols in April–October 2012–2014 using a regional model simulation verified by extensive surface observations throughout the U.S. Simulated CO enhancements on an hourly basis are used to classify the U.S. into weak‐moderate (5 < CO Biomass ‐CO Base  < 20 ppbv) and strongly impacted periods (CO Biomass ‐CO Base  > 20 ppbv). This separation not only facilitates the identification of the spatial frequency of the impact but also helps to filter out nonimpacted periods, enabling us to focus on long‐term contributions. Despite the nonlinear responses of several trace gases to emissions, we observe increases (weak‐moderate and strong) in daily surface SO 4 2− (1.16 ± 0.32 and 6.57 ± 4.65 nmol/m 3 ), NO 3 − (0.36 ± 0.63, 4.70 ± 7.05 nmol/m 3 ), and NH 4 + (2.70 ± 0.92 and 17.82 ± 15.17 nmol/m 3 ) on a national scale. These primarily resulted from (i) increases in daily surface SO 2 (0.02 ± 0.01 and 0.10 ± 0.07 ppbv), afternoon OH (1.28 ± 4.24 and 12.82 ± 23.76 ppqv), and H 2 O 2 (0.06 ± 0.02 and 0.10 ± 0.08 ppbv), which may have accelerated the conversion of S(IV) to S(VI), and (ii) increases in daily surface NH 3 (1.08 ± 0.73 and 8.61 ± 7.73 nmol/m 3 ) and HNO 3 (1.44 ± 0.48 and 7.15 ± 4.25 nmol/m 3 ), which could have produced more particle‐phase NH 4 NO 3 . In the West, where atmospheric moisture is limited, enhanced SO 4 2− leaves less available water for NH 4 NO 3 to become ions. Our results suggest that the major inorganic aerosol enhancement (mass) can reach to 23% of that of the carbonaceous aerosols.